timber frame building

Timber Frame Building

Guide to Platform Frame Construction

Benefits of Timber Frame Timber frame building is simple in concept and well within the scope of anyone with a working knowledge of general building practices. It uses plywood nailed to a timber framework to form a robust structural shell.

Timber frame buildings can be finished with a wide variety of external claddings such as brick, stone, cement based rendering systems, tile hanging, wood or plastic either singly or in combination. Cladding is applied when the timber frame shell is complete. Internal lining, which proceeds at the same time as soon as the building is watertight, is generally with plasterboard, which accepts many different finishes.

To architects, timber frame buildings offer:

• Design flexibility • Variety of form • Use of engineered structural materials with consistent

performance • U values significantly better than statutory minimum values

To builders, timber frame construction means:

• Rapid erection • Efficient use of construction and financial resources • Simplicity of construction • Variation in structural component size to suit handling

by crane or manually

To their owners, timber frame buildings offer:

• Individually designed structures built to the exacting standards of regulatory authorities

• Lower heating costs • High degree of comfort and convenience • Ease of decoration and no internal cracking due to

drying out of walls • Availability in an exceptionally broad range of styles

and designs • Sound insulation superior to British Standards requirements

History and Development In Britain, as in many countries throughout the world, timber was the first material used to build permanent homes. There are many fine examples today of traditional timber frame houses built centuries ago that con- tinue to perform their original function. Other timber frame buildings, constructed more along the lines of the system used today, are still in use in their original form and are over 150 years old.

The timber frame building tradition was taken by British settlers to North America where timber frame building techniques became increasingly refined. Today, timber frame building, with its comfort, economy, energy efficiency and use of renewable resources, is so practi- cal and effective that 90% of North American homes, and increasing numbers elsewhere in the world, are constructed using this building method.

In the 1960s, the Canadian government and wood products industries began a program to reintroduce timber frame housing to the UK. Since that time, the building of timber framed homes in the UK has increased steadily, the total number now over one million.

A well developed, researched and tested building method, timber frame is the most widely used form of low-rise housing construction in the world and its use is being extended into non-residential building. Developments such as hotels and motels, sheltered housing, low-rise commercial properties, community centres and many other building applications are all benefitting from the many advantages that timber frame has to offer.

Guide to Platform Frame Construction The timber frame method outlined in this guide is called Platform Frame Construction. It is the most widely practiced timber frame building method. In this form of construction, each floor is sheathed with

plywood to the perimeter of the building as work proceeds, providing at an early stage in construction a platform on which walls and internal partitions are raised in storey height lifts (Figure 1).

Reference is made throughout this guide to the Building Regulations for England and Wales; the Technical Standards for compliance with the Building Standards (Scotland) Regulations; British Standards and the technical requirements of the National House Building Council (NHBC) and Municipal and Mutual

Insurance (Foundation 15). While every effort has been made to cover all detail presented as completely as possible, building designers and certifiers should ensure full compliance with all statutory and job specific requirements.

This guide is prepared by the the Council of Forest Industries, an industry association representing plywood and timber manufacturers from British Columbia and Alberta, Canada. COFI staff has been closely involved with the reintroduction and subsequent development of timber frame building in the UK for over 30 years. The guide provides information and guidance on the correct application of the techniques and practices necessary to achieve a high degree of success in timber frame building.

8

CLS S-P-F studs COFI plywood floor sheathing Solid blocking Sole plate COFI plywood wall sheathing CLS S-P-F rafters CLS S-P-F floor joists Sleeper wall (honeycomb) Foundation wall

Damp-proof course

9

Figure 1. Elements of Platform Frame Building

CONTENTS

Part 1

Timber Frame — Theory and Practice 5

Structural Materials 6

Part 2 Fabrication and Assembly 9

Erection Sequence 9

Footings and Foundations 10

Floor Framing 14

Walls and Partitions 19

Roof Construction 24

Part 3 Service Installation and Fire Protection 32

Installation of Services 32

Thermal Insulation 33

Vapour Control Layers and Breather Membranes 34

Fire Resistance 35

Part 4 Finishing 37

Internal Finishes 37

Floor Finishes 37

External Wall Finishes 38

TIMBER FRAME — THEORY AND PRACTICE

Timber frame construction is an energy efficient building system in which plywood is nailed to the timber framework of walls, floors and sometimes roof components, forming effective structural diaphragms. These are assembled to form a rigid yet resilient independent structural framework designed to support and transfer to foundations all dead, live and wind loads. Plywood sheathing contributes fundamentally to building stability as well as effecting rapid closure of the frame for weather protection.

The structural integrity of the frame relies on properly nailed connections, in most cases the nails being stressed in shear rather than in withdrawal. With the exception of plywood used as a combination sheathing and cladding, external decorative and weathering f in- ishes have no direct structural function. The fire resistance required by the building regulations is usually achieved by the use of plasterboard internal linings.

The use of panel products for external and internal linings is a natural aid to modular planning and where this is adopted, a major grid of 1200 mm will accommodate both plywood and plasterboard panel sizes (Figure 2).

The space between the framing members in walls, floors and roof permits the easy installation of con-

cealed services and thermal insulation as well as acoustic insulation if required. Internal fire resistance and control of flame spread is provided by internal linings. These are usually of gypsum plasterboard which provides a dry, smooth, seamless base for internal finishes.

Durability of timber framed buildings is ensured by proper detailing, care and attention in construction and the correct specification and application of breather membranes and vapour control layers. Ventilation of timber ground floors, roof spaces and the interiors of buildings will maintain the moisture content of the timber structure well below the level at which fungal attack can occur. Natural durability is further enhanced by the requirements of the NHBC that all timber members in external walls be preservative treated. Plywood does not require treatment.

The structural recommendations in this publication provide a guide to sound, economic construction. De- signers are reminded that engineering calculations for the loadbearing timber structure are required by local authorities and organizations such as the NHBC. The NHBC further requires that the designs are checked by a registered NHBC certifier and that a certificate be

COFI plywood wall sheathing

!Ivj

.—...- ——

Grid line

External corner

Figure 2. Location of Major Grid Lines

issued on Form HB 353b in England, Wales and Northern Ireland and Form HB 210 in Scotland. In most instances, firms offering a timber frame design and fabrication service supply the required structural

compliance documentation.

Modern timber frame buildings in the UK are built to conform not only with building regulation requirements but also the additional requirements of the NHBC and Foundation 15. The framework is designed by structural engineers and construction details checked by independent appraisers. This further serves to ensure compliance with all statutory requirements and satisfactory building performance throughout its life, required to be a minimum sixty years.

Structural Materials

TIMBER

For accuracy in the fabrication of components and to conform with engineering requirements, timber frames should be built only with timber manufactured to precise dimensions and stress graded to determine its structural capacity.

A sustainable and reliable source of such timber is western Canada. Structural timber from British Colum-

Table 1. CLS Timber Sizes

Nominal Size (in.)

Surfaced Dry (actual size)

(mm)

2x3 38x63 2x4 38x89 2x6 38x140 2x8 38x184 2x10 38x235 2x12 38x285

CLS timber is produced to imperial sizes. Metric dimensions are rounded off to the nearest mm for commercial application and are the standard reference sizes in the UK. Surfaced dry timber has a moisture content not exceeding 19%.

bia and Alberta is planed all round with corners slightly rounded, to Canadian Lumber Standards (CLS) dimensions (Table 1). Each piece is stress graded in accordance with National Lumber Grading Authority (NLGA) rules and is stamped (Figure 3) to show grade, grading agency, manufacturer and species group. (Full titles for all standards are given in Standards References, page 41) Canadian CLS material supplied to the UK for timber frame construction is kiln-dried to a moisture content not exceeding 19%.

Figure 3. Timber Grade Stamps

The species group refers to the practice of harvesting and marketing together timber of different species but similar strength properties, appearance and intended end uses. The principal species group used in timber frame construction in the UK is SPF (Spruce-Pine-Fir). Hem-Fir (Western Hemlock and Amabilis Fir) is also used for some applications such as floor framing.

The size and grade of framing members in timber frame construction are determined by structural engineering calculations and other considerations. For instance, framing members must be at least 38 mm thick to accept joints in plasterboard linings. Additional

Table 2. NLGA Grades Commonly Used in Timber Frame Building

Category Grade

CLS Dimensions (Surfaced Dry)

(mm) Recommended Uses

Light Framing Construction Standard

38 x 63 38 x 89

Loadbearing walls and non-loadbearing partitions, plates and noggings

Structural Light Framing No. 1 and No. 2 38 x 63 38 x 89

General construction, principally wall framing. Includes most Ioadbearing walls.Trussed rafters.

Structural Joists and Planks No. 1 and No. 2 Select Structural

38 x 140 38 x 184 38 x 235 38 x_285

Joists, rafters and beams for loadbearing application. Trussed rafters.

Machine Stress Rated (MSR) Various 38 x 89 38 x 140 38 x 184

All the above applications. Particularly suited to trussed rafters. Available with higher permissible stresses than visual grades.

Note: For MSR timber other sizes may also be available depending upon the supplying mill.

.4

A.FPA' 00 S—P—F

1 S-DRY Aibsrta Forest Products Association

1 S-DRY

S-P-F Cartboo Lumber Manufacturers Association

I1fflAsi) 1 00 S—P—F

inturlor Lumber Manufacturers' Association

f77C7 S-P-F (.LJnI. S-DRY

100 NQ1 Northern Interior Lumber S.ctor Council of Forest Industries

width over that required for structural purposes may be needed to accept the specified thickness of insulation or to accommodate services located within wall cavities.

NLGA GRADES

NLGA grades take into account both the size of the timber and its intended use (Table 2). CLS timber is available visually graded and machine stress rated.

NLGA grades are authorized for use by British Standards. Detailed information on permissible stresses may be obtained by referring to the relevant Standards or to the Council of Forest Industries.

BS 4978 GRADES

Timber from western Canada is also imported sawn to the metric dimensions specified in BS 4471 and visually graded to BS 4978. This grading may be done in British Columbia or the UK. Principal visual grades are General Structural (GS) and Select Structural (SS). A number of BS 4978 machine grades using western Canadian timber are also available from both UK and Canadian machine grading sources.

Sawn timber is used principally for floor framing but rarely used in preference to CLS for wall or partition framing.

BS 5268 STRENGTH CLASSES

Canadian structural timber, whether graded to the NLGA, MSR or BS 4978 grading systems, can be categorised in accordance with BS 5268: Part 2 strength classes. SC3 and SC4 are the most commonly specified strength classes for general timber engineering (Table 3).

Most timber frame designs, however, are produced using species/grade combinations as this provides optimum structural design and the most economic and efficient framing solutions.

MOISTURE CONTENT

The use of breather membranes in timber frame construction ensures that any excess moisture in the timber will be dissipated until the moisture content stabilizes to equilibrium conditions. Frames are usually fabricated at approximately 20% moisture content and it is important to ensure that when components are delivered to site they are placed in protective storage prior to erection. Erection and roofing-in is rapid with timber frame construction and moisture levels in framed corn-

ponents do not change appreciably during the course of building.

In timber frame construction, any shrinkage in the joists tends to be concealed between the floor and ceiling linings but shrinkage in the studs subsequent to decorating can lead to defects in the finish. Vapour control layers and internal linings should not be installed until the moisture content of timber frame components is 20% or lower.

PRESERVATIVE TREATMENT

BS 5268: Part 5: categorises the need for treatment based on risk assessment. Treatment is required for roof timbers in those areas of the UK susceptible to infestation by the House Longhorn Beetle; timber in certain flat roofs; sole plates below the damp-proof course; timber set in concrete; loadbearing joinery; timber in ground contact; and timber in contact with brickwork or other materials below the damp-proof course (dpc).

Treatment for timber in other situations is categorised as being either desirable, optional or unnecessary. Additional preservative treatment of external loadbearing walls is deemed necessary by the NHBC and Foundation 15 for the more commonly used species. Treatment of suspended ground floors is indicated as optional where oversite treatment and sub-floor ventilation is adequate.

All softwood joinery exposed to the weather should be preservative treated, as well as softwood claddings with the exception of Western Red Cedar. As with any form of construction, correct detailing of claddings and openings is necessary to prevent weather penetration.

PLYWOOD

Wall, floor and roof sheathing in timber frame construction is an integral element in the stability of the building. The performance and durability of the sheathing are equal in importance to that of the timber itself. Canadian COFI EXTERIOR Quality Certified plywood is the preferred choice of experienced designers, manufacturers and builders. It is permanently bonded with a resin glue that meets the most stringent requirements of BS 6566: Part 8 for Weather and Boilproof' (WBP) bond type which is unaffected by moisture or temperature. COFI EXTERIOR plywood has a high strength to weight ratio, dimensional stability, accepts a wide range of fasteners and adhesives, and resists site damage.

Table 3. Canadian Species Combinations and Grades which Satisfy BS 5268 Strength Classes SC3 and SC4

Strength Class Douglas Fir-Larch (D-F-L) Hemlock-Fir (Hem-Fir) Spruce-Pine-Fir (S-P-F)

SC3 GS (BS 4978) Str. No. 1&2 (NLGA)

GS/MGS/M50 (BS 4978) Str. No. 1&2 (NLGA)

GS/MGS/M50 (BS 4978) Str. No. 1&2 (NLGA)

SC4 SS (BS 4978) Sel. Str. (NLGA)

SS/MSS/M75 (BS 4978) Sel. Str. (NLGA)

SS/MSS/M75 (BS 4978) Sel. Str. (NLGA)

Notes: 1. Appropriate to most commonly used section sizes. 2. M75 Hem-Fir is in SC5 but is included in this table for completeness. 3. The stronger the timber the higher the strength class number. 4. Timber of a higher strength class can always be used in place of lower strength class timber but not vice-versa.

Two types of plywood fully recognized in British Standards for structural uses are manufactured in British Columbia; Douglas Fir plywood manufactured to the Canadian Standard CSA 0121-M1978 and Canadian Softwood plywood manufactured to CSA 0151-M1978 (Figure 4).

LITY CERTIFIED BY COFI

CSAO121-M

BC 100

COFI EXTERIOR

CANADA

0UALI1 CERTIFIEE PAR COFI

ACNOR 0121- CSD

LITY CERTIFIED BY COFI

CSAO151-M

BC 100

COFI EXTERIOR

CANADA

OUALITE CEATIFIEE PAR COFI

ACNORO151- cc

Figure 4. Plywood Grade Stamps

PLYWOOD GRADES AND PRODUCTS

COFI EXTERIOR plywood is manufactured in a number of grades ideal for use in timber frame building. Unsanded plywoods include Sheathing, Select and Select Tight-Face. In all grades, knots, knot-holes and other natural growth characteristics are limited in size and number.

Sheathing grade may be left exposed where good appearance is not essential, but it is used mainly in

applications such as wall and pitched roof sheathing where it is covered by other construction or finishes. Select is a higher grade with more stringent limitations on growth characteristics. It is used typically for flat roofs and some floor sheathing applications. Select Tight-Face is an appearance improved plywood with open face defects excluded. It provides a smooth uniform surface well suited to floors where finishes such as vinyl cushion and tiles or carpeting are to be laid direct.

For the same thickness, number of plies and species, Sheathing, Select and Select Tight-Face grades have identical strength, stiffness and other structural performance properties.

COFI members have developed two innovative sheathing panels with patented tongue and groove profiles. COFI ROOF and COFI FLOOR T&G make roof and floor sheathing faster and easier. They are available in both Douglas Fir and Canadian Softwood plywood.

PLYWOOD DIMENSIONS

COFI EXTERIOR plywood panels are manufactured in 1220 x 2440 mm (imperial) and 1200 x 2400 mm (metric) sizes. Other sizes are available on special order. COFI plywood is produced in a range of thicknesses (Table 4).

COFI FLOOR T&G and COFI ROOF panels are available in imperial and metric sizes. These tongue and groove panels are profiled on the long edges and have standard net widths of 1205 mm (imperial) and 1185 mm (metric).

PLYWOOD PROTECTION

While plywood does not require preservative treatment, the Building Research Establishment deems improved protection is necessary for wall sheathing in some areas. These are where exposure to wind-driven rain is classified as 'very severe" and where shelter is not afforded by adjacent buildings or other features. The improved protection requirement applies to walls facing the direction of prevailing wind. For plywood wall sheathing, the alternatives are: the use of high performance breather membranes or a cavity wall with suitable external cladding other than masonry.

Table 4. Thicknesses and Sizes of Select and Sheathing Grades of COFI EXTERIOR Unsanded Plywood

Panel Thickness Panel Sizes

7.5 mm 9.5 mm Square Edge: 11 mm 1220 x 2440 mm 12.5 mm** 15.5 mm*** 1200 x 2400 mm

18.5 mm*** COFI ROOF and

20.5 mm*** COFI FLOOR T&G:

22.5mm 1220 x 2440 mm

25.5 mm net face width 1205 mm

28.5 mm 1200 x 2400 mm 31.5 mm net face width 1185 mm

COFI ROOF plywood. ** Regular plywood or COFI ROOF plywood. Regular plywood or COFI FLOOR T&G plywood.

Note: All thicknesses and sizes are metric. Some approximate imperial dimensions for example, 9.5 mm (in.), 2440 x 1220 mm (8 ft. x 4 ft.).

Timber frame construction is highly versatile but for simplicity of construction and economical use of materials, a joist and stud spacing of 600 mm on centres is normally used. With an overall plan dimension mea- sured at the inside face of the studs in increments of 1200 mm, stud spacings of 300, 400 or 600 mm are possible while still allowing for optimum utilisation of 1200 mm X 2400 mm panel products.

It is advantageous if any deviation from the 1200 mm dimension made necessary by planning requirements can be made to fall on the stud spacing adopted.

The spacing of studs should be considered when determining the dimensions of openings in external walls. Internal partitions can be positioned without reference to grid lines although additional noggings or studs may be needed to accept finishes and fixings. Loadbearing partitions should where possible be placed to accommodate available lengths of joist stock.

Factory Fabrication In the UK, experience indicates that factory fabrication is ideally suited to the production of structural and non- structural frameworks and panelized components. This includes internal and external wall panels, application of plywood sheathing to made up components, pre- cutting floor joists or fabrication of structural floor components and the fabrication of trussed rafters.

Windows and pre-hung doors may be fixed in the wall panels or delivered to the site as separate assemblies.

Usually, services are installed and finishing operations carried out on-site, though plumbing assemblies and wiring harnesses have been used effectively. The size of wall or floor panels supplied to site is to a large extent determined by handling and transport factors.

Where insulation, internal linings and external claddings are factory applied, the panel size is reduced if manual handling is to be used on site. Where mechanical handling equipment is used, panel size depends upon the stability of the panel in transport and erection and the capacity of handling equipment.

Site Fabrication of Timber Components Although commonplace in North America, on-site building ("stick building") in the timber frame method is seldom done in the UK. In on-site building, timber is either cut on site or supplied pre-cut. Walls are assembled and sheathed on the plywood platform (or con-

crete ground slab) and tilted up into position. Window and door assemblies are normally installed after erection.

It should be noted that the structural frame, when erected, should differ only in minor details whether the components are factory fabricated or site fabricated. Both methods are structurally acceptable.

Erection Sequence The following erection sequence is for a two-storey building with a suspended ground floor, using factory fabricated wall components. The COFI publication Check It Out contains a comprehensive list of erection

requirements in a format designed for use on-site.

Foundations 1. Clear and rough grade the site. 2. Stake out building. 3. Excavate for foundations and services. 4. Lay masonry or pour concrete footings. Build

foundation walls. 5. Install service intakes and sewer outlets. 6. Back-fill and rough grade. 7. Lay compacted fill as required for ground seal slab

and driveway or provide ground cover with polythene membrane and sand or lean mix concrete.

8. Pour slabs and driveway.

Framing 1. Set sole plate dead true and level on cement

mortar bed and damp-proof course and anchor to foundations.

2. Pre-cut floor joists laid and nailed to sole plate and header joists. Fix continuous blocking between joists.

3. Lay plywood structural floor and tack in position. 4. Erect ground-floor external panels, secure to floor

frame and brace temporarily. 5. Erect and secure ground-floor interior panels. 6. Apply second top-plate, check walls for alignment

and nail all vertical elements together. 7. Erect first-floor joists and bridging, structural floor

and partitions - all as for ground floor. 8. Erect trussed rafters and brace or apply plywood

sarking. 9. Install felt, battens and roof tiles.

10. Apply breather membrane if not factory applied, install windows and external doors.

FABRICATION AND ASSEMBLY

Services Fastenings 1. Complete plumbing carcase. 2. Install electrics carcase. 3. Carcase for central heating.

Internal Finishes 1. Install insulation and vapour control layers. 2. Apply plasterboard ceilings followed by walls. 3. Tape and fill joints, lightly sand and apply finishing

coats. 4. Fix interior trim, cabinets and joinery. 5. Paint interior. 6. Install finished electrical items. 7. Complete nailing and clean plywood floors. 8. Fix finished flooring. 9. Install finished plumbing items.

10. Install heating outlet grills and finishing hardware. 11. Clean and touch up paintwork. 12. Inspect.

External Finishes These operations can proceed concurrently with internal finishes, subject to weather conditions.

1. Apply external cladding. 2. Apply trim and rainwater goods. 3. Paint exterior. 4. Carry out final clean-up, grading and landscaping.

Protection of Materials On Site

PLYWOOD

Canadian COFI EXTERIOR plywood requires lithe more than the care given to good grade timber. Panels should always be stacked flat and stored in a dry place. If a natural finish is desired, cover the panels from sun- light to ensure that the surface colouring is retained.

TIMBER

CLS timber and other items of woodwork should be protected from the weather on arrival at the building site. It is best to establish a schedule so that timber, joinery, and other building materials are delivered only as needed, and to follow these simple rules:

1. Keep piles of timber at least 150 mm above the ground and protect them with a waterproof cover.

2. Store window and door frames, timber cladding and external trim inside. Where this is not possible they should be kept off the ground and covered. These items are usually factory-primed when received. Untreated joineiy and mouldings should receive a water repellent treatment or a priming coat of paint on all surfaces upon delivery.

3. Store internal doors, trim, flooring and cabinet work in the buildings. Oversite concrete and wet plaster, if used, should be allowed to dry before internal finishes, cabinets, flooring or panelling are stored or fixed.

As a rule all timber to timber connections in the frame are made with nails. Their number, length and gauge are critical to the structural integrity of the frame.

Common wire nails are used where load is applied at right angles to the nail (shear loading). Other types of nails including those with annular rings or helical grooves are used where higher withdrawal resistance is required such as the fastening of plywood floor sheathing to joists. Nail joints are strongest when the load is acting at right angles to the nails and in timber frame construction nearly all nails are so loaded.

A nailing schedule for CLS timber (38 mm) is shown in Table 5. For thicker timbers, nail specification should be adjusted to ensure adequate structural performance.

Corrosion resistant fixings should be used when directed, such as by NHBC Technical Standards.

Footings and Foundations

Building regulations require that buildings should be constructed so that combined dead, imposed and wind loads are safely transmitted to the ground and that movements of the subsoil caused by swelling, shrinkage or freezing do not impair the stability of the building.

In most respects the foundations used in timber frame building are the same as those used in other forms of construction. However, timber frame is lighter than masonry loadbearing construction and designers should take advantage of this factor in determining the widths of footings.

Using accurately prefabricated frames requires a similar degree of dimensional accuracy for the foundation walls or concrete floor slab. These must also be constructed level to receive the sole plate or wall panels directly if plateless construction is used. Foundation walls may be brick, block or concrete. An alternative, used in many areas in North America, is permanent wood foundations. These provide well-insulated base- ments without the dampness associated with other forms of construction.

Concrete Slab Ground Floors

Although concrete slabs do not have the warmth and comfort of timber they can be an economic form of construction on flat sites with good sub-soil conditions. Slabs must be constructed to prevent the passage of moisture from the ground and incorporate a waterproof membrane in accordance with the recommendations of the relevant building regulations.

Where a slab is not poured integrally with the foundation wall the waterproof membrane can be a well lapped sheet membrane laid on compacted fill blinded with sand, turned up and interconnected with the dpc under the sole plate or bottom plate of the wall panel. Another method is to apply two coats of waterproofing

on top of the slab underneath the screed. In some cases both methods are used as added protection against moisture penetration.

NHBC Technical Standards require that where the depth of fill under a slab exceeds 600 mm, a suspended ground floor must be used which is structurally inde-

pendent of any till. In the case of concrete ground floors, this means the use of a reinforced in-situ concrete ground slab or alternatively, use of a precast concrete floor system.

Examples of foundation details for concrete slab floors are shown in Figure 6.

Table 5. Nailing Schedule for CLS Material Used in Timber Frame Construction1

Construction Detail Number and/or

Nail Length (mm) Spacing

Sole plates See Note 2 Wall panel bottom rail: • direct to foundation • to sole plate • to header/edge joists; blocking (through plywood floor decking)

As for sole plates 75* 300 mm c/c 75 300 mm c/c

Sole plate to header/edge joists (Suspended floor)

75 300 mm c/c and at joists

Wall panel to wall panel 75 2 @ 600 mm c/c

Head binder to wall panel 75 300 mm c/c

Floor joist to sole plate or head binder (skew nailed) 75 2

Header joist: • to joist ends • to sole plate or head binder

75 75

3 300 mm c/c

Edge joist: • to sole plate or head binder 75 300 mm c/c Double joists, trimmers, lintels etc. (fix nails from both faces) 75 2 @ 300 c/c Joist hangers, framing anchors etc. (trimming at openings etc.) Fix in accordance with manufacturer's instruc-

tions. Use nails of correct size in all holes.

Holding down brackets/straps and other fixing devices Fix in accordance with manufacturer's details and project fixing schedule.

Solid blocking 75 2 each end

Noggings 75 2 each end

Herringbone strutting 60 2 each end

Internal partitions: • to wall, floor and roof framing 75 600 c/c

Internal Ioadbearing walls and racking panels . (note any manufacturer s special requirements)

As for wall panels

Gable peak panels As for wall panels Wall studs (site fixed): • single • to form double - to ends

- to adjacent stud Additionally, fix studs through plywood, nailing as below.

75 75 75

2 each end 2 each end 2 @ 600 c/c

COFI plywood wall sheathing COFI plywood roof sheathing COFI plywood floor sheathing

50 50** 50**

150 mm c/c around board edges, 300 mm c/c at intermediate supports

75 mm long nails will generally be 3.35 mm diameter; shorter nails 3.0 mm diameter. All shall be galvanised or with similar treatment. ** Denotes ring-shank nails. If round wire nails are used increase length to 60 mm.

Notes: 1. This schedule represents minimum fixing requirements and does not replace a specific schedule for the project which should be available on-site at all times.

2. Alternative fixings for sole plates include masonry nails, ballistic nails, bolts (cast-in, expansion or through bolt types), metal straps and proprietary fixing clips. All should be fixed in accordance with manufacturers' instructions.

3. For roofing members and all special fixing details refer to project fixing schedule.

NOTE: Examples only. Other combinations and methods are possible.

2 3 4 5 6 7 8 9

10 11

12 13 14 15 16 17 18 19 20 21

22 23 24 25 26 27 28

NOTE: Weep holes to be at least 150 mm below lowest timber.

12.5 mm plasterboard Vapour control layer (or vapour check) Insulation 38 x 89 mm CLS studs COFI plywood sheathing COFI FLOOR 38 x 89 mm CLS plate 12.5 mm diameter anchor bolts or other restraint Damp proof course Tanking system Weep holes 900 mm

Approved ground cover Blinded hardcore Breather membrane Cladding Rigid insulation Concrete slab with or withour insulation

Damp proof membrane Cavity (minimum 50 mm) Facing brick with flexible metal ties Trench fill or strip footing foundation to suit soil conditions CLS joists (depth varies) Sole plate Airbricks for under-floor ventilation CLS header joists Foundation walls Floor insulation if required Precast concrete suspended flooring system

Basement foundation

Figure 5. Foundation Details for Suspended Floors

2

3 4 5

13

21

Timber frame directly clad Brick veneer with precast floor

2 3

4

5 14 15 7 6

25 • 22 23 9

8

26

10

16

Suspended Timber Ground Floors

Suspended timber ground floors provide a good alternative to concrete floors (Figure 5). In accordance with building regulation requirements, resistance to the passage of moisture from the ground in this type of floor can be achieved by a ground cover of 100 mm of concrete of suitable composition. Alternatively, a damp- proof membrane of 1200 gauge polythene sheet can be used. The sheet should be laid with sealed joints on a bed of material that will not damage the sheet and covered with 50 mm of suitable mix concrete. Inert fine aggregate is also suitable. In some cases, reference to Clause 11 of CP 102: 1973 may be necessary.

An air void below the joists of a minimum depth of 150 mm adequately cross ventilated in accordance with the relevant building regulations is required. Joists can be preservative treated for additional protection.

A technical guide to construction of suspended timber ground floors is available from the Council of Forest Industries.

Sole Plates In construction using separate sole plates, the first operation is to securely anchor the sole plate to the completed slab or foundation walls in accordance with structural design specifications. A variety of fixings may be used such as bolts set in concrete, through bolts, steel pins shot fired into concrete or pre-set fixing blocks, galvanized or stainless steel anchors set in masonry, and many others.

The sole plate is usually the same dimension as the studs. It must be pressure treated and installed over an approved dpc. Where it abuts a floor screed the dpc should be turned up and stapled to the sole plate.

Brick veneer

2 3 4 5

11 16

17

7 9

8

1 12.5 mm plasterboard 2 Vapour control layer (or vapour check) 3 Insulation 4 38 x 89 mm CLS studs 5 COFI plywood sheathing 6 COFI FLOOR 7 38x89mmCLSplate 8 Sole plate restraint/fixing 9 Damp proof course

10 Blinded hardcore 11 Breather membrane 12 Cladding 13 Concrete slab with or withour insulation 14 Damp proof membrane 15 Edge plinth detail 16 Cavity (minimum 50 mm) 17 Facing brick with flexible ties 18 bottom plate protection detail and weathering 19 Rigid edge insulation 20 Screed or directly finished slab 21 Trench fill or strip footing foundation to suit soil conditions 22 Foundation walls 23 Floor insulation if required 24 Ventilation space behind cladding if required 25 Cavity flashing and weep holes

FIgure 6. Foundation Details for Concrete Slab Ground Floors

2

3

4

5 11

24 12

7 9

18 8

15

22

19\

20

13

23 14 10

21

Timber frame directly clad

19

Sole plates are set dead level, where necessary on a durable bedding material no more than 20 mm in thickness as recommended by the the NHBC. Where localised shims are used for levelling, all voids must be packed solid with a cement/sand grout. The plate must not overhang the foundation support by more than 12 mm. Sole plates supporting floor joists and internal walls are similarly installed and protected.

Floor Framing

SELECTION AND PLACING OF JOISTS

Load-span tables for regularised sawn timber joists in metric sizes (BS 4471) stress graded to BS 4978 or NLGA rules are given in building regulations' structural

design guidance. The quoted spans are for two qualify- ing strength classes, SC3 and SC4 as defined in BS 5268: Part 2, and relate to timber regulansed across the section width (joist depth). Canadian species groups and

grades that meet SC3 and SC4 specifications for the most commonly used joist sizes are shown in Table 3.

As an alternative to strength classes, the use of specific grade permissible strength properties for an individual species or species group usually provides the most economical use of timber. Load-span tables for

joists produced from Canadian species groups and

grades are available from the Council of Forest Industries. These include metric sawn timber sizes

regularised in width (joist depth) to BS 4471. The use of regularised sawn timber sections is advisable as it makes floor sheathing and ceiling lining application simpler by avoiding the need to pack individual joists.

As with other members used in timber frame construction, the preferred specification is for stress

graded CLS timber, planed all round to ensure precise dimensions. Load-span data for CLS Spruce-Pine-Fir floor joists are given in Table 6. For optimum efficiency, designs using CLS timber are usually based on specific grade strength properties of the species group.

An economic assessment of joist size and spacing must be carried out in combination with the varying thicknesses of floor and ceiling linings. Since the modular panel length is likely to be 2400 mm, the spacing will be either 300 mm, 400 mm, or 600 mm. As the

span table shows, a considerable variation can be achieved in the allowable span by changing the spacing. The thickness of plywood sheathing is governed by the widest spacing. A consistently accurate joist spacing is essential to provide supports for the ends of the panel material whether plywood or plasterboard.

Continuous header joists are installed at the ends of joists bearing on external walls. They perform three functions: providing the cavity barrier required by building regulations, maintaining accurate joist spacing and preventing any tendency for joists to twist when

drying. Solid blockings installed between joists at load-

bearing internal walls perform the same functions.

Unless specific engineering calculations show lesser

support is adequate, the bearing for joists should not be less than 45 mm onto timber plates or rails. This is

to accommodate compression perpendicular to the

grain stresses at bearings. The bearing of joists directly onto masonry support walls should be not less than 90 mm for wall stability purposes. Joists should be framed into the side of steel beams where these are used.

The framed-up floor with attached plywood sheathing provides the basic working platform. It acts as a horizontal diaphragm, stiffening the structure and giving lateral support to the external walls.

BEAMS AND GIRDERS

Beams and girders have several uses. They enable openings to be created in the floor structure and support loadbearing internal walls where the joists do not have sufficient strength and stiffness. They can be used in large open areas to reduce the span and/or dimensions of the joists. They can also be used for their visual appeal.

Smaller beams and trimming and trimmer joists around openings can usually be designed as double or triple joist members which can be concealed in the floor

depth, supporting the joists on ledger rails or joist hangers. The individual members must be fastened

together to act compositely.

Larger beams can be positioned mainly within the

depth of the floor with a limited projection or, where head room allows, may be totally exposed with the

joists supported on top of them.

To avoid the possibility of excessive shrinkage it is important that the beams when installed have a moisture content close to the equilibrium of 12%. As drying large sections of timber can result in unsightly cracks, the use of glue-laminated or plywood web beams is recommended.

STRUTTING TO FLOOR JOISTS

It is now accepted that floor decking effectively dis- tributes concentrated loads over several joists and that

diagonal or solid strutting is not necessary for this function. However, it is essential that the joists have lateral constraint along their span to ensure they do not twist in service and that accurate modular spacing is maintained to provide support for the ends of floor sheathing and ceiling lining. Methods of achieving this are shown in Figure 7.

Strutting should be installed in accordance with the requirements of BS 5268: Part 2 or the NHBC and Foundation 15 technical standards. It should be noted that the NHBC requires solid strutting to be at least

three-quarters of the depth of the joists. The specification for ceiling dry-lining may require that board

edges are supported by noggings. When compatible with spacing, strutting can be installed to coincide with the joints in the plasterboard.

Solid, full joist depth blocking must be used over internal loadbearing walls. In addition to providing restraint and a cavity barrier it also assists in the distribution of load from the construction above.

Table 6. DOMESTIC FLOOR JOISTS. Permissible Clear Spans for Select and No. 1&2 Grades of CLS Spruce-Pine-Fir Structural Joists and Planks and Various Machine Stress-Rated Grades

Conditions of Use

Examples

Imposed Load

Dead Load (kN/m2)

Joist Size (mm)

Permissible Clear Joist Span (m)

Spacing of Joists Centre-to-Centre (mm)

0.25 Select No. 1&2 Select No. 1&2 Select No. 1&2

38x89t 2.032 1.886 1.770 1.641 1.447 1.187 38 x 140 3.333 3.163 3.031 2.876 2.644 2.476 38 x 184 4.360 4.138 3.969 3.766 3.467 3.195 38 x 235 5.300 5.097 4.944 4.754 4.415 4.009 38 x 285 6.113 5.878 5.707 5.487 5.166 4.808

1 450f-1 .3E 1 650t-1 .5E 1 450f-1 .3E 1 650f-1 .5E 1 450f-1 .3E 1 650f-1 .5E 38x89 1.884 2.030 1.638 1.767 1.334 1.443 38 x 140 3.160 3.330 2.872 3.027 2.503 2.639 38 x 184 4.134 4.356 3.761 3.964 3.282 3.460

1 800f-1 .6E 1 950f-1 .7E 1 800f-1 .6E 1 950f-1 .7E 1 800f-1 .6E 1 950f-1 .7E 38x89 2.121 2.165 1.849 1.888 1.513 1.546 38 x 140 3.436 3.486 3.124 3.170 2.725 2.765 38 x 184 4.495 4.560 4.092 4.152 3.574 3.626

38x89t 1.927 1.793 1.686 1.566 1.385 1.111 38 x 140 3.190 3.027 2.898 2.750 2.526 2.278 38 x 184 4.175 3.962 3.798 3.603 3.313 2.991 38 x 235 5.133 4.936 4.785 4.573 4.221 3.756 38 x 285 5.922 5.695 5.525 5.312 4.995 4.507

1 450f-1 .3E 1 650f-1 .5E 1 450f-1 .3E 1 650f-1 .5E 1 450f-1 .3E 1 650f-1 .5E 38x89 1.791 1.924 1.563 1.683 1.279 1.381

38 x 140 3.023 3.186 2.746 2.894 2.391 2.521 38 x 184 3.958 4.171 3.598 3.792 3.135 3.306


1.25 Select No. 1&2 Select No. 1&2 Select No. 1&2

38 x 89t 1.712 1.601 1.508 1.319 1.251 0.958 38 x 140 2.869 2.721 2.602 2.420 2.235 1.834 38 x 184 3.759 3.566 3.412 3.122 2.969 2.554 38 x 235 4.749 4.506 4.347 3.920 3.785 3.210 38 x 285 5.469 5.258 5.093 4.686 4.582 3.842

1450f-1.3E 1650f-1.5E 1450f-1.3E 1650f-1.5E 1450f-1.3E 1650f-1.5E 38x89 1.598 1.710 1.404 1.505 1.159 1.246 38 x 140 2.718 2.865 2.464 2.597 2.084 2.229 38 x 184 3.561 3.754 3.230 3.406 2.806 2.960


t Structural Light Framing designation member.

Notes: 1. CLS sizes and their allowable deviations are defined in Appendix A of BS 4471: 1987. 2. Visual stress grades are in accordance with the NLGA grading rules. 3. Machine Stress-Rated grades are in accordance with BS 5268: Part 2: 1991 and the North American Export Standard for Machine

Stress-Rated Lumber, 1987. 4. The tables are computed on the basis that the specification does not exclude wane at bearings. 5. The spans are calculated in accordance with BS 5268: Part 2: 1991 and BS 5268: Part 7: Section 7.1: 1989. Lateral support should

be provided to joists in accordance with the recommendations in BS 5268: Part 2 or, alternatively for domestic use situations the lateral restraint provision of the National House-Building Council Technical Standards, for example, may be used.

6. Floor joists may be notched and drilled. See page 32. 7 Spans: The calculations of the permissible spans have been based on the following criteria:

— The timber sizes are the metric dimensions shown in the tables. — The loadings shown in the table are in accordance with the recommendations of BS 6399: Part 1: 1984.

8. Deflection: The deflection in any joist has been limited so as not to exceed 0.003 span, up to a maximum deflection of 14 mm. 9. Stresses — Dry: The stresses are in accordance with those shown in BS 5268: Part 2: 1991. Further information on a wider selection

of timber sizes and grades is given in other COFI publications which are available upon request.

Domestic dwellings, houses, flats and bungalows

Uniform: 1.5 kN/m2

Slab: 3.6 kN/m

0.50 Uniform: 1.5 kN/m2

Slab: 3.6 kN/m

Select No. 1&2 Select No. 1&2 Select No. 1&2

Uniform: 1.5 KN/m2

Slab: 3.6 kN/m

Joists butt-jointed over loadbearing partition. Alternatively, joists may be lapped and nailed together.

Double top plate __ loadbearing wall

Double trimming joist

Figure 8. Framing Floor Openings

Diagonal bridging 38 mm x 38 mm minimum section

CLS blocking

Joists

joists

Studs

Double top plate

Full depth solid bridging

Figure 7. Bridging and Blocking Floor Joists

Joist hangers

Double trimmer joist

NOTE: Trimmed joists supported by joist hangers, framing anchors or nails depending on loading and detail.

FRAMING OPENINGS IN FLOORS

Proper arrangement of trimming, trimmer and trimmed

joists accomplishes the framing of openings (Figure 8). Trimming and trimmer joists should be doubled when the span exceeds 1 .2 m. Trimmer joists more than 1.8 m long should be supported at the end by joist hangers or structural framing anchors, unless they are

supported on a loadbearing wall or beam.

Floors, as with other structural elements, are subject to engineering calculations to determine sizes and support requirements. The foregoing provides general guidance only.

SUPPORT OF PARTITIONS

Loadbearing walls should be placed directly over lower walls or girder beams which support the floor framing (Figure 9).

Non-loadbearing partitions running parallel to but not in line with floor joists may be supported on blocking or noggings fixed between adjacent joists where design capacity allows or by incorporation of an additional

single joist below the partition. Alternatively and more common on longer spans, double joists are incorporated under the partitions (Figure 9). These may be spaced apart to facilitate installation of services. Partitions should not be supported only by floor decking.

Where non-loadbearing partitions run perpendicular to the joist span, an additional check will usually be

necessary to substantiate joist size and spacing.

FLOOR OVERHANG

First floor joists can project beyond ground floor walls to accommodate a change in cladding as from brick veneer to tile hanging or to provide design features such as projecting rooms and balconies. Figure 10 shows how simply this effect can be achieved when the joists run at right angles to the supporting wall.

When the front wall of the overhang is parallel to the main joists a double main joist may be incorporated to support cantilevered joists extending over the supporting wall below. The double main joists should be located at a distance of at least twice the overhang back from the lower walls. Cantilever joists may be

38 x

Stud

Attachment of non-loadbearing partitions

Fixing at bottom of partition

Figure 9. Floor Framing

Loadbearing upper wall framework

Modular joists only

Internal loadbearing wall

N Additional joist (off module)

Fixing at top of partition

Additional joist (on module)

framed into the double joists with structural framing anchors.

Engineering calculations to determine stability are essential for any substantial overhang and consideration must be given to any requirements for insulation and firestopping.

FLOOR SHEATHING

Canadian COFI EXTERIOR plywood is the preferred material for structural floors. It is resistant to damage, quickly applied and has inherent strength and stiffness. COFI members have developed COFI FLOOR T&G which has a patented edge profile designed specifically to meet the exacting requirements of floor sheathing. Square-edged plywood can also be used for this application.

Plywood is laid with face grain at right angles to the joists with tongue and groove edges providing joint support between joists. If square-edge panels are used, blocking or noggings are required between joists to provide support for long edges. Short edges must have solid bearing on joists.

The use of annular ring shank nails is strongly recommended to prevent nail-popping. Plywood should be tacked in the first instance then fully nailed in accordance with the nailing schedule (Table 5) before application of finished floorings. Minimum thicknesses of COFI EXTERIOR plywood for structural floor sheathing are shown in Table 7.

A 2 mm gap is recommended between all abutting edges of square edged plywood panels to accommodate movement due to moisture. For tongue and groove joints, the profiles are engineered to provide the required gap and installers should not force the panels together in an attempt to close it.

COMPARTMENT FLOORS

Timber compartment floors (separating floors in Scotland) are required in flats or maisonettes. Building regulations require that the floors have adequate fire resistance and that they provide adequate resistance to the passage of airborne and impact sound when used to separate dwellings.

All flats in Scotland and flats in England and Wales of more than two storeys require one-hour fire resistance.

Figure 10. Framing Details for Floor Overhangs

— Structural framing anchors

Cantilever tailing joists

Table 7. MInimum Thickness of COFI EXTERIOR Plywood used as Structural Floor Sheathing (For residential floors where the superimposed loading will not exceed 1.5 kN/m2)

Maximum Centre-to-Centre Support Spacing

(mm)

Minimum Plywood Thickness (mm)

CSP DFP

Nail Length and Diameter

(mm)

Maximum Nail Spacing

(mm)

300 12.5* 12.5* 50 x 3.0

Annular ringed 150 along edges and 300

along intermediate supports 400t 15.5** 15.5** 600 18.5** 15.5**

f A minimum thickness of 18.5 mm and maximum support spacing of 400 mm are recommended for single layer floors for use under resilient finishes e.g. linoleum, rubber and synthetic tile, and glued-on carpets normally requiring underlayment. For this type of subfloor, elastomeric glue applied between plywood and floor supports and into T&G profiles is recommended. * Regular grades of COFI EXTERIOR unsanded plywood. ** Regular grades of COFI EXTERIOR unsanded plywood and also available as COFI FLOOR T&G plywood with patented T&G edges.

Notes: 1. Plywood shall be applied with face grain perpendicular to supports. 2. Edge supports shall be provided by blocking or use of tongued and grooved edges to prevent differential movement. 3. All end joints shall occur on supports which are not less than 38 mm wide. 4. A 2 mm gap shall be left at all joints which are not COFI tongue and groove profile to allow for movement caused by moisture

variation. 5. If round wire nails are used, they should be 60 mm long.

Stud.

COFI FLOOR sheathing

Plate

Header joistS

COFI FLOOR sheathing

Multiple joist

Cantilever joists

Plates

Stud

Plates

COFI FLOOR Mineral fibre layer (resilient) COFI plywood structural deck Floor joists

ent quilt absorb

________________________________ Battens on _______________________________________ resilient clips __________________________________ or wire hangers

19mm and 12.5mm plasterboard

Figure 11. Compartment Floor with Ceiling Supported by Battens on Resilient Clips

COFI plywood structural deck Floor joists Sound absorbent quilt 19mm and 12.5 mm plasterboard

NOTE: Floating floors should be isolated from structural floors and walls.

Figure 12. Compartment Floor with Independent Ceiling Joists

Floor joists

Sound absorbent quilt

Figure 13. COFI Timber Compartment Floor

One-hour fire resistance can be achieved using two layers of 12.5 mm plasterboard fixed with staggered joints. Fire resistant floors should be supported by walls with the same level of fire resistance.

There are a number of methods for achieving the required sound transmission reduction. Provisions made in building regulations regarding sound prescribe methods which can be used without the need for further tests. Any other method which is adopted must be supported by evidence showing that the construction has been performance tested and meets the required levels of sound insulation.

Figures 11 and 12 illustrate two alternatives which have been used successfully but which are not described in the building regulations. These rely on separation of the ceiling construction from the structural floor.

One of the prescribed specifications has been further developed by COFI as an efficient and economical method of construction (Figure 13). This comprises a structural timber joisted floor with plasterboard layers forming a ceiling fixed directly to the underside of the joists with sound absorbent quilt supported above. The floor deck comprises a sandwich construction of plywood, plasterboard and mineral fibre resilient layer. This floor meets the fire and sound requirements of the building regulations. Details are available from the Council of Forest Industries.

Timber Framed Walls and Partitions In timber frame construction all loads are carried on walls fabricated with timber members and, as with other structural components, designed by a structural engineer in accordance with well established principles and current codes of practice.

Wall frames consist of vertical members (studs) uni- formly spaced to a module determined by the dimensions of the panel materials used for external sheathing and internal linings. This is to ensure that joints can be made on the centre line of the studs. The standard metric panel size is 1200 mm x 2400 mm and stud spacing is 300 mm, 400 mm or 600 mm, depending on loading conditions.

Where imperial size panels and modules are used, stud spacing must be amended accordingly and a maximum centre-to-centre stud spacing of 610 mm is acceptable. For two-storey residential buildings, a wall stud spacing of 600 mm or 610 mm is normal practice (Figure 14).

Top and bottom plates, of the same dimension as the studs, are nailed to the stud ends. Additional studs are incorporated in the frame where necessary to support point loads from supported components such as beams and trimmers. Additional studs are also used at corners and intersections to provide support for the edges of the wall linings (Figure 15). Horizontal noggings are installed as required between studs to provide support at partition junctions and for wall hung fixtures such as cupboards and basins. Horizontal noggings between studs should be fixed with the wide face vertical and

aligned flush to the internal stud line.

Independent ceiling joists

COFI plywood flooring spot

mm Plasterboard

i— Mineral fibre layer (resilient) COFI plywood structural deck

19 mm plasterboard

'"Width of stud for overlap joint

Lintel

Cripple stud

Dimension may vary for non-modular length panels

Dimension opening with tolerance to suit manufactured dimension of window

NOTES: 1. Where possible keep windows at least 600 mm from ends of panels. 2. Keeping openings to grid lines optimises number of studs used. 3. Dimension of first stud in from corners may vary from 600 mm to suit

detailing/installation of plasterboard lining.

Plate

Figure 15. WaIl Framing Details — Corners and Junctions Corner assembly

Sheathing, Corner stud assembly

Figure 14. Wall Framing

Timber spacer

Studs

Stud

Plate -

Stud

Alternate methods of corner stud assembly

38 x 89 CLS noggings

Stud assembly at junction of internal partition with external wall

Window and door openings in loadbearing walls are formed using timber lintels with their ends supported on shorter length studs called cripple studs which are nailed to adjacent full height studs (Figure 16). An alternative form of lintel in light load conditions comprises a framed-up head where the plywood sheathing is carried over the opening head to form an effective single skin plywood web beam.

A second top plate is added on-site to provide an additional tie between panel components and as an aid in correctly aligning them. The use of the second top plate makes it unnecessary in many cases for joist and rafter spacing to coincide with the stud positions. However, in some loading situations, for example with long span trussed rafters, attic trusses and heavy tile finishes, lining up trusses with wall studs may be necessary even with a double top plate. In some variants of platform frame building, this second top plate is omitted. In this case, it is necessary to align supported members such as joists and trussed rafters directly over stud positions.

Studs are usually NLGA No. 2 Structural Light Framing grade, CLS dimension 38 mm x 89 mm, though they may be increased to 38 mm x 140 mm to accommodate increased loads or thicker insulation.

EXTERNAL WALLS

Timber frame construction allows for openings and partitions to be located independently of the modular spacing of the studs. As shown in Figure 14, the mod-

Table 8. COFI EXTERIOR DFP and CSP for Wall Sheathing


(mm)

Recommended Plywood Thickness

(mm)


(mm)


(mm)

400 7.5 or 9.5 50 x 3.0* 150 along edges and 300 600 9.5 Round Wire along intermediate supports

*Although these are recommended, other nail lengths and diameters can be used.

Notes: 1. Plywood is usually applied with face grain parallel to supports. 2. All edges shall be supported and separated by a 2 mm gap. 3.See BS 5268: Part 6: Section 6.1: 1988 for design information. 4. Corrosion resistant nails shall be used if required by the specification. 5.COFI plywood is a Category 1 racking resistance sheathing as noted in BS 5268: Part 6: Section 6.1.

Table 9. COFI EXTERIOR DFP and CSP for Combined Wall Sheathing and Cladding

Maximum Centre-to-Centre Recommended Nail Length Maximum Nail Support Spacing Plywood Thickness and Diameter Spacing

(mm) (mm)* (mm) (mm)

400 8 50 x 3.0** Round Wire

150 along edges and 300 along intermediate supports 600 11

* Minimum net thickness in grooved panels. ** Although these are recommended, other nail lengths and diameters can be used.

Notes: 1. Plywood is usually applied with face grain parallel to supports. 2. All edges shall be supported and separated by a 2 mm gap. 3. Nails shall be corrosion resistant. To reduce risk of nail popping, annularly grooved nails may be used. 4. All plywood thicknesses are for sanded panels. Medium Density Overlay (MDO) panels can also be used. 5.See BS 5268: Part 6: Section 6.1: 1988 for design information. 6. COFI plywood is a Category 1 racking resistance sheathing as noted in BS 5268: Part 6: Section 6.1.

Double lintel

Stud

C ripple stud.

Plate

Door opening in Ioadbearing wall

Double lintel

Stud

Cripple stud

Plate

Window opening in loadbearing wall

Figure 16. Framing Door and Window Openings

ular spacing is continued below the window sill to provide for the joints in the linings.

Where the overall length of the wall is not a multiple of the stud module, only one spacing needs to vary. In Figure 14, the modular grid line is on the outside face of the studs and the wall component can be reduced in length by the width of the adjoining component to allow the external sheathing to overlap providing an additional connection. Some methods of corner stud assembly are given in Figure 15. Partition junctions can be accommodated by installing horizontal noggings or additional studs as shown.

WALL SHEATHING

For site assembled panels, once the external wall framing has been nailed together and squared up by check- ing the diagonals, wall sheathing is fixed by nailing in accordance with the nailing recommendations given in Table 5. Plywood sheathing turns the frames into rigid vertical diaphragms, ensuring not only stability in transport and erection but providing the inherent strength of the completed structure that is characteristic of timber frame construction.

Sheathing grade Canadian COFI EXTERIOR plywood is the preferred sheathing material. It is strong, stable, easy to work with and has a history of proven structural performance.

Plywood may be applied either vertically or horizontally with a 2 mm expansion gap left between sheets. Horizontal application with the bottom edge over- lapping and nailed to the sole plate may be specified for areas where high winds or seismic disturbances require increased strength.

Where horizontally applied plywood sheathing is used, horizontal noggings of full stud section must be incorporated to provide support and fixing for unsupported plywood edges.

Table 8 gives COFI EXTERIOR plywood thicknesses for wall sheathing. Thicknesses for COFI plywood used as combined wall sheathing and cladding are given in Table 9.

INTERNAL PARTITIONS

These are fabricated in the same way as external walls and often from the same dimension CLS timber, though some manufacturers and builders use CLS 38 mm x 63 mm studs. The design parameters for 38 x 63 mm partitions are the same as external walls with the exception that sheathing is not usually applied unless the structural engineer has designed an internal partition as a shear diaphragm.

Non-loadbearing partitions may be made from lower grade studs but must still be at least 38 mm thick to accept joints in the plasterboard and provide firmness and rigidity. In these partitions neither lintels nor cripple studs are structurally required at openings but double studs are recommended to stiffen door frames.

The use of 38 x 63 mm CLS for partition framing is now common for internal loadbearing walls as tests

co-sponsored by COFI have shown these walls achieve half-hour fire resistance. BS 5268: Part 6 for the design of timber frame walls provides guidance on the racking resistance afforded by 38 x 63 mm CLS framed walls.

ERECTION

Factory fabricated components are usually manufactured with a small minus tolerance in length and should be identified with a stamp showing their proper site location on the erection drawings. Ideally they should be off-loaded from the delivery vehicle to suit the erection sequence.

Two components forming an external corner are nailed to the sole plates (if used) and to each other, becom-

ing mutually self-supporting. Internal components are delivered to their respective positions while the perimeter framing is completed.

Internal components must be plumbed vertical before being nailed in position. After the second top plate (if used) has been nailed in position and the floor joists installed the whole frame must be checked for alignment and plumb before the floor sheathing is applied.

All framework nails specified in Table 5 must be driven as the work proceeds to ensure stability during the erection process.

Construction time varies with the complexity of design, the size of the components and the number of field operatives. On average, a single detached house should be framed up ready for roofing in two days. It is a para- mount principle that the frame should be made weather tight as soon as possible so that internal work can

proceed without being affected by weather conditions.

TIMBER SEPARATING WALLS

Compartment and separating walls between adjoining houses and flats are required to have adequate resistance to the spread of fire and resistance to airborne sound transmission.

Tests in laboratories and in the field have proved that the standard construction of a twin leaf timber frame compartment/separating wall exceeds the requirements of the regulations in both respects. Compartmentl separating wall construction details are shown in

Figure 17.

In timber frame compartment/separating walls, the layers of plasterboard provide the required fire resistance while the spatial separation of the two wall frames in conjunction with the construction, provides the required sound resistance. Although not a requirement of the building regulations, timber framed compartment/ separating walls also reduce impact sound and noise from the operation of lights and taps. Surveys carried out by the Building Research Establishment confirm that timber frame compartment/separating walls perform consistently well and are one of the best forms of construction for reducing sound transmission between dwellings.

Floor joists at right angles to separating wall

FIgure 17. Timber Compartment/Separating Walls

Firestop

External wall plans

Tile battens bedded in non-combustible material

mineral fibre insulation or mortar

-Fire stop and cavity tie

Floor joists parallel to Sections separating wall

Compartment/separating walls must be continuous from the ground seal or concrete slab to the underside of the roof covering with no combustible material carried across the cavity between the two timber framed elements. Although two layers of 12.5 mm plasterboard can achieve the required fire resistance this is usually increased to a 19 mm layer overlaid and stagger-jointed with 12.5 mm of plasterboard to achieve the required sound resistance. In some cases, three layers of 12.5 mm plasterboard are used as the wall lining.

Incombustible quilt is also installed in the stud cavity to reduce sound transmission. This enhances the fire resistance of the walls and reduces the transmission of heat from one dwelling to another. Fire stops and cavity barriers must be installed in the cavity between the two timber framed walls at intermediate floors, at the top ceiling level and at the junction between the cavity and the roof covering according to specific building regulation requirements. The cavity must also be fire stopped at the junction with the external wall and a barrier installed in any cavity behind the external finish.

Plumbing and drainage services should not be installed in compartment/separating walls and electrical services kept to a minimum. Where service boxes are installed they should not be located to coincide with boxes on the adjoining wall and must be surrounded by adequate fire-stopping to maintain the integrity of the wall.

The sound resistance of the wall is further enhanced if the bottom plate of the wall is fixed over a foamed plastic strip or the joist is sealed with a bead of acoustic sealant.

As with compartment floors, details of approved construction of timber framed compartment/separating walls are shown in the relevant building regulations. These are now included as deemed-to-satisfy construction.

Roof Construction As with other methods of construction, roofs for timber framed buildings must be weathertight, durable and structurally designed in accordance with BS 5268: Parts 2 and 3 to support all the loads to which they are subjected. Although some buildings involve on-site roof construction in the traditional pattern, most utilize light, factory-made trussed rafters set at 600 mm centres and providing a clear span across the floor area.

Trussed rafters provide economical use of timber, reduced site labour and a method of rapidly making buildings weatherproof. A straight gable roof is the easiest to frame with trusses, and gable overhangs can be formed as shown in Figure 18 which also shows a typical eaves detail. Other configurations incorporating hips, valleys and other complex configurations can also be built with prefabricated trusses. Typical framing details for flat and pitched roofs are given in Figure 19.

Designing prefabricated trussed rafters to conform with the British Standard is usually the responsibility of the

manufacturer. The design of the roof as a total structure with respect to wind and stability bracing, tank support and connections to the remainder of the timber frame structure is the responsibility of the building designer. Where diagonal bracing to trussed rafters is used, this must be in accordance with the requirements of BS 5268: Part 3 (which also contains other information required by both trussed rafter and building designers). Where plywood sarking is used this provides the bracing to the rafter planes.

Fascia

Typical eaves detail for trussed rafters

Notch

Figure 18. Roof Construction Details

Rafter

Plates

Double top plates

Screened roof vent

Soffit lining'

lining

Stud

Trussed rafter

Gable ladder ex 38 x 89 CLS

Roof overhang at gable end trussed rafter roof construction

Stud

Gable end conventional cut root construction

fl9ure !9. lyplcai Framing betaiis — Flat and Pitched oofs

Ridge board

Top plates

•Jack rafter

Valley board

• Jack rafter

Rafter

Hip board

-Jack rafter

Top plates

Multiple roof joists

Ridge board

Header

imposed

Corner framing for flat roof

plate

Studs •Rafter

Ceiling joist

Stud

In common with other structural elements of timber frame construction, trussed rafters are engineered components in which individual members can be highly stressed. The correct specification of timber is there- fore essential. Where trussed rafters are manufactured by a truss fabricator, the main chords are likely to com- prise high visual grade or MSR timber. An increasingly popular alternative is to specify CLS timber which has been machine stress rated. A number of grades and section widths are available in thicknesses of 38 mm. This thickness is well able to handle site handling and provides a stiffer truss component with a wide nailing face for fixing tiling battens and plasterboard ceiling linings. Visually graded 38 mm CLS is an alternative. Additional details of timber specification can be obtained from trussed rafter manufacturers.

Preservative treatment is not a requirement of the regulations except in those areas in southern England where infestation by the House Longhorn Beetle is known to occur.

Ventilation of the roof space above the insulation in the ceiling is provided by the equivalent of 10 mm of continuous eaves ventilation in roofs of more than 150 and 25 mm for those of a lower pitch. Ridge ventilation is also required to provide natural stack effect and continuous movement of air through the roof space.

An alternative to the use of pressed metal plate con- nectors for the joints between truss members is the use of nailed plywood gussets (Figure 20). Plywood gussets are preferred by some designers because of their additional strength and resistance to accidental damage and by some builders who prefer to carry out their own fabrication.

The CMHC publication Canadian Wood-Frame House Construction gives information on the design of plywood gussetted trussed rafers in accordance with Canadian practice. It is essential that roofs constructed with COFI plywood gussetted trussed rafter are braced in accordance with the requirements of BS 5268: Part 3. Plywood sheathing to rafter planes is a superior alternative to diagonal rafter bracing.

ROOF SHEATHING

Sheathing a roof with plywood creates a structural diaphragm with strength greatly exceeding that of a normally braced roof. For maximum strength, plywood sheets must be placed with face grain at right angles to (i.e. across) the rafters as shown in Figure 21.

Unsupported edges must be provided with noggings to accommodate nail fixing. Square edged panels should be spaced 2 mm apart to allow for slight movement due to moisture. Profiled tongue and groove plywood like COFI ROOF is designed for the correct

spacing and panels should not be forced tightly together. The fascia board should be applied first; the first course of plywood sheathing panels is then nailed over it to make a true roof edge.

Minimum thicknesses of COFI plywood for structural

pitched roof sheathing are given in Table 10.

FLAT ROOFS

Flat or very low pitched roofs rarely feature as a design element in modern housing although a greater under- standing of the factors involved in determining their

durability and functional use has led to a considerable

improvement in their performance. Permissible clear spans for CLS S-P-F flat roof joist are given in Table 11. Minimum COFI plywood thicknesses for structural flat roof sheathing are given in Table 12.

Panel edges supported by blocking or H-clips where tongue and groove plywood or COFI ROOF not used. Blocking will be neccessary where plywood is

to act a structural diaphragm.

NOTE: Where plywood roof sheathing with adequate fixing is used on trussed rafter roofs, permanent bracing at rafter level may be omitted.

Figure 21. Roof Sheathing with COFI EXTERIOR Plywood

COFI plywood roof sheathing

Figure 20. Trussed Rafter with Plywood Gussets

Flat roofs can generally be defined as either cold deck or warm deck. In cold deck roofs the required thickness of insulation is placed in the joist cavity immedi- ately above the ceiling lining and over the vapour control layer. All cavities above this insulation must be cross ventilated with the equivalent of a minimum of 25 mm continuous strip. Where flat roofs are bounded by a higher part of the building, roof vents may be required to provide this ventilation.

In warm deck roofs, either sandwich or inverted, rigid insulation is placed on top of the deck over an impervious vapor control membrane and no ventilation of the roof space is required.

ALTERNATIVE ROOF STRUCTURES

Homes can nearly always benefit from additional space. This may be needed for storage or living area or both. Today's small plot sizes can make it difficult to add extensions, and the use of trussed rafters and low pitched roofs effectively eliminates the attic as a source of useable space. However, there are methods of roof construction which, for a relatively small initial increase in overall building cost, can provide space for immediate habitation or future expansion. The principal ones and their structural components are described below.

STRESSED SKIN PANEL ROOFS

Stressed skin roof panels consist of solid timber webs (longitudinal framing members) which are factory fabricated with plywood skins glued to one or both sides (Figure 22). The strength of the timber and plywood combine to provide an efficient structural unit allowing for clear spans of considerable length.

For pitched roofs, the panels are supported on the external walls and are mutually self-supporting at the ridge, providing a roof space that can be completely lined and insulated and which is free from obstruction by structural members. Overhangs can be incorpo-

rated at eaves and gables and provision made for installing roof lights or windows. Insulation can be factory installed within the panel with provision made to provide cross-ventilation through the panel cavities above the insulation. Floor joists, which can be trimmed to provide stair openings, provide a horizontal tie between panel ends at eaves level.

Stressed skin panels can also be used for long span flat roofs and intermediate flooring systems where normal joisted and sheathed construction is uneco- nomical. Detailed information on the design and construction of stressed skin panels is available from the Council of Forest Industries.

Panel ventilation where — required by means of holes or notches in end blockings (other methods are possible)

Figure 22. Typical Four Web Stressed Skin Panel

Table 10. COFI EXTERIOR Plywood for Pitched Roof Sheathing (For pitched roof sheathing where the roofing plus superimposed snow distributed loading does not exceed 1.25 kN/m2 OR where battens are used to support a tiled or slated roof.)


(mm)

Minimum Plywood Thickness

(mm) CSP and DFP


(mm)


(mm)

300 75* 50 x 3.0 Annularly Grooved

150 along edges and 300 along

intermediate supports

400 75*

600 95* or

11** * Regular grades of COFI EXTERIOR unsanded plywood. ** COFI ROOF plywood. Notes: 1. Plywood shall be applied with face grain perpendicular to supports.

2. Edge supports shall be provided by blocking or use of tongued and grooved edges to prevent differential movement. 3. All end joints shall occur on supports which are not less than 38 mm wide except for the top chords of trussed rafters which may

be 35 mm or wider. 4.A 2 mm gap shall be left at all joints to allow for plywood movement caused by moisture variation. For COFI ROOF plywood this

spacing is inherent in the edge profile. 5. If round wire nails are used, they should be 60 mm long.

Upper skin of COFI EXTERIOR plywood

CLS S-P-F timber webs

Lower skin of

COFI EXTERIOR plywood (Not always required depending upon panel span and loading)

Table 11. FLAT ROOF JOISTS. Permissible Clear Spans for CLS Spruce-Pine-Fir Structural Joists and Planks and Various Machine Stress-Rated Grades (For roofs where there is no access other than for cleaning and repair purposes)

Imposed Load

Dead Load (kN/m2)

Joist Size (mm)

Permissible Clear Joist Span (m) Uniform (kN/m2)

Concentrated

(kN) Spacing of Joists Centre-to-Centre (mm)

300 400 600

0.75 0.9 0.25 Select No. 1&2 Select No. 1&2 Select No. 1&2 38x89t 1.792 1.659 1.768 1.638 1.723 1.599 38 x 140 3.385 3.147 3.313 3.083 3.187 2.971

38 x 184 4.872 4.542 4.743 4.427 4.178 3.964 38 x 235 6.590 6.210 6.037 5.729 5.311 5.040 38 x 285 7.926 7.524 7.274 6.904 6.412 6.085

1 450f-1 .3E 1 650f-1 .5E 1 450f-1 .3E 1 650f-1 .5E 1 450f-1 .3E 1 650f-1 .5E

38x89 1.658 1.791 1.636 1.766 1.598 1.722

38 x 140 3.146 3.384 3.082 3.312 2.969 3.185 38 x 184 4.541 4.870 4.425 4.741 3.961 4.174

1800f-1.6E 1950f-1.7E 1800f-1.6E 1950f-1.7E 1800f-1.6E 1950f-1.7E 38 x 89 1.874 1.914 1.848 1.887 1.800 1.837 38 x 140 3.533 3.604 3.455 3.523 3.291 3.338 38 x 184 5.075 5.173 4.906 4.977 4.307 4.370

1.00 0.9 0.25 Select No. 1&2 Select No. 1&2 Select No. 1&2

38 x 89t 1.792 1.659 1.768 1.638 1.723 1.599 38 x 140 3.385 3.147 3.313 3.083 2.964 2.812 38 x 184 4.856 4.542 4.432 4.206 3.883 3.684 38x235 6.158 5.845 5.629 5.342 4.941 4.687 38 x 285 7.417 7.040 6.791 6.445 5.970 5.664

1 450f-1 .3E 1 650f-1 .5E 1 450f-1 .3E 1 650f-1 .5E 1 450f-1 .3E 1 650f-1 .5E 38 x 89 1.658 1.791 1.636 1.766 1.598 1.722 38 x 140 3.146 3.384 3.082 3.312 2.809 2.961

38 x 184 4.541 4.853 4.203 4.428 3.680 3.879 1 800f-1 .6E 1 950f-1 .7E 1 800f-1 .6E 1 950f-1 .7E 1 800f-1 .6E 1 950f-1 .7E

38x89 1.874 1.914 1.848 1.887 1.800 1.837 38 x 140 3.533 3.604 3.455 3.523 3.056 3.101

38 x 184 5.006 5.079 4.569 4.635 4.003 4.062


38 x 89t 1.792 1.659 1.768 1.638 1.723 1.599 38 x 140 3.385 3.147 3.192 3.028 2.788 — 2.645 38 x 184 4.584 4.350 4.178 3.964 3.655 3.467 38 x 235 5.819 5.523 5.311 5.040 4.653 4.414 38 x 285 7.016 6.659 6.412 6.085 5.625 5.336

1 450f-1 .3E 1 650f-1 .5E 1 450f-1 .3E 1 650f-1 .5E 1 450f-1 .3E 1 650f-1 .5E 38 x 89 1 .658 1.791 1.636 1.766 1.598 1 .722

38 x 140 3.146 3.384 3.026 3.189 2.642 2.785 38 x 184 4.347 4.581 3.961 4.174 3.463 3.650

1 800f-1 .6E 1 950f-1 .7E 1 800f-1 .6E 1 950f-1 .7E 1 800f-1 .6E 1 950f-1 .7E 38 x 89 1.874 1.914 1.848 1.887 1.800 1.837 38 x 140 3.533 3.604 3.291 3.338 2.875 2.917 38 x 184 4.726 4.794 4.307 4.370 3.768 3.823


38x89t 1.723 1.599 1.684 1.564 1.616 1.503 38 x 140 3.187 2.971 3.080 2.875 2.906 2.717 38 x 184 4.524 4.230 4.344 4.067 3.883 3.684 38 x 235 6.102 5.719 5.629 5.342 4.941 4.687 38 x 285 7.417 7.040 6.791 6.445 5.970 5.664


1800f-1.6E 1950f-1.7E 1800f-1.6E 1950f-1.7E 1800f-1.6E 1950f-1.7E 38 x 89 1.800 1.837 1.758 1.793 1.684 1.718 38 x 140 3.320 3.384 3.206 3.267 3.021 3.077 38 x 184 4.705 4.792 4.514 4.597 4.003 4.062

Table 11. — (Continued)

Imposed Load

Dead Load

(kN/m2)

Joist Size (mm)

Permissible Clear Joist Span (m) Uniform

(kN/m2)

Concentrated

(kN) Spacing of Joists Centre-to-Centre (mm)

300 400 600

1.00 0.9 0.50 Select No. 1&2 Select No. 1&2 Select No. 1&2 38 x 89f 1.723 1.599 1.684 1.564 1.616 1.503 38 x 140 3.187 2.971 3.080 2.875 2.788 2.645 38 x 184 4.524 4.230 4.178 3.964 3.655 3.467 38 x 235 5.819 5.523 5.311 5.040 4.653 4.414 38 x 285 7.016 6.659 6.412 6.085 5.625 5.336



1.25 0.9 0.50 Select No. 1&2 Select No. 1&2 Select No. 1&2 38 x 89t 1.723 1.599 1.684 1.564 1.616 1.503 38 x 140 3.187 2.971 3.033 2.875 2.647 2.510 38 x 184 4.363 4.140 3.972 3.769 3.470 3.291 38 x 235 5.543 5.260 5.053 4.794 4.420 4.192 38 x 285 6.688 6.347 6.104 5.792 5.346 5.071

1 450f-1 .3E 1 650f-1 .5E 1 450f-1 .3E 1 650f-1 .5E 1450f-1 .3E 1 650f-1 .5E 38x89 1.598 1.722 1.563 1.683 1.502 1.614 38 x 140 2.969 3.185 2.873 3.030 2.507 2.643 38 x 184 4.137 4.359 3.765 3.968 3.287 3.465


t Structural Light Framing designation member.

Notes: 1. CLS sizes and their allowable deviations are defined in Appendix A of BS 4471: 1987. 2. Visual stress grades are in accordance with the NLGA grading rules. 3. Machine Stress-Rated grades are in accordance with BS 5268: Part 2: 1991 and the North American Export Standard for Machine

Stress-Rated Lumber, 1987. 4.The tables are computed on the basis that the specification does not exclude wane at bearings. 5.The spans are calculated in accordance with BS 5268: Part 2: 1991 and BS 5268: Part 7: Section 7.2: 1989. Lateral support should

be provided to joists in accordance with the recommendations in BS 5268: Part 2 or, alternatively for domestic use situations the lateral restraint provision of the National House-Building Council Technical Standards, for example, may be used.

6. Roof joists may be notched and drilled. See page 32. 7. Flat Roofs Without Access: The calculations of the permissible spans have been based on the following criteria:

— The timber sizes are the metric dimensions shown in the tables. — The loadings shown are compatible with the recommendations of BS 6399: Part 3: 1988 for small buildings' where snow drift-

ing loads need not be separately calculated. The designer will need to determine actual site loading criteria. 8. Deflection: The deflection in any joist has been limited so as not to exceed 0.003 span. 9. Stresses—Dry: The stresses are in accordance with those shown in BS 5268: Part 2: 1991. Further information on a wider selec-

tion of timber sizes and grades is given in other COFI publications which are available upon request.

Table 12. COFI EXTERIOR Plywood for Flat Roof Sheathing (For flat roof sheathing where no access is provided other than for the purpose of cleaning and repair and where the total dead plus superimposed snow distributed loading on the plywood does not exceed 2.0 kN/m2 with a maximum dead load allowance of 0.25 kN/m2. The table also allows for a concentrated load of 0.9 kN maximum in conjunction with the dead load of 0.25 kN/m2.)


(mm)

Minimum Plywood Thickness (mm)

CSP DFP


(mm)


(mm)

300 12.5* 95* or or ii 11 50 x 3.0

Annular Grooved

150 along edges and 300 along

intermediate supports 400 12.5* 12.5* or or

12.5** 11**

600 15.5* 15.5* *

Regular grades of COFI EXTERIOR unsanded plywood. ** COFI ROOF plywood.

Notes: 1. Plywood shall be applied with the face grain perpendicular to supports. 2. Edge supports shall be provided by blocking or use of tongued and grooved edges to prevent differential movement. 3. All end joints shall occur on supports which are not less than 38 mm wide. 4. A 2 mm gap shall be left at all joints to allow for plywood movement caused by moisture variation. For COFI ROOF plywood this

spacing is inherent in the design of the edge profile. 5. If round wire nails are used, they should be 60 mm long.

PLYWOOD WEB BEAMS

Plywood web beams consist of timber top and bottom

flanges to which plywood webs are fixed by nailing or gluing to form a boxed beam. The beams are economical, easily fabricated and have high strength to weight ratio. They are particularly useful in the roof construction of narrow fronted houses where they act

as purlin beams spanning across the house and supporting rafter roofs of traditional construction or stressed skin panels (Figure 23). They can also provide support for otherwise long span floor joists with the use of joist hangers. This can eliminate the need for an intermediate loadbearing wall or reduce the size of the joists.

Figure 23. Plywood Web Beam — Cross Wall Purlins

Dormer window Timber rafters or stressed skin panels

ATTIC TRUSSES

An attic truss (Figure 24) is a factory-fabricated trussed rafter designed as clear spanning or with intermediate support depending on span. Installation is at 600 mm maximum centres. The ridge component is often supplied separately to reduce the height during transport. The timber members are approximately twice the size of those in a 'W' truss and weight is a major consid-

eration. Depending upon the span and pitch, it may be necessary to use a crane for installation.

In accordance with engineering design the spacings may be varied to accommodate dormer windows, roof lights and stairs. Designers should note that stairs should run parallel to the trusses.

Figure 24. Attic Truss

MANSARD TRUSSES

These are clear span, site-fabricated trusses with joints formed by plywood gussets nailed to each side of the timber members (Figure 25). They provide almost total utilization of the roof space. The floor is not an integral part of the truss and is designed to be

Figure 25. Mansard Root Truss

supported conventionally on loadbearing walls. Windows may be installed in the plane of the roof or inset. Wind bracing is provided by plywood sheathing applied to wall and roof slopes.

-Floor joists

Dormer window

Separate ridge component fixed to main truss on site

Rooflight

Supporting walls

Living space

Framing for inset windows

COFI plywood sheathing

Timber member

Supporting walls

SERVICE INSTALLATION AND FIRE PROTECTION

Installation of Services While external service connections in timber frame building are essentially the same as for other forms of construction, internal services are more easily installed.

Wiring and piping are accommodated in the stud spaces without the expense of chipping and chasing masonry or providing specialist fixings for steel or concrete framed buildings. Regulations concerning concealed plumbing and drainage vary, and designers should check with local authorities and utility companies.

Electrics

Wiring is run from the mains connection in non- metallic flexible cables through holes drilled in accordance with normal practice in the timber framing. Metal boxes for switches, outlets and junction boxes are fixed to the sides of joists or studs or to noggings installed to receive them. In loadbearing walls, electrical services must be fire stopped. Wiring should not be stapled to the sides of studs and should be kept clear of hot water pipe runs. Where installed in insulated walls, wiring should be adequately de-rated to prevent over-heating.

Gas Installations In ground floor slab construction, gas carcasing can be incorporated in the screed or concrete base. Upstands should be accurately located to clear walls and linings with sufficient space for the use of a span- ner. Where the appliance requires a flue, this can be a prefabricated insulated metal flue. Gas boilers can be served by balanced flues which should not be located adjacent to windows or at internal angles.

For additional information consult the British Gas Publication Gas Installation in Timber Frame Houses.

Plumbing and Heating Small bore pipes may be accommodated by notching or drilling studs and joists but only where specifically allowed for in building design. The location of pipes accommodated in notches should be shown on the wall or ceiling linings and the floor sheathing to preclude the possibility of having nails driven through them.

Notches and holes in structural members are permitted providing they conform to the requirements of the structural design as outlined below. Vertical wastes and stacks can also be located in the stud cavity with larger diameter pipes accommodated as necessary by increasing the dimension of the studs or by furring them.

Both hot and cold water pipes should be lagged or insulated to prevent the loss or gain of heat. All items in the installation of plumbing and heating should be tested for leaks before being covered by internal linings.

Access Panels Where service runs are accommodated in timber floors, panels in the floor may be used to provide access. Such panels should be of square edged plywood (tongue removed from tongue and groove sheets) and be fully supported along all edges by joists or noggings. Access panels should be fixed with woodscrews to allow easy removal.

Notching and Drilling Joists and Studs Holes drilled for service runs in joists and studs should be made close to the centre line of the member depth to minimize the effect on the strength of the member and reduce the possibility of services being punctured by nails.

BS 5268: Part 2 recommends that in calculating the strength of notched joists, the effective depth is taken as the depth of the residual section. BS 5268: Part 2 and NHBC Technical Standards recommend that the effect of notches and holes need not be calculated in

simply supported floor and roof joists of not more than 250 mm in depth if the limits shown in Figure 26 are not exceeded. If other conditions apply, structural calculations must be undertaken.

BS 5268: Part 2 recommends that when it is necessary to notch or drill compression members such as studs and columns, allowance for the notches or holes should be made in the design (Figure 27). The only exception is where circular holes with diameters not

exceeding one-quarter of the width of the member are positioned on the neutral axis at between .25 and .40 of the actual length from the end or from a support. In this case, calculations are not required.

Framing should be inspected after the installation of services to ensure that the structural integrity of the timber frame has not been affected.

Notches and holes are not permitted in trussed rafters or other structural members unless allowed for in

design calculations.

13 F

* NOTE: NHBC Technical Standards allow 0.15D if contained within 0.1 to 0.20 of span from support.

Rules apply to maximum

depth of 250 mm joist

Maximum hole diameter-D/4

Thermal Insulation

Support end Maximum notch depth D/8*

value — is the calculated performance figure for each combination of materials. The lower the U value the better the thermal efficiency.

To meet the requirements of the current regulations for 38 mm

housing in England and Wales, roofs should have a U value of 0.25 Wm2K and walls and ground floors a U value of 0.45 Wm2K. In addition the combined areas of windows and roof lights should not exceed 15% of the total floor area if single glazed, or 30% if double glazed.

The U value of a brick veneered timber framed wall with 90 mm insulation quilt is approximately 0.35 Wm2K; a considerable improvement on the minimum legal requirements and a worthwhile investment in comfort and economy.

Insulation for timber framed houses is usually mineral wool or fibreglass quilt, although rigid and semi-rigid insulation may be used. All insulation should be cut accurately to size to ensure it fits snugly. Badly fitted insulation can lead to cold bridges and the possibility of condensation.

Concrete ground floors may have the insulation located either below the slab or above the slab and under the screed. In both cases it is subject to loading and must be adequate for strength and water resistance. An alternative is to float a panel flooring off resilient insulation placed over the concrete base slab. Various options are available. Reference should be made to the Building Research stablishment publication Thermal Insulation — Avoiding Risks for guidançe on these and other methods of insulation.

Keep holes apart by three -times the hole diameter

Keep holes and notches at least 100 mm apart

Notches cut only in this shaded area

0.25 x span*

0.07 x span*

Holes drilled only in this shaded area on joist centreline

NOTE: Joists must be subject to specific design where any cutting, drilling or notching other than that indicated is carried out.

Figure 26. Permissible Notching and Drilling of Joists

Holes drilled only in this shaded area on stud or column centrelines through thickness of member

'1 89mm

Figure 27. Drilling Studs and Columns

Building regulations require that reasonable provision shall be made for the conservation of fuel and power in buildings. The regulations are revised as the energy efficiency of buildings is up-graded and current requirements should be consulted.

Energy loss through the fabric of a building is largely controlled by the thermal insulation of the roof, walls and floors. The thermal transmittance coefficient — U

For suspended timber ground floors, insulation is either mineral fibre quilt installed between the joists and supported on a nylon mesh or rigid insulation boards supported on nails or battens.

Where an exceptionally high standard of insulation is required, increasing stud size to 38 mm x 140 mm and

installing 140mm quilt improves the U value to approximately 0.21 Wm2K.

To insulate a ceiling below an unheated attic space, insulation is laid between the ceiling joists extending over the top of the external wall insulation taking care to avoid blocking the ventilation air path. Insulation is omitted from the area below insulated cold water tanks so that heat rising from below will help prevent freezing.

In rooms in the roof where insulation is installed in the space between rafters there should be sufficient depth for the required thickness of insulation with a clear 50 mm air path above it. Cross battens may be required to increase the depth.

An important consideration in thermal heat loss is the uncontrolled movement of air through leakage around window and door openings. These should incorporate effective draught-proofing. Double glazing should be considered a minimum standard and consideration given to both triple glazing and insulated doors. Where solid fuel burning fire places are built they should incorporate an effective damper to close the flue when not in use.

Vapour Control Layers and Breather Membranes As a result of high humidity output from showers, washers and dryers, the air inside houses can become moisture laden. This moisture is held in suspension in the form of water vapour. During warmer penods of the

year, moisture is vented to the outside through open doors and windows. During the heating season when ventilation can be limited in a well-sealed house, moisture in the air can condense on low temperature surfaces.

Surface condensation is most likely to occur on windows, particularly if single glazing is used, or on wall

surfaces with inadequate insulation. This can result in mould growth.

The internal vapour pressure in a heated house is

higher than the external vapour pressure causing vapour to migrate through the fabric of the building (Figure 28). If this movement is not restricted the vapour can condense within the structure on those surfaces with a temperature below dew point. This is known as interstitial condensation.

Interstitial condensation is prevented by the correct specification and installation of a vapour control layer (also referred to as a vapour check or barrier) and a breather membrane. A vapour control layer of impervious sheet material is placed under the linings on the warm side of the insulation in external walls and ceilings to minimize the passage of water vapour from the interior of the house into the structure (Figure 28). Holes made in the vapour control layer to accommodate services should be kept to a minimum and where they do occur should be effectively sealed to restrict the flow of air.

Experience in the field and in laboratories shows that properly constructed timber framed structures are not at risk from interstitial condensation. Where surface condensation does occur it can be controlled by either improved ventilation, more adequate heating or a combination of both.

Kitchens and bathrooms are the principal sources of moisture in a home and these rooms should be equipped with extract fans which remove much of the moisture at source. Building regulations specify where extract fans are mandatory.

Clothes driers should be vented to the exterior of the building. Other sources of moisture can be controlled by adequate ventilation defined as one air change per hour for habitable rooms and three changes per hour for bathrooms and kitchens. Reference should be made to building regulations for information on methods of ventilation.

Vapour control layers are unlikely to be perfect and any moisture which passes them or is residual in the structure must be allowed to dissipate through the breather membrane installed on the external face of the sheathing. This membrane helps to protect and weatherproof

Vapour control layer prevents movement of water vapour from the interior into the wall and roof structure.

Exterior Low vapour pressure — Insulation

Sheathing Cladding Any vapour that passes the barrier must be allowed to escape — Breather membrane — Vapour control layer

NOTE: Where adequate ventilation is provided to large unheated roof spaces, vapour control layer to ceiling is not required

Figure 28. Controlling Moisture in Timber Frame Buildings

—*1 Interior High vapour pressure H

Outside

houses at an early stage of construction and with lightweight cladding provides a second line of defence against wind-driven rain and snow. The breather membrane, although impervious to water, is permeable to water vapour which can escape to the outside.

It is common practice to omit the vapour control layer at the ceiling level where there is a well ventilated roof space above. This reduces the positive vapour pressure on the walls and the possibility of interstitial condensation.

It is important that the separate functions of the vapour control layer and breather membrane should be understood and that the following specifications are adhered to:

VAPOUR CONTROL LAYERS

1. Vapour control layers shall be impervious sheet materials, 500 gauge polythene or equal and approved.

2. Install on the warm side of all insulation as near to the internal surface as possible.

3. Locate all joints over supporting members and lap at least 100 mm.

4. The entire surface including framing members shall be protected with the vapour control layer so that no gaps occur.

5. Openings shall be cut in such a manner that the vapour control layer fits snugly around electrical outlets, etc. without damaging the insulation.

6. Damaged vapour control layers shall be replaced or repaired.

BREATHER MEMBRANES

1. Breather membranes shall be adequately permeable to vapour transmission from within the timber frame and be resistant to the passage of water from external sources both during and after construction. Polyolefin membrane materials are now exten- sively used for the purpose and have the added advantage of being self-extinguishing to fire. Guidance on the selection of breather membrane materials is given in the Building Research Establishment Information Paper lP6/87 Fire Behaviour of Breather Membranes.

2. Install over the whole of the exterior wall surface.

3. Where sheathing or a sheet cladding is used apply one layer of paper lapped 100 mm to horizontal laps and 150 mm to vertical laps. At horizontal joints, lap upper sheets over lower.

4. Where no sheathing is used apply two layers of membrane. Vertically lap joints occurring at studs with roofing nails or staples spaced 75 mm on centres around the edges and 150 mm on centres in the field. Laps to occur only over studs and structural members.

Fire Resistance The requirements of the building regulations are designed to ensure the safety of the occupants and to restrict the spread of fire from one building or compartment to another. Means of meeting the requirements are detailed in the regulations.

There is a requirement to ensure a safe means of escape in three storey houses and flats. This can be met in single family three storey houses by providing on the ground and first floors a staircase enclosed by full half-hour fire-resistant partitions with fire-resistant self closing access doors. In three storey flats, stair- cases should be enclosed with one-hour fire-rated partitions with self closing fire-resistant doors.

Internal fire spread along surfaces is controlled by the use of wall and ceiling finishes that are resistant to the spread of flame and which do not make a significant contribution to the products of combustion. Finishes with a Class 1 spread of flame classification will normally meet the requirements; Class 3 rated finishes are permitted for defined uses. The standard wall and ceiling finish in timber frame is gypsum plasterboard with a flame spread classification of 0.

Internal fire spread through structures is covered by the following requirements of the regulations:

1. The building shall be so constructed that in the event of fire its stability shall be maintained for a reasonable period.

2. The building shall be divided into compartments where it is necessary to inhibit the spread of fire within the building.

3. Concealed spaces within the building shall be lined and subdivided where it is necessary to inhibit the unseen spread of fire and smoke.

4. Buildings in different occupancy shall be separated by walls and floors which offer a reasonable degree of resistance to the spread of fire and smoke.

In timber frame construction these requirements are usually met by the application of gypsum plasterboard as an internal lining to ceilings and walls. In two storey houses, modified half-hour fire resistance (i.e., 30 minutes loadbearing capacity, 15 minutes insulation and 15 minutes integrity) is allowed for floors. This is achieved by the application of 12.5 mm plasterboard with taped and filled joints. In three storey houses this can be up-graded to a full half-hour resistance by apply- ing a 5 mm gypsum plaster finish or by using, for example, 12.5 mm fireline board with all joints taped and filled.

Full half-hour resistance for walls and partitions is achieved by the application of 12.5 mm plasterboard. In separating or compartment floors or walls requiring one- hour fire resistance this is achieved by the application of two layers of 12.5 mm plasterboard with staggered joints. In separating or compartment wall construction the plasterboard thickness is increased to 32 mm to provide the required resistance to the passage of sound.

Designers should note that construction with a fire resistant requirement must be supported on elements with the same or greater fire resistance. For example, a loadbearing partition supporting a floor with one-hour fire resistance must also have one-hour fire resistance.

External Fire Spread The external walls and roof of a building must offer adequate resistance to the spread of fire from one building to another. Suitable cladding materials are determined primarily by the distance of the building from the relevant boundaries.

As the distance from the boundary increases, taking into account the length and height of the building, the area of unprotected construction which can be defined as combustible claddings or openings increases. The formula and tables for calculating this area are detailed in the building regulations.

Roof coverings are classified in terms of fire penetration from the outside and flame spread. The use of clay or concrete tiles is not restricted but where combustible coverings, such as cedar shingles or thatch, are used reference should be made to the building regulations for suitability of use.

Fire-Stops and Cavity Barriers Concealed cavities in timber frame construction are required to be sealed against the passage of fire and smoke by the use of effective cavity barriers. These must be positioned and spaced in accordance with

building regulation requirements. Timber frame wall top and bottom plates, the perimeter header joist around external walls and full depth solid blocking between floor joists and over loadbearing external walls fulfill this function.

Recent changes to the England and Wales Building Regulations have greatly reduced the need for cavity barriers in house and flat construction, although other purpose group buildings still require them. These changes have removed the need to provide cavity barriers horizontally at floors and vertically in the cavity between brickwork veneer and timber frame structure in houses and flats. Where cavity barriers are required between a brick external cladding and timber frame, these can be of 38 mm minimum thickness timber. Proprietary systems are also available.

Fire stops are required for houses and flats at the junctions of separating and compartment walls and floors with elevation walls and with each other. Fire stops must be of incombustible materials.

Fire Safety In a fire safety study the Building Research Establishment found that timber frame is as safe as

any other form of construction. This view is shared by major house insurers who charge the same premiums for brick clad timber framed houses as for masonry construction.

Research on house fires shows that the most common path for the spread of fire originating on the ground floor is through doorways, up stairs and into the roof, with fire penetration occurring through the floors before it penetrates the walls. Providing that the walls have the required fire resistance their method of construction is not significant during the period of required fire resistance.

Most casualties caused by fire result from noxious fumes, gases and smoke produced by burning house- hold furnishings. These casualties can and do occur well within the required period of fire resistance. As an essential element of fire safety, the England and Wales Building Regulations require houses be fitted with smoke alarms.

Internal Finishes Since gypsum plasterboard combines required fire protection with ease of application and decoration, economy and negligible water content, it is the most widely used interior finish in timber frame building. It should be specified and applied strictly in accordance with BS 8212.

Plasterboard should provide a smooth flat surface accurate to line without surface defects or cracks. In order to achieve this the application must have wall and ceiling framing true to line without projecting or recessed studs or joists. The timber must be at a moisture content of 20% or less, since any substantial shrinkage after application can cause nail heads to protrude through the face of the board and cracking where architraves or skirtings have been installed over the plasterboard.

Plasterboard is available in various widths and lengths, the most common dimension being 1200 mm x 2400 mm. Boards should be carried in a vertical position and stored horizontally on battens to prevent ground contact. Plasterboard is a hygroscopic material and must be protected at all times from moisture. Prior to application it should be stored in a dry watertight building. If it is necessary to store outdoors, it must be off the ground and fully protected by securely anchored polythene sheets or tarpaulins. Tacking the plasterboard should be done by the dry-lining contractor. Ceiling linings should be fixed first, followed by wall linings installed in firm contact with the ceiling. The framing should provide a firm support around the perimeter of each board.

For most installations plasterboard is applied with a paper bound depression on the edges to accommodate jointing tape and filling compound. These edges should be lightly butted together with gaps, if any, not exceeding 3 mm.

Cut edges make plasterboard joints difficult to conceal and should only occur at openings and corners. In par- ticular the common practice of cutting boards to align vertically with openings should be avoided. Nails or screws used to fasten the boards should not be closer than 10 mm to paper bound edges or 13 mm from cut ends. Boards must be in firm contact with the back- ground when fixed, starting from the centre of the board and working towards the perimeter.

Fixings must be corrosion resistant with gauge, length and spacing as required in BS 8212. Where single nail-

ing is used the spacing is usually at 150 mm centres around the perimeter and at intermediate supports. Where double nailing is used at intermediate supports the spacing is 300 mm. Nails must be driven straight with the head left slightly below the surface without breaking the paper face. This allows for concealment with spotting compound.

Jointing All external angles should be protected by corner tape. Jointing tape is used for internal angles and the paper- bound edges of board joints. Jointing tape is set in a jointing compound and covered by two coats of finishing compound to provide a band approximately 300 mm wide with feathered edges. This should be followed by sanding with a very fine paper.

These operations may be done by hand or by taping and finishing machines. The drying of these compounds and the speed of the operation will be faster in winter conditions if the temperature can be maintained above 10°C.

Finishing The lining should be protected as soon as possible after installation by the application of one or two coats of dual purpose pigmented primer. This will prevent water absorption, act as an additional vapor control layer, facilitate stripping of wallpaper at some future time and provide a better base for a paint finish, On large ceiling areas where some imperfections are likely due to the necessity of making joints between the cut ends of boards, the use of a textured finish applied either by brush or spray will help to reduce the visual effect. Other finishing coats may be applied in accordance with manufacturers' instructions.

Wallpaper, ceramic tiles and plastic finishes may be plastered or glued directly to the plasterboard. Where designers wish to use timber or plywood panelling, reference should be made to the building regulations covering internal flame spread to determine the allowable application and the area of panelling permitted. These finishes may be nailed through the plasterboard to the framing or to battens nailed to the studs.

Floor Finishes All types of floor finishes can readily be used in timber frame construction. The plywood structural floor should have all nails driven and counterpunched, cracks or

FINISHING

holes in excess of 2 mm wide filled, and the plywood surface well cleaned. COFI EXTERIOR Select Tight- Face grade provides a surface highly suitable for receiving floor finishes.

Carpets and carpet underlay, linoleum, thermoplastic and cork tiles should be laid directly on the plywood in accordance with manufacturers' instructions.

External Wall Finishes External wall finishes are not an essential element in the structural integrity of timber frame houses so the basic criteria governing the choice of finish are aesthetics, economy, maintenance and durability. Providing that the cladding material complies with the requirements of the building regulations for fire spread, designers have a wide range of finishes from which to choose. Examples are shown in Figure 29.

It should be noted that window and door frames are supported on the timber framed walls of brick veneered houses. This allows the buildings to be made weathertight before the cladding is applied and prevents damage to internal linings caused by a differential movement between masonry and the timber frame. Lintels installed at openings in the brickwork must be supported independently on the brick veneer and not by the timber frame.

Brick Veneer Brick remains the most popular cladding for residential and other forms of construction, including timber frame. It is often used in conjunction with lightweight claddings, particularly when the design incorporates overhangs or set-backs for the upper floors.

Brick is applied in a single half-brick thickness along the foundation wall with a 50 mm cavity between the brickwork and the frame. Brickwork is tied back to the frame at every sixth course with flexible stainless steel ties. These are twice nailed with stainless steel fastenings through sheathing to the studs at a maximum of 600 mm horizontal centres with additional ties located to the vertical perimeters of openings. In some severe wind exposure locations, ties may need to be spaced at 600 mm horizontal centres and 225 mm vertically (every third course).

Ties are laid in the mortar bed joint, carried across the cavity and fixed to the wall studs. The use of flexible ties accommodates any differential movement between the brick and the timber frame which could be caused by timber shrinkage or brick movement. Allowance for shrinkage must also be made between the top of the masonry at overhanging roof members and under widow sills.

The shrinkage allowance at each level of a timber frame building can be predicted as follows: studs do not shrink in length but in cross section as the timber dries down to equilibrium moisture content. Timber decreases approximately 1% in dimension for each 4% decrease in moisture content.

Timber installed at a moisture content of 20% will dry down to approximately 10% in a centrally heated building. In a three storey platform frame building with a suspended timber ground floor, and taking into account the combined dimension of plates and joists, this will amount to a shrinkage of about 18 mm maximum at eaves level, reducing progressively at each lower storey in the building.

The NHBC and Foundation 15 technical details give guidance on allowances for differential movement.

Where cavity barriers are installed in accordance with the building regulations, weep holes must be provided in cross joints to drain moisture to the outside. Cavities should be kept clear of mortar droppings and on no account should any form of cavity insulation be installed.

Designers should take into account both the vertical coursing and horizontal dimension of brickwork when locating the openings in a brick veneered timber framed building since the timber framework provides a template to which the bricklayer must work. The timber framework must be accurate in line and plumb to ensure that the 50 mm cavity can be maintained with the brick veneer aligned plumb.

Timber Cladding Timber cladding may be applied vertically, horizontally or diagonally. It is fixed to 22 x 38 mm treated battens attached vertically over the breather membrane, through the sheathing to the studs. In accordance with NHBC requirements and good practice, the space behind the cladding should be ventilated.

Many board profiles are available, such as rebated, feather-edged and shiplap, and should be a minimum thickness of 16 mm which may be reduced to 6.5 mm at the thinner edge of feather-edged boards. Boards should be fixed with only one nail per board at each batten and nails must be corrosion resistant, such as double dipped galvanized or aluminum. Where Copper Chrome Arsenate (CCA) treated timber is used, the nails should be silicon- or phosphor-bronze.

The surface finish of the timber cladding may be planed or saw-textured, the latter being preferred both for aesthetic reasons and because it provides a better base for the finish. Timber claddings are usually finished with either heavy bodied pigmented stain or semi- transparent, water repellent, anti-fungicidal stains.

As the colour is bleached out over time, boards can be re-finished with additional coats with very little surface preparation since no blistering or flaking will occur. Where a paint finish is applied to surfaced cladding it should be micro-porous since this allows any moisture inside the cladding to migrate to the outside and gives maximum paint life.

Among the various choices of timber cladding available, Western Red Cedar is an excellent one due to its aesthetic appeal, stability and durability. Western Red Cedar accepts a wide range of finish coatings but can also be left to weather naturally. In this case, a Coat of clear anti-fungicidal water repellant should be applied to resist mildew.

.COFI plywood sheathing

Plasterboard and vapour check

•WalI studs

Vertical battens

• Breather membrane

lathing

Vertical

and

Timber cladding

Horizontally boarded details

Wall studs

(25 or 50 mm)

Plasterboard and vapour check

Wall studs

Cement render detail

Plasterboard and• vapour check

Air space (mm. 10 mm)

Breather membrane

plywood sheathing

Vertical battens

Horizontal counter battens •Vertical timber cladding

Vertically boarded details

Brick veneer

Horizontal rebated drop cedar

Tile Hanging

Vertical rebated cedar

FIgure 29. External Claddings

Tile Hanging Tiles in a wide variety of patterns and sizes are a popular cladding material particularly above a brick veneered ground floor. They are fixed to preservative treated battens with corrosion resistant nails which should be silicon bronze or phosphor bronze if the preservative is CCA.

To provide adequate ventilation behind the tile, gaps are left between the butting ends of the battens or alternatively the battens are fixed to vertical counter battens. Where the bottom of the tile hanging abuts a brick veneer finish, sprockets may be used to let the tiles overlap the brickwork although this can introduce detailing problems at corners.

A quarry tile sill combined with flashing may also be used to accommodate the difference in thickness of the claddings but the simplest and best solution for tiles and other light weight claddirigs above brick veneer is to cantilever the first floor joists a sufficient distance to provide a straight-forward flush overlap.

Rendering Properly applied cement or plaster rendering is economical and durable, requiring only limited maintenance. It is applied to metal lathing which may be backed or unbacked. The NHBC requires a ventilated space behind the finish and preservative treated battens should be used. The manufacturers' directions should be strictly adhered to in the lapping and fixing of metal lathing.

Render, which is a mixture of sand, Portland cement and lime, is applied in two or three coats, the last coat providing the required texture and colour. Horizontal expansion joints should be located in line with the floor joists. Where vertical joints are required, these may be located behind rainwater pipes.

Living In a Timber Frame House

The fail-safe methods of design and construction of timber framed houses provide more than adequate safe- guards against any deterioration of the structure. Home owners will be chiefly concerned with internal and external finishes. Internally (see Vapour and Moisture Control Membranes), the primary source of potential problems is excessive moisture vapour in the air. The first sign of this is condensation on the windows which is easily removed by increased ventilation. Condensation can occur on the linings of external walls; if this occurs the heating in the room should be turned up and trickle ventilators in the windows left open.

Internal maintenance of a timber framed house is normally limited to periodic painting and decorating. In a correctly built house there should be no cracks due to shrinkage or movement. Plasterboard is a robust material and unlikely to be subject to accidental damage. If the surface is damaged it can be repaired with proprietary fillers. Holes can be repaired by bonding a strip of plasterboard to the inside of the lining to support a piece carefully matched to the hole.

In almost all respects external maintenance is the same as for other forms of construction. The claddings should be inspected for cracks or other damage; gutters, gullies and rainwater pipes should be kept clear as should ventilation openings, including those of underfloor and roof areas. Seals around openings should be repaired or replaced as necessary and flash-

ings checked to ensure that they are not defective. Stains and modern micro-porous paints applied to timber should not blister, peel or crack and should be refinished at the owner's discretion in accordance with the manufacturer's recommendations.

Fixing objects to timber framed walls is simple but the type of fixing should be matched to the weight of the

object. Lightweight fittings, such as small pictures, can be supported on nails driven at a downwards angle into the plasterboard. Heavier pictures and mirrors can be supported by patented fixture fittings such as toggle bolts. Heavy fittings such as cupboards and shelving must be supported by backing studs or noggings.

As with any building type, extensions, modifications to the plan or new openings in internal or external walls should not be carried out without seeking professional advice on structural aspects. However, the simplicity and light weight of timber frame construction makes most of the work easier than in other forms of building.

The information contained in this guide is comprehensive enough for most purposes of the designer or builder bearing in mind that framing will in all proba- bility be designed, fabricated and erected by special- ists in this field. As well, in the case of houses covered by the NHBC or Foundation 15 warranty, all aspects of the design and construction will be subjected to independent appraisal and inspection.

COFI EXTERIOR plywood is manufactured by these members of the Council of Forest Industries:

Ainsworth Lumber Co. Ltd. Evans Forest Products Limited Federated Co-operatives Limited Northwood Pulp and Timber Limited Richmond Plywood Corporation Ltd. Riverside Forest Products Limited Tolko Industries Ltd. Weldwood of Canada Limited West Coast Plywood Company Ltd.

Spruce-Pine-Fir is manufactured by members of the following western Canadian timber associations:

Alberta Forest Products Association Cariboo Lumber Manufacturers' Association Interior Lumber Manufacturers' Association Northern Interior Lumber Sector

REFERENCES

Standards References BS CP1 02: 1973: Protection of Buildings Against Water from the Ground

BS 4471: 1987: Specification for Sizes of Sawn and Processed Softwood

BS 4978: 1988: Specification for Softwood Grades for Structural Use BS 5268: Structural Use of Timber: Part 2: 1991: Code of Practice for Permissible Stress Design, Materials and Workmanship BS 5268: Structural Use of Timber: Part 3:1985: Code of Practice for Trussed Rafter Roofs

BS 5268: Structural Use of Timber: Part 5: 1989: Preservative Treatment for Structural Timber

BS 5268: Structural Use of Timber: Part 6: Section 6.1: 1988: Code of Practice for Timber Frame Walls

BS 5268: Structural Use of Timber: Part 7: Section 7.1: 1989: Recommendations for the Calculation Basis for Span Tables — Domestic Floor Joists

BS 5268: Structural Use of Timber: Part 7: Section 7.2: 1989: Recommendations for the Calculation Basis for Span Tables — Joists for Flat Roofs

BS 6399: Part 1: 1984: Code of Practice for Dead and Imposed Loads

BS 6399: Part 3:1988: Code of Practice for Imposed Roof Loads

BS 6566: Part 8: 1985: Specification for Bond Perform- ance of Veneer Plywood BS 8212: 1988: Code of Practice for Dry Lining and Partitioning Using Gypsum Plasterboard

Canadian Standards Association 0121-Mi 978 Douglas Fir Plywood Canadian Standards Association 0151-M1978 Cana- dian Softwood Plywood National Lumber Grades Authority. Standard Grading Rules for Canadian Lumber. 1987

NLGA, et al. North American Export Standard for Machine Stress Rated Lumber, 1987

Other References British Gas Guide for Gas Installations in Timber Framed Housing, Second Edition, 1984

Building Regulations (England and Wales) 1992

Building Regulations (Northern Ireland) 1990

Building Standards (Scotland) 1990

Building Research Establishment Report Thermal Insulation: Avoiding Risks 1989

Building Research Establishment Paper 1P6/87 Fire Behaviour of Breather Membranes 1987

Canada Mortgage and Housing Corporation. Canadian Wood-Frame House Construction

Municipal Mutual Insurance Ltd. Foundation 15 Tech- nical Manual

National House-Building Council Standards 1991

COFI Literature The Council of Forest Industries produces a range of literature concerning western Canadian timber and plywood and timber frame construction. A selection from the available material is listed below:

British Columbia Timber and British Standard BS 5268: Part 2 Canadian COFI EXTERIOR Quality Certified Plywood Canadian Sawn Structural Timber and the U.K. Strength Class System CLS Kiln-Dried Spruce-Pine-Fir

Sheathing with Canadian COFI EXTERIOR Plywood Stressed Skin Panel Construction Guide

Stressed Skin Panels for Floors and Flat Roofs

Stressed Skin Panels for Pitched Roofs

Suspended Timber Ground Floor Guide Timber Compartment Floor Guide

Timber Frame House Construction: Check It Out

NOTES

S

Council of Forest Industries

Canada

Tileman House 131/133 Upper Richmond Road

London SW15 ZFR, UK Tel: 081-788-444 Fax: 081-789-0148

The Council of Forest Industries does not warrant the accuracy of any information contained herein. The Council of Forest Industries, its directors, officers, employees, servants and agents shall not be respon- sible or liable for any cause of action, loss, damage,

injury or death in any way connected with the information herein even though such cause of action, loss, damage, injury or death arises from the negligence or default of the Council of Forest Industries, its directors, officers, employees, servants or agents.

© copynght 92-490 December 1992 Printed in Canada

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