wood-frame construction, fire resistance and sound ... · pdf filewith exterior brick ......

For many years, wood-frame construction has been pro-

viding Canadians with high-quality, affordable housing.

This goes from town houses to multi-storey apartment

buildings, and from single-family bungalows to large

luxury houses.

The secret of this success lies with many factors, and speed of erection isdefinitely one of them, not to mention the fact that occupants can moveinto the building almost as soon as it is completed. In addition, wood-frame construction uses a renewable resource; and its design flexibilityallows for efficient building concepts in terms of energy conservation aswell as safety and reliability under extreme conditions such as earthquakes.

North American fire-loss statistics reveal that death and injury in build-ing fires are usually caused by smoke inhalation, and occur long beforestructural failure. With large numbers of residential fires resulting fromthe cooking of meals or the ignition of furnishings by cigarettes, it isclear that people are just as safe in a wood-frame house as they wouldbe in a house built of stone, bricks, steel or any other material.

Research and experience confirm that fire safety in a house or apartmenthas little to do with the combustibility of the structural materials usedin its construction. In fact, the occupants’ safety is far more dependenton their own awareness of fire hazards (open flames, etc.), the contentsof their homes, and the fire protection measures designed into the build-ing. The main objectives of fire protection are a) to confine any fire to itsarea of origin and ensure that the structural integrity of the building ismaintained during evacuation, and b) to ensure that measures are inplace to allow for the safe exit of all occupants.

The intent of this brochure is to provide some background on fire safeconstruction concepts, as well as examples of wood-based light-framebuilding systems designed to maximize fire safety.

Wood-frame Construction, Fire Resistance and Sound Transmission

Wood frame single family construction with exterior brick

Wood frame multi-family construction


Fire resistance in single-family construction

For single-family houses, Canadian build-ing codes contain measures to limit therisk of fires spreading from one house toanother. These measures usually involveminimum permitted distances between

houses, combined with restrictions on cladding com-bustibility and opening sizes (windows or doors) in crit-ical walls. Regardless of the materials used in construc-tion of the dwellings, electrical and heating systems mustbe installed and maintained according to approved pro-cedures; and minimum permitted distances are pre-scribed between combustible materials and heat sources(cooking range to kitchen cabinets, wood stove to walllining, etc.). However, Canadian codes do not requirefire-rated floors or walls in single-family construction. Itis assumed that people within an individual dwelling aregenerally aware of each other’s activities and will react ina mutually responsible manner to the occurrence of fireand smoke alarms within their dwelling. Regular gradesof gypsum board used to sheath walls and ceilings offermeaningful measures of fire containment and protectionof wood structural members while contributing time foroccupants to safely evacuate the house; basements arefrequently left without a ceiling. In countries wherebuilding codes may require fire-rated construction insingle-family houses, the techniques described for multi-family construction would be equally applicable.

Fire resistance in multi-family construction

In Canada as in most countries, fire-resistance-rated building assemblies in multi-familydwellings are used to prevent, for a certainperiod of time, the spread of fire, smokeand heat from one unit to another (essen-

tially through walls and floors), and to ensure that thestructural integrity of the building is maintained. Fire-resistance-rated walls and floors are also required for exitcorridors and stairways to ensure that people can safelyleave the building in the event of a fire.

[photo of multi-storey multi-family building]

Fire-resistance rated gypsum board

The fire-resistance of wood-frame assemblies (walls orfloors) depends almost entirely on the gypsum board(also called wallboard or plasterboard) used to shieldstructural wood members from the effects of heat. Whenexposed to fire, the gypsum absorbs large amounts ofheat as its water content is released.

There are different kinds of gypsum board on the market,but the construction of fire-rated wood-frame assembliesrequires specially manufactured fire-rated panels. Thesepanels contain, among other constituents, glass fibresthat improve their dimensional stability and nail-headpull-through resistance, allowing them to remain inplace for longer periods of time when exposed to fire.Specific construction practices and design details are usedto maximize the length of time that the gypsum boardremains in place.

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Wood frame single family construction with wood exterior siding

Wood frame multi-story condominium units, in construction

Fire-resistance ratings provide a measure of the time that anassembly will withstand the passage of flame and smoke, andthe transmission of heat when exposed to fire under specifiedfire conditions, including structural loads if applicable. Theyare generally based on tests conducted in conformance withone of two standards: International Standards Organisation(ISO); ISO 834 Fire Resistance Tests – Elements of BuildingConstruction, or American Standard Testing Method(ASTM); ASTM E-119 Standard Test Methods for Fire Tests ofBuilding Construction and Materials. Fortunately, the fireexposures prescribed in the two standards are essentially iden-tical. Therefore, fire-resistance ratings determined in one coun-try are often accepted by building officials in others.

Fastening gypsum board to walls orceilings

Attaching gypsum board to walls or ceilings usingresilient metal channels reduces sound transmission aswell as the stresses on gypsum caused by movement inthe joists or studs during a fire. Keeping wallboard fas-teners 38 to 50 mm from the edge of the panel allowsmuch more shrinkage to occur before the panel pullsaway from the fasteners. The fasteners holding the gyp-sum board in place on a wall or ceiling should be spacedno further than 200 mm apart along each resilient metalchannel, stud or joist. The screws used to attach gypsumboard to resilient metal channels should be long enoughto penetrate 10 mm through the channel. The screwsused to attach resilient metal channels to the wall studsor ceiling joists, or to attach gypsum board directly to the

studs or joists should penetrate 32 mm into the woodmembers. If nails are used, these should penetrate at least45 mm into the wood members. Screws used to fastengypsum board to resilient channels should never comeinto contact with structural members2.

For ceilings consisting of a single layer of gypsum board,this design detail requires two resilient channels to beplaced back-to-back at joints between the butt ends ofadjacent gypsum panels (See Figure at top of next page).For ceilings consisting of two layers of gypsum board,there is no need to keep the fasteners back from the butt-end edges of the base layer, but they should be kept 38 to50 mm from the butt-end edges of each panel in the facelayer. The easiest way to do this is to fasten the butt-endsof the face layer of gypsum board to the base layer usingType G wallboard screws (thicker shank, coarser thread).When ceilings consist of two layers of gypsum board,joints between adjacent panels of gypsum board in theface layer should be centred on the panels in the baselayer.

1 The screws used to attach the gypsum board to resilient channels must belocated away from the studs, as any direct contact with structural memberswould create a path for sound transmission.

2 Additional information on gypsum installation for fire resistance andsound control is available from the Canada Mortgage and HousingCorporation (CMHC), Ottawa, Canada, and the Gypsum Association,Washington, D.C., USA.


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Typical gypsum board installation on walls, with resilient channels

Fire chamber at the National Research Council of Canada


Use of insulation in floors or wallsseparating different dwellings

Placing glass-fibre or rock-fibre insulation between thejoists of wood-frame floor assemblies reduces soundtransmission. It also restricts heat transfer from the ceil-ing into the joist cavities and, for a time, shields the sidesof the joists and subfloor from the effects of the fire.However, the additional heat retained in the gypsumboard reduces the time it will remain in place. Once thegypsum board falls from the ceiling, the insulation isexposed directly to the fire. Glass-fibre insulation meltsafter a few minutes. Rock-fibre insulation does not meltand only shrinks slightly; bats of rock-fibre insulation

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3 The screws used to attach the gypsum board to resilient channels must belocated away from the joists, as any direct contact with structural memberswould create a path for sound transmission.

Typical gypsum board installation on ceilings – single layer onresilient channels (top) – double layer on resilient channels(bottom)

supported between the joists by the resilient channelswill therefore shield the joists and subfloor from fire fora significant period of time after the ceiling falls away.

Insulating materials are installed in close contact withthe sides of the studs or joists, as gaps would allow hotfire gases to penetrate into the cavity and attack the sidesof the wood members.

Fire-rated wall construction

In multi-family construction, load-bearing walls (sup-porting upper floors) and party walls (separating twodwelling units, or public corridors and exits from the restof the building) are designed to provide specific fire-resistance ratings as per applicable building codes.

Fire-rated gypsum board is available in two thicknesses:12.7 mm and 15.9 mm. The thicker panels present asomewhat better thermal barrier and allow significantlygreater fire resistance. On the other hand, the stud spac-ing (400 mm or 600 mm on-centre) does not signifi-cantly change the fire resistance of a wall assembly.

Horizontal installation of gypsum board (i.e. with thelong direction perpendicular to the studs) results inunbacked horizontal joints. Since the joints between gyp-sum board panels are weak points in fire-resistance ratedwalls, the panels should be installed vertically so jointsmay be centred on studs. There are two exceptions: 1) ifwall cavities are filled with rock-fibre insulation, and 2) if the wall includes a double layer of gypsum board(with the joints in the face layer offset from those in thebase layer); then, the gypsum board may be applied ineither direction.

Some countries’ building codes require horizontal block-ing at 400 or 600 mm spacing between the studs. Thesecomponents provide additional opportunities to fastenthe gypsum panels to the wall when resilient channelsare not used, thereby increasing the fire resistance of theassembly. They also serve to increase the fire resistance oftaller walls, where the mode of failure is likely to be dueto wall buckling.

In the construction of shear walls, plywood or oriented-strandboard (OSB) panels nailed to the studs and cov-ered with gypsum board add 5 to 10 minutes to the over-all fire resistance of the assembly, depending on whichside of the wall they are placed (ambient or fire-exposedside). There are no significant differences between thefire-resistance ratings of wall assemblies constructed withplywood or OSB shear panels.

Firewalls are fire separations of non-combustible construction.They have fire-resistance ratings as prescribed in buildingcodes, and structural properties such that they will remainintact under fire conditions for the required fire-rated time.Firewalls are commonly used to divide row-housing blocks intosmaller groups and resist the spread of fire from one group toanother. They are also used to divide a large building intosmaller units where standard fire protection measures areapplicable.

Fire-rated floor construction

Fire-rated floors, used in most types of multi-storey,multi-family buildings, are constructed with joists, a ceil-ing and a subfloor.

In ceiling applications, there is little advantage in using15.9-mm gypsum board panels as their greater weightcauses them to fall off during fires at about the same timeas the thinner panels, so that the fire resistance of thefloor assembly is not substantially increased.

When the gypsum board ceiling is attached with resilientmetal channels, the use of blocking or bridging betweenthe joists will not enhance the fire resistance of a floorassembly. If, on the other hand, the ceiling is attacheddirectly to the bottom of the joists, the use of blocking orbridging between the joists along the centreline of a floorassembly will reduce twisting of the joists during a fire,thereby helping the gypsum board to remain in placeand protect the assembly.

When a gypsum board ceiling is attached directly to thebottom of the joists, the fire resistance of that floorassembly may be significantly reduced if the spacingbetween the joists is increased (from 400 to 600 mm, forexample) because of the corresponding increase in dis-tances between the rows of fasteners. However, if the gyp-sum board is attached to the joists through resilientmetal channels spaced at 400 mm on-centre, increasingthe joist spacing may actually increase the fire resistancerating of the assembly because the structural loads arereduced and wider joist spacing requires a thickersubfloor.


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Subfloors usually consist of 1220 mm x 2440 mm pan-els, either tongued-and-grooved plywood or OSB. Thelong directions of the panels are oriented perpendicularto the joists. The 1220-mm butt ends of the panels arecentred on joists. Floor assemblies with joists spaced400 mm on-centre are constructed with 15.9-mm thickpanels; those with joists spaced 600 mm on-centre use19.0-mm panels. There are no significant differencesbetween the fire resistance rating of floor assemblies con-structed with plywood or OSB.

For a given floor span, the fire resistance of a floor assem-bly constructed with I-Joists will generally be three to fiveminutes less than that of a similar assembly constructedwith solid wood joists (which is insignificant in most sit-uations). This can be attributed to the thinner profile ofthe structural members, particularly the webs. Overall,there is no significant difference in the fire resistance of I-Joists manufactured with laminated-veneer-lumber(LVL) or solid lumber flanges; nor is there any significantdifference in the fire resistance of I-Joists manufacturedwith plywood or OSB webs. Finally, there is no signifi-cant difference in the fire performances of I-Joists ofidentical profile fabricated by different manufacturers.

Fire resistance ratings for floor assemblies constructedwith parallel-chord wood trusses (glued or connectedwith metal or gusset plates) differ very little from thosefor floors constructed with solid wood joists. While fail-ure of wood trusses in fires usually results from the teethof the gusset plates pulling out of charred wood in thebottom chord, the metal teeth do not accelerate char for-mation in the underlying lumber.

Finally, there is no scientific evidence to support theanecdotal claims that floors constructed with wood I-Joists or metal-plate-connected wood trusses fail cata-strophically and without warning during fires.

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Typical fire-rated floor assembly. Subfloor may be OSB or plywood. Note resilient channels supporting the ceiling.

Solid wood joist

Open web finger joined wood trusses

Metal plate connected wood truss

I-Joist

Sound transmission

Wood-frame construction is particularlyefficient in row houses and small apart-ment buildings where high levels of soundinsulation are desired between adjoiningdwelling units.

In Canada, airborne-sound-transmission measurements aremade in accordance with ASTM E 90 Standard Test Methodfor Laboratory Measurement of Airborne SoundTransmission Loss of Building Partitions. Sound-transmissionclass (STC) is determined in accordance with ASTM E 413Standard Classification for Rating Sound Insulation.Transmission of impact sound through floors is measured inaccordance with ASTM E 492 Standard Test Method forLaboratory Measurement of Impact Sound Transmissionthrough Floor-ceiling Assemblies Using the TappingMachine. Impact-insulation class (IIC) is calculated accord-ing to ASTM E 989 Standard Classification forDetermination of Impact Insulation Class.

The National Building Code of Canada currently specifiesSTC 50 for partywalls in multifamily dwellings. However,individual sensitivities to noise differ, in terms of both volumeand frequency ranges. Researchers at the National ResearchCouncil Canada suggest that music or sounds from a televisionset could be transmitted through a wall with STC 45, but thatonly a bit of the base beating might be heard if the STC of thewall was 50. Following a number of homeowner acoustic-comfort surveys, Canada Mortgage and Housing Corporationidentified the following sound-insulation objectives for multi-family buildings: STC >55 for inter-unit walls and floors,impact insulation class (IIC) >55 for inter-unit “hard” floorsand IIC >65 for inter-unit carpeted floors.

The reduction of sound transmission between dwellingunits relies on three factors: 1) decoupling the two sidesof walls or floors from each other; 2) increasing theassembly’s overall mass; and 3) filling the joist or studcavities with sound-absorbing insulation. Resilient metalchannels used to attach gypsum board to wood-frameassemblies, double layers of gypsum in ceilings andwalls, and insulation in the wall or floor cavities simulta-neously enhance fire resistance and minimize soundtransmission. Furthermore, wood-frame constructiondoes not present the impact-noise transmission prob-lems commonly noted with concrete construction.


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Acoustic chambers at the National Research Council of Canada

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Examples of fire-rated assemblies

The following are examples of sound-transmission-class(STC), impact insulation class (IIC) and fire-resistanceratings assigned to specific wood-frame walls and floor

designs approved by the National Building Code ofCanada (NBCC). Many other designs exist to achievesimilar ratings.

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Forintek is Canada’s wood products research insti-tute. A not-for-profit independent organization,Forintek has a history, dating back to 1913, of pro-viding leading-edge technical support to the woodproducts industry. Forintek is North America’s fore-most industry-funded fire research organization forwood construction systems. For more information:www.forintek.ca

Canada Mortgage and Housing Corporation is theFederal Government’s housing agency. For over 50years, CMHC has been helping provide Canadianswith housing quality, affordability and choice. CMHCis also the Canadian housing industry’s export partner,bringing Canadian expertise to foreign markets. Inaddition, CMHC is Canada’s largest publisher of hous-ing information. For more information: www.cmhc.ca

The Quebec Housing Corporation (Société d’habitationdu Québec) is the government agency responsible for allmatters related to housing in the province. Its mandatecovers primarily social housing and home improvementactivities. The Corporation also supports research activi-ties in collaboration with a variety of partners as well as the development and promotion of provincial expertise in housing both in Quebec and abroad. Tel.: 1-800-463-4315. Website: www.shq.gouv.qc.ca

©2002 Forintek Canada Corp., Société d’habitation du Québec and Canada Mortgage and Housing Corporation.

In conclusion

The fire performance of wood-frame construction isbased on many years of North American experience. It hasbeen documented through extensive testing, includingtests on full-size buildings.

Fire loss statistics and research demonstrate that peopleare just as safe from fire in a wood-frame house, whethersingle family or low-rise multi-family, as they would be inhouses built with any other material. Whatever the mate-rial used, the fire protection measures laid out by modernbuilding codes such as the National Building Code ofCanada ensure adequate building integrity and safe evac-uation for all occupants.

Wood-frame construction also provides a superior levelof comfort with respect to sound transmission, and it canbe designed to accommodate the broadest range of cli-matic, cultural, regulatory and economic conditions.Housing units exported from Canada are designed tomeet clients’ performance requirements anywhere in theworld.

For more information

Fire Safety Design in Buildings is a reference for applyingthe fire safety requirements of the National Building Codeof Canada to building design. This publication is availablefrom the Canadian Wood Council. Tel.: 1-800-463-5091.Website: www.cwc.ca

Ce document est aussi disponible en français.Este documento se encuentra disponible también en español.

wood-frame construction, fire resistance and sound ... · pdf filewith exterior brick ......

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