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WOODWhat is Wood?

Wood is a hard, fibrous structural tissue found in the stems and roots of trees and other woody plants. It hasbeen used for thousands of years for both fuel and as a construction material. It is an organic material

natural composite of cellulose fibers (which are strong in tension) embedded in a matrix of lignin which resists

compression.

What is Timber?

Trees or wooded land considered as a source of wood.

What is Lumber?

Lumber is wood material that has been manufactured. Lumber is supplied either rough or finished. Roughlumberis the raw material for furniture-making and other items requiring additional cutting and shaping

Finished lumberis supplied in standard sizes, mostly for the construction industry.

PROPERTIES OF WOODPHYSICAL PROPERTIES

1. Appearance

From left to right: Woods can have different appearances in terms of their color, texture, sizes and

shape.

2. Moisture Content defined as the weight of water in wood over the weight of ovendry wood. Wood ishygroscopic. It picks up or gives off moisture to equalize with the relative humidity and temperature in

the atmosphere. As a result, a change in the strength is evident.

Fiber Saturation Point is the moisture content below which the physical and mechanical properties of

wood begin to change. It is usually taken as 30% moisture content on oven-dry basis.

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6. Electrical Conductivity The electrical conductivity of wood varies slightly with applied voltage andapproximately doubles for each temperature increase of 10

oC. It varies greatly with moisture content

It increases as the moisture content increases.

7. CombustibilityIn general, woods are combustible but there are some special treatments to aid thisproperty.

MECHANICAL PROPERTIES

1. Orthotropicity It is the uniqueness of the mechanical properties in the direction of the threemutually perpendicular axes: longitudinal, tangential and radial.

2. Elasticity it implies that deformations produced by low stress are completely recoverable after loadsare removed. When loaded to higher stress levels, plastic deformation or failure occurs. There are 12

constants are needed to describe the elastic behaviour of wood

a. 3 Modulus of Elasticity (E) It is determined from bending rather than from axial test forwoods.

b. 3 Modulus of Rigidity (G) also called as shear modulus, indicates the resistance to deflectionof a member caused by shear stress.

c. 6 Poissons Ratio (v) the ratio of traverse strain to axial strain.

When a member is loaded axially, the

deformation perpendicular to the direction

of the load is proportional to the

deformation parallel to the direction of the

load.

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3. Strength It is defined as the ability of a member to sustain stress without failure. The followingcriterion is considered for the determination of the strength of wood:

a. HardnessIt is the resistance to denting and wear. It is the resistance to indentation usingtheJanka Hardness test, measured by the load required to embed an 11.28-mm (0.444 in)

to half of its diameter.

Janka Harndess and other Mechanical Properties of common woods in the Philippines

b. Stiffness it is the resistance to deflection and bending when loaded. Stiff woods are notnecessarily strong. They may resist bending up to a point and then brek suddenly.

c. Toughness - It is the resistance to shock loading. Tough woods willdeflect before breaking. Even before fracturing, the fibers tend to

hang together and resist separation.

d. Resistance to Warping - Warping is the twisting, bending or bowingdistortions shown by some woods. It was greatly affected by the

method of sawing and curing.

e. Nail-holding Resistance - Nail-holding resistance for harwoods isgreater than for softer woods. However, woods that are so hard that

they tend to split when nailed, lose much of their holding ability.

Preboring to 75% of the nail size avoids splitting.

f. Workability - It is the ease in sawing, shaping and nailing. It is moreevident in soft, low-density woods than for hardwoods, but usually

they cannot be given a high polish.

Common Modes ofWarping of Woods

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Common wooden furniture has high degree of workability

to the extent that they can be shaped into different forms.

g. Natural-decay Resistance Resistance of woods to decay caused by extremeenvironmental conditions and termites is a good criteria of the strength of wood.

h. Paint-holding Ability Discoloration of wood due to time is a great deal for maintaining thebeauty and aesthetics of wood.

SOURCES OF WOODSSUSTAINABLE SOURCES

Sustainable sources have a constantly

regenerating supply over the course of a year.

These sustainable sources can exhibit seasonality,

but regenerate nonetheless.

- Woods from logging- Woods from saw mills- Woods from pallet plants

NON-SUSTAINABLE SOURCES

Non-sustainable sources occur one time. These

sources can provide a spike in the overall wood

availability, but are short-lived.

- Clearing a site for development- Trimmings from trees and woody bushes

CLASSIFICATION OF WOOD

1. Hardwood2. Softwood

HARDWOOD

Hardwoods are generally broad-leaved deciduous trees. Hardwoods are porous.

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Hardwoods are angiosperms. Contract with heat and moisture Stronger, denser, & homogeneous than softwoods (good for framing system) Good for interior finishes & for furniture Resist fire better than softwood Nearly impervious to water (some varieties) Cost: high

WOODS CLASSIFIED AS HARDWOOD

NARRA

Scientific Name: Pterocarpus Indicus

Color: Deep Orange Golden To Darker Red Tones

Description: Narra is considered the most valuable wood in the Philippines, and

is therefore very restricted. Special permits are required for export of finished

products. - Most often used for furniture, flooring, and panels.

MAPLE

Scientific Name: Acer Saccharum

Color: Creamy White To Light Reddish Brown

Description: White Maple is widely used for furniture, and is often used for very

pale products with a soft sanded surface.

OAK

Scientific Name: Quercus Rob.

Color: Pale/light

Description: Lead time for production in Oak will most often be about 6 months,

until producion is stable, after which 3 months production time is normal. It is

good decay resistance.

MAHOGANY

Scientific Name: Swietenica Macrophylla

Color: Red brownish with Orange Tone

Description: Mahogany has been planted in the Philippines since the 70s.

Originating in Brazil, where it is now an endangered species, this plantation

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species can now be acquired on sustainable basis. It may contain some small firm

knots, but is available in good quality for furniture purposes.

ACACIA

Scientific Name: Acacia Auriculaeformi, Racosperma Aurculiforme

Color: Dark Brown, With Very Distingt Sap Wood (yellow)

Description:The Acasia grows wild everywhere in the Philippines, and is often

used for local handicrafts, and especially suited for turning into bowls and plates.

YAKAL

Scientific Name: Shorea Laevis

Color: Yellow To Golden Red

Description: Yakal is a hard and golden Mahogany type which is used for

frequently used products and surfaces. Ideal for outdoor use also.

ROSEWOOD

Scientific Name: Petersianthus Quadrialatus

Color: Very Dark With Lighter Flames Naturally Occuring

Description: Philippine Rosewood is a very beautiful dark and flamy wood. It has

for many years been used for local boat making due to its strength and

durability. This wood is primarily used for interiors and flooring.

TEAK

Scientific Name: Tectona Grandis

Color: Brown

Description: Teak is one of the world's best timbers. It's usage is multiple, but

mainly furniture, decking, and various kitchen accessories. Especially well suited

for outdoor use.

SOFTWOOD

Softwoods are generally needle-leaved coniferous trees that bear their seeds in cones.

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Softwoods are nonporous. Softwoods are gymnosperms. Contract with heat and moisture Solid but soft Have lower density than hardwoods (good for acoustic & thermal insulation) Good for interior finishes & for furniture Flexible and strong Not water resistance Cost: low

WOODS CLASSIFIED AS SOFTWOOD

PINE

Pine is a soft, white or pale yellow wood which is light weight, straight grained

and lacks figure. It resists shrinking and swelling. Knotty pine is often used for

decorative effect.

Pickled, whitened, painted and oil finishes are often used on this wood.

ASH

It has a prominent grain that resembles oak, and a white to light brown color.

Ash burls have a twisted, interwoven figure.

Ash is widely used for structural frames and steam bent furniture pieces. It is

often less expensive than comparable hardwoods.

CEDAR

Cedar is a knotty softwood which has a red-brown color with light streaks. Its

aromatic and moth repellent qualities have made it a popular wood for lining

drawers, chests and boxes.

Simple cases and storage closets are also constructed from this light, brittle

wood.

BIRCH

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Birch is a hard, heavy with a light brown or reddish colored heartwood and

cream or light sapwood.

Birch is often rotary or flat sliced, yielding straight, curly or wavy grain patterns.

It can be stained to resemble mahogany or walnut.

HEMLOCK: Light in weight, uniformly textured. It machines well and has low resistance to decay and

nonresinous. Used for construction lumber, planks, doors, boards, paneling, sub flooring and crates.

FIR: Works easy and finishes well. Uniform in texture and nonresinous. Has low resistance to decay. Used in

furniture, doors, frames, windows, plywood, veneer, general millwork and interior trim.

SPRUCE: Strong and hard. Finishes well and has low resistance to decay. Has moderate shrinkage and light in

weight. Used for masts and spars for ships, aircraft, crates, boxes, general millwork and ladders.

REDWOOD: The best quality redwood comes from the heartwood which is resistant to deterioration due to

sunlight, moisture and insects. It is used to craft outdoor furniture and decorative carvings. Redwood burlshave a "cluster of eyes" figure. They are rare and valuable.

SOFTWOOD VS. HARDWOOD

PROPERTY SOFTWOOD HARDWOOD

Colour Lighter Darker

Growth Faster Slower

Weight Lighter Heavier

Density Low High

Annual Rings Distinct Indistinct

Heart wood and sap wood Cannot be distinguished Can be distinguished

Strength Strong along the grainsStrong along and across

the grains

Conversion Easy Difficult

USES (FIELD APPLICATIONS) OF WOOD1. Foundations

Treated wood is used for basement foundation walls. Basically, such foundations consist of wood-

frame wall sections with studs and plywood sheathing supported on treated wood plates, all of which

are preservatively treated to a specified level of protection. Because a foundation wall needs to

be permanent, the preservative treatment of the plywood and framing and the type of fasteners used

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for connections are very important. A special foundation treatment has been established for the

plywood and framing, with strict requirements for depth of chemical penetration and amount of

chemical retention. Corrosion-resistant fasteners (for example, stainless steel) are recommended for

all preservatively treated wood.

2. FloorsTypically consists of wood joists on 400- or 600-mm (16- or 24-in.) centers supported by the foundation

walls and the center girder

Joist- One of a series of parallel beams used to support floor and ceiling loads and supported in

turn by larger beams, girders, or bearing walls. Joist size depends on the anticipated loading,

spacing between joists, distance between supports (span), species, and grade of lumber.

3. Exterior WallsExterior walls of light-frame structures are generally load bearing; they support upper floors and the

roof.

4. Ceiling and RoofRoof systems are generally made of either the joists-and-rafter systems or with trusses. Engineered

trusses reduce on-site labor and can span greater distances without intermediate support, thus

eliminating the need for interior load-carrying partitions. This provides greater flexibility in the layout

of interior walls. Prefabricated roof trusses are used to form the ceiling and sloped roof of more than

two-thirds of current light-frame buildings.

5. Wood Deck

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Decks are made of preservatively treated lumber, which is generally available from local building

supply dealers. To ensure long life, acceptable appearance, and structural safety, several important

guidelines should be followed. Proper material selection is the first step. Then, proper design and

construction techniques are necessary. Finally, proper maintenance practices are necessary.

ADVANTAGES OF WOOD1. Wood stores carbon

Trees and wood products have a unique ability to store carbon. When trees are harvested and used to

make wood products, the carbon remains stored in the wood for the life of the product. 50% of the dry

weight of wood is carbon.

2. Wood produces less carbon dioxide in its production than many other major building materials

The production and processing of wood uses less energy than most other building materials, giving

wood products a very low carbon footprint. Wood can often be used in place of materials like steel, aluminum

concrete or plastics that require large amounts of energy to produce. This means that there are less carbon

dioxide emissions associated with wood products than other major building materials.

Substituting a cubic meter of wood for other construction materials (concrete, blocks or bricks) could save upto 1 tonne of CO2 emissions.

3. Wood is renewable

Responsibly sourced wood is renewable. Responsibly, well-managed forests and plantations wil

regrow to provide a wide range of other benefits such as further carbon storage, oxygen generation and forest

habitat. Additionally, after decades or even centuries of use, wood buildings can be easily adapted or

deconstructed and reused, which means they can continue to store carbon indefinitely.

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4. Wood is durable

Wood is a durable material for both homes and commercial buildings. When properly looked after it

can last hundreds of years. Modern wood preservatives enhance natural durability. Wood has been used for

thousands of years and is resistant to heat, frost, corrosion and pollution. The only factor that needs to be

controlled is exposure to weathering.

5. Wood is structurally very strong

Wood is very strong structurally. A comparison with steel and concrete shows that radiata pine

structural timber, for example, has a strength for weight ratio 20 percent higher than structural steel and four

to five times better than non reinforced concrete in compression.

6. Wood is a natural insulator

Wood itself is a natural insulator due to air pockets within its cellular structure. As an insulator wood is

15 times better than masonry, 400 times better than steel, and 1,770 times better than aluminum. In addition,

lightweight wood framing methods allow easy installation of additional fiber or foil insulation. Wooden

windows are more thermally efficient as they do not form 'thermal-bridges' between the cold outside air and

warm air inside (and vice versa in summer).

7. Wood is fast and efficient to build with

Wood construction is fast and efficient. Wooden buildings can be built year-round in most climates.

8. Wood is naturally beautiful

Wood is naturally beautiful and aesthetically pleasing. It is available in a large range of colors, grains

and textures making wood visually attractive. Wood is also a very tactile product, making it nice to touch,

dependent on the finish.

DISADVANTAGES OF WOOD1. Shrinkage and Swelling of Wood:

Wood is a hygroscopic material. This means that it will adsorb surrounding condensable vapors and

loses moisture to air below the fiber saturation point.

2. Deterioration of Wood:

The agents causing the deterioration and destruction of wood fall into two categories: Biotic

(biological) and abiotic (non-biological).

Biotic agents include decay and mold fungi, bacteria and insects.

Abiotic agents include sun, wind, water, certain chemicals and fire.

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a. Termites

There are two types of termites: Subterranean termites damage wood that is untreated, moist,

in direct contact with standing water, soil, other sources of moisture.

Dry wood termites attack and inhabit wood that has been dried to moisture contents as low as

5 to 10%. The damage by dry wood termites is less than subterranean termites.

b. Powderpost beetles

Powderpost beetles attack hardwood and softwood. At risk is well seasoned wood as well as

freshly harvested and undried wood.

c. Carpenter ants

Carpenter ants do not feed on wood. They tunnel through the wood and create shelter. They

attact most often wood in ground contact or wood that is intermittently wetted.

d. Carpenter bees

They cause damage primarly to unpainted wood by creating large tunnel in order to lay eggs.

e. Marine borers

They attack and can rapidly destroy wood in salt water and brackish water.

B. Abiotic Deterioration of wood:

i. Fire:

Another disadvantage of wood is that it easily catches fire. Wood consists of organic compounds which

are composed mainly of carbon and hydrogen. They can combine with oxygen and burns. Because of

these properties, wood is classified as a combustible material.

If the temperature of a inflammable gas is between 225-260C, it burns with a touch of flame. After

the withdrawal of flame it will stop burning. If the temperature increases to 250-270C, it burns with a

touch of flame and goes on to burn without a flame. If the temperature increases to 330-520C, wood

begins to burn spontaneously. Chemical materials, especially extractives in woods structure cause the

burning point to change. For example, a resinous piece of pinewood can catch fire in lower

temperatures. In addition to this, specific gravity and surface mass (m2/kg) affect the duration offlame. Wood burns harder when the specific gravity and surface mass and moisture content increase

and vice versa.

Using thick wood as a structure element is another way of extension of burning point. Outer surface

burns and turns into charcoal. Charcoal, which forms on the surface of wood as it burns is a very

effective heat insulator. Therefore large timbers burn very slowly. In addition to this, wood is very good

heat insulator too. The outer surface of the wood is 1000C and the interior part is still 40C when a

piece of thick wood is burning. For this reason, buildings with thick structure elements such as beams

and columns do not collapse easily on fire. On the other hand, in steel constructions, as heat increases,

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Waterborne Preservatives

Waterborne preservatives are often used when cleanliness and paintability of the treated wood are required

Several formulations involving combinations of copper, chromium, and arsenic have shown high resistance to

leaching and very good performance in service. Waterborne preservatives are included 1410 in specifications

for items such as lumber, timber, posts, building foundations, poles, and piling.

Acid Copper Chromate

Tests on stakes and posts exposed to decay and termite attack indicate that wood well-impregnated

with ACC gives acceptable service, but it is more prone to leaching than are most other waterborne

preservatives. Use of ACC is generally limited to cooling towers that cannot allow arsenic leachate in

cooling water.

Ammoniacal Copper Zinc Arsenate

This preservative is used most commonly to treat refractory species, such as Douglas-fir. Service

records on structures treated with ACA show that this preservative provides protection against decay

and termites. High retention levels of preservative will provide extended service life to wood exposed

to the marine environment, provided pholad-type borers are not present.

Preservative Effectiveness

Preservative effectiveness is influenced not only by the protective value of the preservative chemical, but also

by the method of application and extent of penetration and retention of the preservative in the treated wood

Even with an effective preservative, good protection cannot be expected with poor penetration or

substandard retention levels. The species of wood, proportion of heartwood and sapwood, heartwood

penetrability, and moisture content are among the important variables that influence the results of treatment

Preparation of Timber for Treatment

Peeling

Peeling round or slabbed products is necessary to enable the wood to dry quickly enough to avoid

decay and insect damage and to permit the preservative to penetrate satisfactorily. Even strips of the

thin inner bark may prevent penetration. Patches of bark left on during treatment usually fall off in

time and expose untreated wood, thus permitting decay to reach the interior of the member.

Drying

Drying of wood before treatment is necessary to prevent decay and stain and to obtain preservative

penetration. However, for treatment with waterborne preservatives by certain diffusion methods, high

moisture content levels may be permitted. For treatment by other methods, however, drying before

treatment is essential. Drying before treatment opens up the checks before the preservative is applied,

thus increasing penetration, and reduces the risk of checks opening after treatment and exposing

unpenetrated wood. Good penetration of heated organic-based preservatives may be possible in wood

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with a moisture content as high as 40% to 60%, but severe checking while drying after treatment can

expose untreated wood.

Conditioning of Green Products

Plants that treat wood by pressure processes can condition green material by means other than air and

kiln drying. Thus, they avoid a long delay and possible deterioration of the timber before treatment.

Incising

Wood that is resistant to penetration by preservatives may be incised before treatment to permit

deeper and more uniform penetration. To incise, lumber and timbers are passed through rollers

equipped with teeth that sink into the wood to a predetermined depth, usually 13 to 19 mm (1/2 to

3/4 in.). The teeth are spaced to give the desired distribution of preservative with the minimum

number of incisions. A machine of different design is required for deeply incising the butts of poles,

usually to a depth of 64 mm (2.5 in.)

Cutting and Framing

All cutting and boring of holes should be done prior to preservative treatment. Cutting into the wood in

any way after treatment will frequently expose the untreated interior of the timber and permit ready

access to decay fungi or insects.

APPLICATION OF PRESERVATIVESTwo general types: (a) pressure processes, in which the wood is impregnated in closed vessels under pressures

considerably above atmospheric, and (b) nonpressure processes, which vary widely in the procedures and

equipment used.

Pressure Processes

In commercial practice, wood is most often treated by immersing it in a preservative in a high pressure

apparatus and applying pressure to drive the preservative into the wood. Pressure processes differ in details,

but the general principle is the same. The wood, on cars or trams, is run into a long steel cylinder (Fig. 143)

which is then closed and filled with preservative. Pressure forces the preservative into the wood until the

desired amount has been absorbed. Considerable preservative is absorbed, with relatively deep penetration.

Three pressure processes are commonly used: full-cell, modified full-cell, and empty-cell.

Full-Cell

The full-cell (Bethel) process is used when the retention of a maximum quantity of preservative is

desired. It is a standard procedure for timbers to be treated full-cell with creosote when protection

against marine borers is required.

Modified Full-Cell

The modified full-cell process is basically the same as the full-cell process except for the amount of

initial vacuum and the occasional use of an extended final vacuum. The modified full-cell process uses

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lower levels of initial vacuum; the actual amount is determined by the wood species, material size, and

final retention desired.

Empty-Cell

The objective of the empty-cell process is to obtain deep penetration with a relatively low net

retention of preservative. For treatment with oil preservatives, the empty-cell process should always

be used if it will provide the desired retention. Two empty-cell processes, the Rueping and the Lowry,

are commonly employed; both use the expansive force of compressed air to drive out part of the

preservative absorbed during the pressure period.

Nonpressure Processes

The numerous nonpressure processes differ widely in the penetration and retention levels of preservative

attained, and consequently in the degree of protection they provide to the treated wood. When similar

retention and penetration levels are achieved, wood treated by a nonpressure method should have a service

life comparable to that of wood treated by pressure.

Surface Applications

The simplest treatment is to apply the preservative to the wood with a brush or by dipping.

Preservatives that are thoroughly liquid when cold should be selected, unless it is possible to heat the

preservative. The preservative should be flooded over the wood rather than merely painted. Every

check and depression in the wood should be thoroughly filled with the preservative, because any

untreated wood left exposed provides ready access for fungi.

Cold Soaking and Steeping

Cold soaking well-seasoned wood for several hours or days in low viscosity preservative oils or steeping

green or seasoned wood for several days in waterborne preservatives has provided varying success on

fence posts, lumber, and timbers.

Diffusion Processes

In addition to the steeping process, diffusion processes are used with green or wet wood. These

processes employ waterborne preservatives that will diffuse out of the water of the treating solution

or paste into the water of the wood.

Vacuum Process

The vacuum process, or VACVAC as referred to in Europe, has been used to treat millwork with

water-repellent preservatives and construction lumber with waterborne and water-repellent

preservatives. In treating millwork, the objective is to use a limited quantity of water-repellent

preservative and obtain retention and penetration levels similar to those obtained by dipping for 3

min.

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Effect on Mechanical Properties

Coal-tar creosote, creosote solutions, and pentachlorophenol dissolved in petroleum oils are practically inert

to wood and have no chemical influence that would affect its strength. Chemicals commonly used in

waterborne salt preservatives, including chromium, copper, arsenic, and ammonia, are reactive with wood

Thus, these chemicals are potentially damaging to mechanical properties and may also promote corrosion of

mechanical fasteners. Significant reductions in mechanical properties may be observed if the treating and

subsequent drying processes are not controlled within acceptable limits. Factors that influence the effect of

the treating process on strength include (a) species of wood, (b) size and moisture content of the timbers

treated, (c) type and temperature of heating medium, (d) length of the heating period in conditioning the

wood for treatment and time the wood is in the hot preservative, (e) post-treatment drying temperatures, and

(f) amount of pressure used. Most important of those factors are the severity and duration of the in-retort

heating or post-treatment redrying conditions used.

Factors Affecting Finish Performance

Satisfactory performance of wood finishes is achieved when the many factors that affect these finishes are

given full consideration. These factors include the effect of the wood substrate, properties of the finishing

material, details of application, and severity of exposure.

Wood Properties

Wood surfaces that have the least tendency to shrink and swell are best for painting. For this reason, vertical-

or edge-grain surfaces are far better than flat-grain surfaces especially when the wood is used outside where

wide ranges of relative humidity and periodic wetting can produce wide ranges of swelling and shrinking.

Wood Extractives

Water-soluble colored extractives occur naturally in the heartwood of such species as western redcedar,

cypress, and redwood. These substances give the heartwood of some species their attractive color, water

repellency, and natural decay resistance. However, discoloration of paint may occur when the extractives are

dissolved and leached from the wood by water.

Wood Product Characteristics

Five general categories of wood products are commonly used in exterior construction: (a) lumber, (b

plywood, (c) fingerjointed wood, (d) reconstituted wood products (such as hardboard, oriented strandboard

(OSB), and particleboard), and (e) preservativefire-retardant-treated wood. Each product has unique

characteristics that affect the application and performance of finishes.

Lumber

Although several alternative materials are being used for siding (such as vinyl, aluminum, OSB, and

hardboard), lumber is still the preferred choice for siding in many areas of the country and for a variety of

architectural designs. Many older homes have wood siding. The ability of lumber to retain and hold a finish is

affected by species, grain orientation, and surface texture.

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Plywood

Plywood for exterior use nearly always has a flat-grain surface, and if it is used for exterior wood siding, the

surface is rough sawn. Smooth-sanded plywood is not recommended for siding, although it is often used for

soffits. The flat-grain pattern in nearly all plywood can contribute to early paint failure. Therefore, if plywood

is to be painted, take special care to prepare the surface and use high quality latex paint. Rough-sawn plywood

holds paint much better than does smooth plywood.

Fingerjointed Lumber

In recent years, many mills have been producing lumber that consists of many small pieces of wood that are

glued together and have fingerjoints to improve strength. This process is done to eliminate knots and other

defects from the lumber. The lumber is commonly used for fascia boards, interior and exterior trim, windows

and doors, and siding.

Particleboard and Similar Reconstituted Wood Products

Reconstituted wood products are those made by forming small pieces of wood into large sheets, usually 1.2 by

2.4 m (4 by 8 ft) or as required for a specialized use such as clapboard siding. These products may be classified

as fiberboard or particleboard, depending upon the nature of the basic wood component Particleboard is

manufactured from whole wood in the formof splinters, chips, flakes, strands, or shavings. Flakeboard is a

type of particleboard made from relatively large flakes or shavings.

Treated Wood

Wood used in severe outdoor exposures requires special treatment for proper protection and best service.

The most common hazard in such exposures is decay (rot) and insect attack, particularly by termites. Marine

exposure also requires wood to be protected with special treatment. Many building codes require fire-

retardant treatment of wood for some uses.

Types of Exterior Wood Finishes

Weathered Wood as Natural Finish

The simplest finish for wood is that created by the weathering process. Without paint or treatment of

any kind, wood surfaces gradually change in color and texture, and they may stay almost unaltered for

a long time if the wood does not decay. Generally, dark-colored woods become lighter and light-

colored woods become darker. As weathering continues, all woods become gray because of the loss of

colored components from the wood surface and the growth of mildew. As the surface erodes, it

becomes uneven because of the different erosion rates of earlywood and latewood.

Penetrating Wood Finishes

Penetrating finishes constitute a broad classification of natural wood finishes that do not form a film

on the wood surface. Penetrating finishes are classified as (a) transparent or clear systems, (b) lightly

colored systems, (c) pigmented or semitransparent systems, and (d) oils.

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Film-Forming Finishes

o Clear Varnisho Pigmented Varnisho Solid-Color Stainso Painto Fire-Retardant Coatings

APPLICATION OF WOOD FINISHESTypes of Finish

Water-Repellent Preservatives

The most effective method of applying a water repellent or water-repellent preservative is to dip the

entire board into the solution.

Semitransparent Penetrating Stains

Semitransparent penetrating stains may be brushed, sprayed, or rolled on, but they must be back-

brushed. Brushing works the finish into the wood and evens out the application so that there is less

chance for lap marks. Semitransparent penetrating stains are generally thin and runny, so application

can be messy

Waterborne Semitransparent Stains

Waterborne semitransparent stains do not penetrate the wood surface as well as oilborne

semitransparent stains, but they are easy to apply and less likely to form lap marks. These stains form a

thin film, and a second coat will improve their durability. Apply the second coat any time after the first

has dried.

Solid-Color Stains Paint

Solid-color stains may be applied to a smooth wood surface by brush, spray, or roller; if the finish is

applied by spray or roller, it is necessary to back-brush immediately after application.

Paint

Wood and wood-based products should be protected from sunlight and water while stored prior to

delivery to a construction site and while stored on the construction site. The finish should be applied assoon as possible after the wood is installed.

Refinishing

Exterior wood surfaces need to be refinished only when the old finish has worn thin and no longer

protects the wood. In repainting, one coat may be adequate if the old paint surface is in good

condition. Dirty paint can often be renewed and cleaned by washing with detergent. Too-frequent

repainting with an oil-based system produces an excessively thick film that is likely to crack abnormally

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across the grain of the wood. Complete removal of the paint and repainting are the only cure for cross-

grain cracking (see subsection on crossgrain cracking under Finish Failure or Discoloration).

Back-Priming

Back-priming simply means the application of a primer or water-repellent preservative to the back side

of wood (usually wood siding) before the wood is installed. Back-priming retards absorption of water

thus improving dimensional stability and extending the service life of the paint. It improves the

appearance of the wood by decreasing extractives staining, particularly run-down extractives bleed.

Types of Finish

Sealers and Drying Oils Sealers and drying oils penetrate the wood surface, then solidify to form a

barrier to liquid water. Many commercial sealers are similar to thinned varnish. These finishes can

include a wide range of formulations including polyurethane, alkyds, and modified oils. Unmodified oils

such as tung, linseed, and walnut oil can also be used as sealers if they are thinned to penetrate the

wood.

Nondrying Oils

Nondrying oils simply penetrate the wood. They include both vegetable and mineral oils. Vegetable

oils (such as olive, corn, peanut, and safflower) are edible and are sometimes used to finish wood

utensils. Mineral (or paraffin) oil is a nondrying oil from petroleum. Since it is not a natural product, it is

not prone to mildew or to harbor bacteria.

Paraffin Wax

Paraffin wax is similar to paraffin oil but is solid at room temperature. Paraffin wax is one of the

simplest ways to finish wood utensils, especially countertops, butcher blocks, and cutting board