There are more than a hundred softwood species in North America. To simplify the supply and use of structural softwood lumber, species having similar strength characteristics, and typically grown in the same region, are combined. Having a smaller number of species combinations makes it easier to design and select an appropriate species and for installation and inspection on the job site. In contrast, non-structural wood products are graded solely on the basis of appearance quality and are typically marked and sold under an individual species (e.g., Eastern White Pine, Western Red Cedar).
The Spruce-Pine-Fir (S-P-F) species group grows abundantly throughout Canada and makes up by far the largest proportion of dimension lumber production. The other major commercial species groups for Canadian dimension lumber are Douglas Fir-Larch, Hem-Fir and Northern Species.
The four species groups of Canadian lumber and their characteristics are shown below.
Species Combination: Douglas Fir-Larch
Abbreviation: D.Fir-L or DF-L
Species Included in Combination
Growth Region
Douglas Fir Western Larch
Characteristics
Colour Ranges
Reddish brown to yellow
High degree of hardness
Good resistance to decay
Species Combination: Hem-Fir
Abbreviation: Hem-Fir or H-F
Species Included in Combination
Growth Region
Pacific Coast Hemlock Amabilis Fir
Characteristics
Colour Ranges
Yellow brown to white
Works easily
Takes paint well
Holds nails well
Good gluing characteristics
Species Combination: Spruce-Pine-Fir
Abbreviation: S-P-F
Species Included in Combination
Growth Region
White Spruce Engleman Spruce Red Spruce Black Spruce
Jack Pine Lodgepole Pine Balsam Fir Alpine Fir
Characteristics
Colour Ranges
White to pale yellow
Works easily
Takes paint well
Holds nails well
Good gluing charateristics
Species Combination: Northern Species
Abbreviation: North or Nor
Species Included in Combination
Growth Region
Western Red Cedar
Characteristics
Colour Ranges
Reddish brown heartwood, light sapwood
Exceptional resistance to decay
Moderate strength
High in appearance qualities
Works easily
Takes fine finishes
Lowest shrinkage
Also Included in Northern Species
Species Included in Combination
Growth Region
Red Pine
Characteristics
Colour Ranges
Works easily
Also Included in Northern Species
Species Included in Combination
Growth Region
Ponderosa Pine
Characteristics
Colour Ranges
Takes finishes well
Holds nails well
Holds screws well
Seasons with little checking or cupping
Also Included in Northern Species
Species Included in Combination
Growth Region
Western White Pine Eastern White Pine
Characteristics
Colour Ranges
Creamy white to light straw brown heartwood, almost white sapwood
Works easily
Finishes well
Doeasnât tend to split or splinter
Holds nails well
Low shrinkage
Takes stain, paints & varnishes well
Also Included in Northern Species
Species Included in Combination
Growth Region
Trembling Aspen Largetooth Aspen Balsam Poplar
Characteristics
Colour Ranges
Works easily
Finishes well
Holds nails well
Below is a map of the forest regions in Canada and the principal tree species that grow in each region.
Many engineered wood products, including finger-joined lumber, plywood, oriented strand board (OSB), glulam, cross-laminated timber (CLT), wood I-joists and other structural composite lumber products, require the use of adhesives to transfer the stresses between adjoining wood fibres. Waterproof adhesives and heat resistant adhesives are commonly used in the manufacture of structural wood products.
Advances in adhesive technology to address challenges associated with increased production rates, visual appearance, process emissions and environmental impact concerns, have resulted in a wider range of innovative structural adhesive products. It is imperative that this new generation of adhesives achieve the same level of performance as traditional structural wood product adhesives such as phenol-formaldehyde (PF) or phenol-resorcinol formaldehyde (PRF).
Examples of different structural wood product adhesives families include, but are not limited to:
Emulsion polymer isocyanate (EPI);
One-component polyurethane (PUR);
Phenolic resins such as phenol-formaldehyde (PF) and phenol-resorcinol formaldehyde (PRF).
Various types of extenders such as walnut shell flour, Douglas fir bark flour, alder bark flour, and wood flour are sometimes used to reduce cost, control penetration into the wood fibre or moderate strength properties for the specific materials being bonded.
There are several industry standards that may be used to evaluate the performance of structural wood product adhesives, including:
CSA O112.6
Phenol and phenol-resorcinol resin adhesives for wood (high-temperature curing)
CSA O112.7
Resorcinol and phenol-resorcinol resin adhesives for wood (room- and intermediate-temperature curing)
CSA O112.9
Evaluation of adhesives for structural wood products (exterior exposure)
CSA O112.10
Evaluation of adhesives for structural wood products (limited moisture exposure)
CAN/CSA O160
Formaldehyde emissions standard for composite wood products
ASTM D7247
Standard Test Method for Evaluating the Shear Strength of Adhesive Bonds in Laminated Wood Products at Elevated Temperatures
ASTM D7374
Standard Practice for Evaluating Elevated Temperature Performance of Adhesives Used in End-Jointed Lumber
Bolts are widely used in wood construction. They are able to resist moderately heavy loads with relatively few connectors.
Bolts may be used in wood-to-wood, wood-to-steel and wood-to-concrete connection types. Some typical structural applications for bolts include:
purlin to beam connections
beam to column connections
column to base connections
truss connections
timber arches
post and beam construction
pole-frame construction
timber bridges
marine structures
Several types of bolts as shown in Figure 5.10 below, are used for wood construction with the hexagon head type being the most common. Countersunk heads are used where a flush surface is desired. Carriage bolts can be tightened by turning the nut without holding the bolt since the shoulders under the head grip the wood.
Bolts are commonly available in imperial diameters of 1/4, 1/2, 5/8, 3/4, 7/8 and 1 inch. Bolts are installed in holes drilled slightly (1 to 2 mm) larger than the bolt diameter to prevent any splitting and stress development that could be caused by installation or subsequent wood shrinkage. Depending on the diameter, bolts are available in lengths from 75 mm (3″) up to 400 mm (16″) with other lengths available on special order.
Bolts can be dipped or plated, at an additional cost, to provide resistance to corrosion. In exposed conditions and high moisture environments, corrosion should be resisted by using hot dip galvanized or stainless steel bolts, washers and nuts.
Washers are commonly used with bolts to keep the bolt head or nut from crushing the wood member when tightening is taking place. Washers are not required with a steel side plate, as the bolt head or nut bears directly on the steel. Common types of washers are shown in Figure 5.11 below.
Design information provided in CWC’s Wood Design Manual is based on bolts conforming to the requirements of ASTM A307 Standard Specification for Carbon Steel Bolts, Studs, and Threaded Rod 60 000 PSI Tensile Strength or Grade 2 bolts and dowels as specified under SAE J429 Mechanical and Material Requirements for Externally Threaded Fasteners.
Framing connectors are proprietary products and include fastener types such as; framing anchors, framing angles, joist, purling and beam hangers, truss plates, post caps, post anchors, sill plate anchors, steel straps and nail-on steel plates. Framing connectors are often used for different reasons, such as; their ability to provide connections within prefabricated light-frame wood trusses, their ability to resist wind uplift and seismic loads, their ability to reduce the overall depth of a floor or roof assembly, or their ability to resist higher loads than traditional nailed connections. Examples of some common framing connectors are shown in Figure 5.6, below.
Framing connectors are made of sheet metal and are manufactured with pre-punched holes to accept nails. Standard framing connectors are commonly manufactured using 20- or 18-gauge zinc coated sheet steel. Medium and heavy-duty framing connectors can be made from heavier zinc-coated steel, usually 12-gauge and 7-gauge, respectively. The load transfer capacity of framing connectors is related to the thickness of the sheet metal as well as the number of nails used to fasten the framing connector to the wood member.
Framing connectors are suitable for most connection geometries that use dimensional lumber that is 38 mm (2″ nom.) and thicker lumber. In light-frame wood construction, framing connectors are commonly used in connections between joists and headers; rafters and plates or ridges; purlins and trusses; and studs and sill plates. Certain types of framing connectors, manufactured to fit larger wood members and carry higher loads, are also suitable for mass timber and post and beam construction.
Manufacturers of the framing connectors will specify the type and number of fasteners, along with the installation procedures that are required in order to achieve the tabulated resistance(s) of the connection. The Canadian Construction Materials Centre (CCMC), Institute for Research in Construction (IRC), produce evaluation reports that document resistance values of framing connectors, which are derived from testing results.
Figure 5.6 Framing Connectors
For more information, refer to the following resources:
Canadian Construction Material Centre, National Research Council of Canada
Truss Plate Institute of Canada
CSA S347 Method of Test for Evaluation of Truss Plates used in Lumber Joints
ASTM D1761 Standard Test Methods for Mechanical Fasteners in Wood
Nailing is the most basic and most commonly used means of attaching members in wood frame construction. Common nails and spiral nails are used extensively in all types of wood construction. Historical performance, along with research results have shown that nails are a viable connection for wood structures with light to moderate loads. They are particularly useful in locations where redundancy and ductile connections are required, such as loading under seismic events.
Typical structural applications for nailed connections include:
wood frame construction
post and beam construction
heavy timber construction
shearwalls and diaphragms
nailed gussets for wood truss construction
wood panel assemblies
Nails and spikes are manufactured in many lengths, diameters, styles, materials, finishes and coatings, each designed for a specific purpose and application.
In Canada, nails are specified by the type and length and are still manufactured to Imperial dimensions. Nails are made in lengths from 13 to 150 mm (1/2 to 6 in). Spikes are made in lengths from 100 to 350 mm (4 to 14 in) and are generally stockier than nails, that is, a spike has a larger cross-sectional area than an equivalent length common nail. Spikes are generally longer and thicker than nails and are generally used to fasten heavy pieces of timber.
Nail diameter is specified by gauge number (British Imperial Standard). The gauge is the same as the wire diameter used in the manufacture of the nail. Gauges vary according to nail type and length. In the U.S., the length of nails is designated by “penny” abbreviated “d”. For example, a twenty-penny nail (20d) has a length of four inches.
The most common nails are made of low or medium carbon steels or aluminum. Medium-carbon steels are sometimes hardened by heat treating and quenching to increase toughness. Nails of copper, brass, bronze, stainless steel, monel and other special metals are available if specially ordered. Table 1, below, provides examples of some common applications for nails made of different materials.
TABLE 1: Nail applications for alternative materials
Material
Abbreviation
Application
Aluminum
A
For improved appearance and long life: increased strain and corrosion resistance.
Steel – Mild
S
For general construction.
Steel – Medium Carbon
Sc
For special driving conditions: improved impact resistance.
Stainless steel, copper and silicon bronze
E
For superior corrosion resistance: more expensive than hot-dip galvanizing.
Uncoated steel nails used in areas subject to wetting will corrode, react with extractives in the wood, and result in staining of the wood surface. In addition, the naturally occurring extractives in cedars react with unprotected steel, copper and blued or electro-galvanized fasteners. In such cases, it is best to use nails made of non-corrosive material, such as stainless steel, or finished with non-corrosive material such as hot-dipped galvanized zinc. Table 2, below, provides examples of some common applications for alternative finishes and coatings of nails.
TABLE 2: Nail applications for alternative finishes and coatings
Nail Finish or Coating
Abbreviation
Application
Bright
B
For general construction, normal finish, not recommended for exposure to weather.
Blued
Bl
For increased holding power in hardwood, thin oxide finish produced by heat treatment.
Heat treated
Ht
For increased stiffness and holding power: black oxide finish.
Phoscoated
Pt
For increased holding power; not corrosion resistant.
Electro galvanized
Ge
For limited corrosion resistance; thin zinc plating; smooth surface; for interior use.
Hot-dip galvanized
Ghd
For improved corrosion resistance; thick zinc coating; rough surface; for exterior use.
Pneumatic or mechanical nailing guns have found wide-spread acceptance in North America due to the speed with which nails can be driven. They are especially cost effective in repetitive applications such as in shearwall construction where nail spacing can be considerably closer together. The nails for pneumatic guns are lightly attached to each other or joined with plastic, allowing quick loading nail clips, similar to joined paper staples. Fasteners for these tools are available in many different sizes and types.
Design information provided in CSA O86 is applicable only for common round steel wire nails, spikes and common spiral nails, as defined in CSA B111. The ASTM F1667 Standard is also widely accepted and includes nail diameters that are not included in the CSA B111. Other nail-type fastenings not described in CSA B111 or ASTM F1667 may also be used, if supporting data is available.
Wood screws are manufactured in many different lengths, diameters and styles. Wood screws in structural framing applications such as fastening floor sheathing to the floors joists or the attachment of gypsum wallboard to wall framing members. Wood screws are often higher in cost than nails due to the machining required to make the thread and the head.
Screws are usually specified by gauge number, length, head style, material and finish. Screw lengths between 1 inch and 2 ¾ inch lengths are manufactured in ¼ inch intervals, whereas screws 3 inches and longer, are manufactured in ½ inch intervals. Designers should check with suppliers to determine availability.
Design provisions in Canada are limited to 6, 8, 10 and 12 gauge screws and are applicable only for wood screws that meet the requirements of ASME B18.6.1. For wood screw diameters greater than 12 gauge, design should be in accordance with the lag screw requirements of CSA O86.
Screws are designed to be much better at resisting withdrawal than nails. The length of the threaded portion of the screw is approximately two-thirds of the screw length. Where the wood relative density is equal to or greater than 0.5, lead holes, at least the length of the threaded portion of the shank, are required. In order to reduce the occurrence of splitting, pre-drilled holes are recommended for all screw connections.
The types of wood screws commonly used are shown in Figure 5.4, below.
For more information on wood screws, refer to the following resources:
Many historic structures in North America were built at a time when metal fasteners were not readily available. Instead, wood members were joined by shaping the adjoining wood members to interlock with one another. Timber joinery is a traditional post and beam wood construction technique used to connect wood members without the use of metal fasteners.
Timber joinery requires that the ends of timbers are carved out so that they fit together like puzzle pieces. The variations and configurations of wood-to-wood joints is quite large and complex. Some common wood-to-wood timber joints include mortise and tenon, dovetail, tying joint, scarf joint, bevelled shoulder joint, and lap joint. There are many variations and combinations of these and other types of timber joinery. Refer to Figure 5.18, below, for some examples of timber joinery.
For load transfer, timber joinery relies upon the interlocking of adjoining wood members. The mated joints are restrained by inserting wooden pegs into holes bored through the interlocked members. A hole about an inch in diameter is drilled right through the joint, and a wooden peg is pounded in to hold the joint together.
Metal fasteners require only minimal removal of wood fibre in the area of the fasteners and therefore, the capacity of the system is often governed by the moderate sized wood members to carry horizontal and vertical loads. Timber joinery, on the contrary, requires the removal of a significant volume of wood fibre where joints occur. For this reason, the capacity of traditional timber joinery construction is usually governed by the connections and not by the capacity of the members themselves. To accommodate for the removal of wood fibre at the connection locations, member sizes of wood construction systems that employ timber joinery, such as post and beam construction, are often larger than wood construction systems that make use of metal fasteners.
Wood engineering design standards in Canada do not provide specific load transfer information for timber joinery due to their sensitivity to workmanship and material quality. As a result, engineering design must be conservative, often resulting in larger member sizes.
The amount of skill and time required for measuring, fitting, cutting, and trial assembly is far greater for timber joinery than for other types of wood construction. Therefore, it is not the most economical means of connecting the members of wood buildings. Timber joinery is not used where economy is the overriding design criteria. Instead, it is used to provide a unique structural appearance which portrays the natural beauty of wood without distraction. Timber joinery offers a unique visual appearance exhibiting a high degree of craftmanship.
For further information, refer to the following resources:
Oriented Strand Board (OSB) is a widely used, versatile structural wood panel. OSB makes efficient use of forest resources, by employing less valuable, fast-growing species. OSB is made from abundant, small diameter poplar and aspen trees to produce an economical structural panel. The manufacturing process can make use of crooked, knotty and deformed trees which would not otherwise have commercial value, thereby maximizing forest utilization.
OSB has the ability to provide structural performance advantages, an important component of the building envelope and cost savings. OSB is a dimensionally stable wood-based panel that has the ability to resist delamination and warping. OSB can also resist racking and shape distortion when subjected to wind and seismic loadings. OSB panels are light in weight and easy to handle and install.
OSB panels are primarily used in dry service conditions as roof, wall and floor sheathing, and act as key structural components for resisting lateral loads in diaphragms and shearwalls. OSB is also used as the web material for some types of prefabricated wood I-joists and the skin material for structural insulated panels. OSB can also be used in siding, soffit, floor underlayment and subfloor applications. Some specialty OSB products are made for siding and for concrete formwork, although OSB is not commonly treated using preservatives. OSB has many interleaved layers which provide the panel with good nail and screw holding properties. Fasteners can be driven as close as 6 mm (1/4 in) from the panel edge without risk of splitting or breaking out.
OSB is a structural mat-formed panel product that is made from thin strands of aspen or poplar, sliced from small diameter roundwood logs or blocks, and bonded together with a waterproof phenolic adhesive that is cured under heat and pressure. OSB is also manufactured using the southern yellow pine species in the United States. Other species, such as birch, maple or sweetgum can also be used in limited quantities during manufacture.
OSB is manufactured with the surface layer strands aligned in the long panel direction, while the inner layers have random or cross alignment. Similar to plywood, OSB is stronger along the long axis compared to the narrow axis. This random or cross orientation of the strands and wafers results in a structural engineered wood panel with consistent stiffness and strength properties, as well as dimensional stability. It is also possible to produce directionally-specific strength properties by adjusting the orientation of strand or wafer layers. The wafers or strands used in the manufacture of OSB are generally up to 150 mm (6 in) long in the grain direction, 25 mm (1 in) wide and less than 1 mm (1/32″) in thickness.
In Canada, OSB panels are manufactured to meet the requirements of the CSA O325 standard. This standard sets performance ratings for specific end uses such as floor, roof and wall sheathing in light-frame wood construction. Sheathing conforming to CSA O325 is referenced in Part 9 of the National Building Code of Canada (NBC). In addition, design values for OSB construction sheathing are listed in CSA O86, allowing for engineering design of roof sheathing, wall sheathing and floor sheathing using OSB conforming to CSA O325.
OSB panels are manufactured in both imperial and metric sizes, and are either square-edged or tongue-and-grooved on the long edges for panels 15 mm (19/32 in) and thicker. For more information on available sizes of OSB panel, refer to the document below.
For more information on OSB, please refer to the following resources:
Plywood is a widely recognized engineered wood-based panel product that has been used in Canadian construction projects for decades. Plywood panels manufactured for structural applications are built up from multiple layers or plys of softwood veneer that are glued together so that the grain direction of each layer of veneer is perpendicular to that of the adjacent layers. These cross-laminated sheets of wood veneer are bonded together with a waterproof phenol-formaldehyde resin adhesive and cured under heat and pressure.
Plywood panels have superior dimensional stability, two-way strength and stiffness properties and an excellent strength-to-weight ratio. They are also highly resistant to impact damage, chemicals, and changes in temperature and relative humidity. Plywood remains flat to give a smooth, uniform surface that does not crack, cup or twist. Plywood can be painted, stained, or ordered with factory applied stains or finishes. Plywood is available with squared or tongue and groove edges, the latter of which can help to reduce labour and material costs by eliminating the need for panel edge blocking in certain design scenarios.
Plywood is suitable for a variety of end uses in both wet and dry service conditions, including: subflooring, single-layer flooring, wall, roof and floor sheathing, structural insulated panels, marine applications, webs of wood I-joists, concrete formwork, pallets, industrial containers, and furniture.
Plywood panels used as exterior wall and roof sheathing perform multiple functions; they can provide resistance to lateral forces such as wind and earthquake loads and also form an integral component of the building envelope. Plywood may be used as both a structural sheathing and a finish cladding. For exterior cladding applications, specialty plywoods are available in a broad range of patterns and textures, combining the natural characteristics of wood with superior strength and stiffness properties. When treated with wood preservatives, plywood is also suitable for use under extreme and prolonged moisture exposure such as permanent wood foundations.
Plywood is available in a wide variety of appearance grades, ranging from smooth, natural surfaces suitable for finish work to more economical unsanded grades used for sheathing. Plywood is available in more than a dozen common thicknesses and over twenty different grades.
Unsanded sheathing grade Douglas Fir Plywood (DFP), conforming to CSA O121, and Canadian Softwood Plywood (CSP), conforming to CSA O151, are the two most common types of softwood plywoods produced in Canada. All structural plywood products are marked with a legible and durable grade stamp that indicates: conformance to either CSA O121, CSA O151 or CSA O153, the manufacturer, the bond type (EXTERIOR), the species (DFP) or (CSP), and the grade.
Plywood can be chemically treated to improve resistance to decay or to fire. Preservative treatment must be done by a pressure process, in accordance with CSA O80 standards. It is required that plywood manufacturers carry out testing in conformance with ASTM D5516 and ASTM D6305 to determine the effects of fire retardants, or any other potentially strength-reducing chemicals.
For further information, refer to the following resources:
The National Building Code of Canada (NBC) requires that some buildings be of ‘noncombustible construction’ under its prescriptive requirements.
Noncombustible construction is, however, something of a misnomer, in that it does not exclude the use of ‘combustible’ materials but rather, it limits their use. Some combustible materials can be used since it is neither economical nor practical to construct a building entirely out of ‘noncombustible’ materials.
Wood is probably the most prevalent combustible material used in noncombustible buildings and has numerous applications in buildings classified as noncombustible construction under the NBC. This is due to the fact that building regulations do not rely solely on the use of noncombustible materials to achieve an acceptable degree of fire safety. Many combustible materials are allowed in concealed spaces and in areas where, in a fire, they are not likely to seriously affect other fire safety features of the building.
For example, there are permissions for use of heavy timber construction for roofs and roof structural supports. It may also be used in partition walls and as wall finishes, as well as furring strips, fascia and canopies, cant strips, roof curbs, fire blocking, roof sheathing and coverings, millwork, cabinets, counters, window sashes, doors, and flooring.
Its use in certain types of buildings such as tall buildings is slightly more limited in areas such as exits, corridors and lobbies, but even there, fire-retardant treatments can be used to meet NBC requirements. The NBC also allows the use of wood cladding for buildings designated to be of noncombustible construction.
In sprinklered noncombustible buildings not more than two-storeys in height, entire roof assemblies and the roof supports can be heavy timber construction. To be acceptable, the heavy timber components must comply with minimum dimension and installation requirements. Heavy timber construction is afforded this recognition because of its performance record under actual fire exposure and its acceptance as a fire-safe method of construction. Fire loss experience has shown, even in unsprinklered buildings, that heavy timber construction is superior to noncombustible roof assemblies not having any fire-resistance rating.
In other noncombustible buildings, heavy timber construction, including the floor assemblies, is permitted without the building being sprinklered.
In sprinklered buildings permitted to be of combustible construction, no fire-resistance rating is required for the roof assembly or its supports when constructed from heavy timber. In these cases, a heavy timber roof assembly and its supports would not have to conform to the minimum member dimensions stipulated in the NBC.
NBC definitions:
Combustible means that a material fails to meet the acceptance criteria of CAN/ULC-S114, “Test for Determination of Non-Combustibility in Building Materials.”
Combustible construction means that type of construction that does not meet the requirements for noncombustible construction.
Heavy timber construction means that type of combustible construction in which a degree of fire safety is attained by placing limitations on the sizes of wood structural members and on thickness and composition of wood floors and roofs and by the avoidance of concealed spaces under floors and roofs.
Noncombustible construction means that type of construction in which a degree of fire safety is attained by the use of noncombustible materials for structural members and other building assemblies.
Noncombustible means that a material meets the acceptance criteria of CAN/ULC-S114, “Test for Determination of Non-Combustibility in Building Materials.”
For further information, refer to the following resources:
Wood Design Manual, Canadian Wood Council
National Building Code of Canada
CAN/ULC-S114 Test for Determination of Non-Combustibility in Building Materials
Stairs and storage lockers in noncombustible buildings
Stairs within a dwelling unit can be made of wood, as can storage lockers in residential buildings. These are permitted, as their use is not expected to present a significant fire hazard.
Wood roofing materials in noncombustible buildings
In the installation of roofing, wood cant strips, roof curbs, nailing strips, and similar components may be used. Wood roofs defined as ‘heavy timber construction’ in the NBC are permitted in any noncombustible building two-storeys or less in height when the building is protected by a sprinkler system.
Roof sheathing and sheathing supports of wood are permitted in noncombustible buildings provided:
they are installed above a concrete deck;
the concealed space does not extend more than 1 m (39 in) above the deck;
the concealed roof space is compartmented by fire blocks;
openings through the concrete deck are located in noncombustible shafts;
parapets are provided at the deck perimeter extending at least 150 mm (6 in) above the sheathing; and
no building services are located on the roof other than those placed in noncombustible shafts.
The noncombustible parapets and shafts are required to prevent roof materials igniting from flames projecting from openings in the building face or roof deck. Roof coverings have often been contributing factors in conflagrations. Most roof coverings, even today, are combustible by the very nature of the materials used to make them waterproof.
The objective of the NBC is to require that the risks associated with a roof covering be minimized for the type of building, its location and use.
The NBC permits roof coverings that meet a Class C rating to be used for any building regulated by Part 3, including any noncombustible building, regardless of height or area.
This C rating can be met easily using fire-retardant-treated wood (FRTW) shakes or shingles, asphalt shingles, or roll roofing.
In buildings that are required to be of noncombustible construction, the roof coverings must have a fire classification of Class A, B or C. In such cases, the use of FRTW shakes and shingles on sloped roofs is allowed.
Small assembly occupancy buildings not more than two-storeys in building height and less than 1000 m2 (10,000 ft2) in building area do not require a classification for the roof covering. In these traditional cases, untreated wood shingles are acceptable if they are underlaid with a noncombustible material to reduce the potential for burn through.
Wood partitions in noncombustible buildings
Wood framing has many applications in partitions in both low-rise and high-rise buildings required to be of noncombustible construction. The framing can be located in most types of partitions, with or without a fire- resistance rating.
Wood framing and sheathing is permitted in partitions, or alternatively, solid lumber partitions at least 38 mm (2 in nominal) thick are permitted, provided:
the partitions are not used in a care, treatment or detention occupancy;
the area of the fire compartment, if not sprinklered, is limited to 600 m2 (the area of the fire compartment is unlimited in a floor area that is sprinklered); and,
the partitions are not required by the Code to be fire separations.
Alternatively, wood framing is permitted in partitions throughout floor areas, and can be used in most fire separations with no limits on compartment size or a need for sprinkler protection provided:
the buildings is not more than three-storeys in height;
the partitions are not used in a care, treatment or detention occupancy; and,
the partitions are not installed as enclosures for exits or vertical service spaces.
Similarly, as a final option, wood framing is permitted in buildings with no restriction on building height provided:
the building is sprinklered;
the partitions are not used in a care, treatment or detention occupancy;
the partitions are not installed as enclosures for exits or vertical service spaces; and,
the partitions are not used as fire separations to enclose a mezzanine.
These allowances in the code are based on the performance of fire-rated wood stud partitions compared to steel stud partitions. This research showed similar performance for wood and steel stud assemblies.
Also, the increase in the amount of combustible framing material permitted is not large compared to what is permitted as contents. In many cases, the framing is protected and only burns later in a fire once all combustible contents have been consumed, by which time the threat to life safety is not high. The exclusion of the framing in care and detention occupancies and in applications around critical spaces such as shafts and exits are applied to keep the level of risk as low as practical in these applications.
Wood furring in noncombustible buildings
Wood is particularly useful as a nailing base (also called a nailer) for different types of cladding and interior finishes.
Wood furring strips can be used to attach interior finishes such as gypsum wallboard, provided:
The strips are fastened to noncombustible backing or recessed into it.
The concealed space created by the wood elements is not more than 50 mm (2 in) thick.
The concealed space created by the wood elements is fire blocked.
Experience has shown that a lack of oxygen in these shallow concealed spaces prevents rapid development of fire.
Wood nailer strips can also be used on parapets, provided the facings and any roof membrane covering the facings are protected by sheet metal. This is permitted because it is considered that a nailing base such as plywood or oriented strand board (OSB) does not constitute an undue fire hazard.
Wood flooring and stages in noncombustible buildings
Combustible sub-flooring and finished flooring, such as wood strip or parquet, is allowed in any noncombustible building, including high rises. Finished wood flooring is not a major concern. During a fire, the air layer close to the floor remains relatively cool in comparison with the hot air rising to the ceiling.
Wood supports for combustible flooring are also permitted provided:
they are at least 50 mm but no more than 300 mm high;
they are applied directly onto or are recessed into a noncombustible floor slab; and,
the concealed spaces are fire blocked (as in Figure 1 below)
This allows the use of wood joists or wood trusses, the latter providing more flexibility for running building services within the space.
Since stages are normally fairly large and considerably higher than 300 mm which creates a large concealed space. Because of this, wood stage flooring must be supported by noncombustible structural members.
Figure 1. Raised wood floor
Fire stops in noncombustible buildings
Wood is commonly used for fire stops in combustible construction and it may also be used in noncombustible assemblies. Wood is permitted as a fire stop material for dividing concealed spaces into compartments in roofs of combustible construction.
However, wood fire stops must must meet the criteria for fire stops when the assembly is subject to the standard fire test used to determine fire resistance.
Interior wood finishes in noncombustible buildings
Wood finishes may be used in noncombustible buildings on walls and partitions within and outside suites and to a lesser extent, in areas such as exits and lobbies. The use of interior finishes is mostly regulated by restrictions on their flame-spread rating (FSR). Wood finishes not exceeding 25 mm (1 in) in thickness and having a FSR of 150 or less may be used extensively in noncombustible buildings that are not considered high buildings. However, where finishes are used as protection for foamed plastic insulation, they are required to act as a thermal barrier.
Some restrictions do apply in certain areas of a building. The area permitted to have a FSR of 150 or less is limited as follows:
in exits – only 10 percent of total wall area
in certain lobbies – only 25 percent of total wall area
in vertical spaces – only 10 percent of total wall area
The use of wood finishes on the ceilings in noncombustible buildings is much more restricted, but not totally excluded. In such cases, the FSR must be 25 or less. In certain cases, ordinary wood finishes (FSR of 150 or less) can also be used on 10 percent of the ceiling area of any one fire compartment, as well as on the ceilings of exits, lobbies and corridors.
Fire-retardant-treated wood (FRTW) must be used to meet the most restrictive limit of FSR 25. Consequently, it is permitted extensively throughout noncombustible buildings as a finish. The only restriction is that it cannot exceed 25 mm (1 in) in thickness when used as a finish, except when used as wood battens on a ceiling, in which case no maximum thickness applies. The NBC requirement for interior finishes in non-combustible buildings requires that the FSR be applicable to any surface of the material that may be exposed by cutting through the material. FRTW is exempted from this requirement because the treatment is applied through pressure impregnation. Fire retardant coatings are not exempt because they are surface applied only.
The FSR 75 limit for interior wall finishes in certain corridors does not exclude all wood products. For example, western red cedar, amabilis fir, western hemlock, western white pine and white or sitka spruce all have FSR at or lower than 75.
Corridors requiring FSR 75 include:
public corridors in any occupancy;
corridors used by the public in assembly or care or detention occupancies;
corridors serving classrooms; and,
corridors serving sleeping rooms in care and detention occupancies.
If these corridors are located in a sprinklered building, wood finishes having FSR 150 or less may be used to cover the entire wall surface.
In high rise buildings regulated by NBC (Division B, Subsection 3.2.6.), wood finishes are permitted within suites or floor areas much as for other buildings of noncombustible construction. However, certain additional restrictions apply for:
exit stairways;
corridors not within suites;
vestibules to exit stairs;
certain lobbies;
elevators cars; and,
service spaces and service rooms.
Wood cladding in noncombustible buildings
The NBC contains rules on the use of combustible claddings and supporting assemblies on certain types of buildings required to be of noncombustible construction. Specifically, the use of wall assemblies containing both combustibles cladding elements and non-loadbearing wood framing members is allowed.
These wall assemblies can be used as in-fill or panel type walls between structural elements, or be attached directly to a load-bearing noncombustible structural system. This applies in unsprinklered buildings up to three- storeys and sprinklered buildings of any height.
The wall assembly must satisfy the criteria of a test that determines its degree of flammability and the interior surfaces of the wall assembly must be protected by a thermal barrier (for example, 12.7 mm gypsum board) to limit the impact of an interior fire on the wall assembly.
These requirements stem from fire research that indicated that certain wall assemblies containing combustible elements do not promote exterior fire spread beyond a limited distance.
Each assembly must be tested in accordance with CAN/ULC-S134 to confirm compliance with fire spread and heat flux limitations specified in the NBC.
Fire-retardant-treated wood (FRTW) decorative cladding is permitted on first floor canopy fascias. In this case, the wood must undergo accelerated weathering before testing to establish the flame-spread rating. A FSR of 25 or less is required.
Millwork and window frames in noncombustible buildings
Wood millwork such as interior trim, doors and door frames, show windows and frames, aprons and backing, handrails, shelves, cabinets and counters are also permitted in noncombustible construction. Because these elements contribute minimally to the overall fire hazard it is not necessary to restrict their use.
Wood frames and sashes are permitted in noncombustible buildings provided each window is separated from adjacent windows by noncombustible construction and meets a limit on the aggregate area of openings in the outside face of a fire compartment.
Glass typically fails early during a fire, allowing flames to project from the opening and thereby creating serious potential for the vertical spread of fire. The requirement for noncombustible construction between windows is intended to limit fire spread along combustible frames closely set into the outside face of the building.
The National Building Code of Canada (NBC) contains requirements regarding the engineering design of structural wood products and systems. The CSA O86 standard is referenced in Part 4 of the NBC and in provincial building codes for the engineered design of structural wood products. The first edition of CSA O86 was published in 1959.
CSA O86 provides criteria for the structural design and evaluation of wood structures or structural elements. It is written in the limit states design (LSD) format and provides resistance equations and specified strength values for structural wood products, including: graded lumber, glued-laminated timber, cross-laminated timber (CLT), unsanded plywood, oriented strandboard (OSB), composite building components, light-frame shearwalls and diaphragms, timber piling, pole-type construction, prefabricated wood I-joists, structural composite lumber (SCL) products, permanent wood foundations (PWF), and their structural connections.
The CSA O86 provides rational approaches for structural design checks related to ultimate limit states, such as flexure, shear, and bearing, as well as serviceability limit states, such as deflection and vibration. The CSA O86 also contains strength modification factors for behaviour related to duration of load, size effects, service condition, lateral stability, system effects, preservative and fire-retardant treatment, notches, slenderness, and length of bearing.
Structural design of wood buildings and components is undertaken using the loads defined in Part 4 of the NBC and the material resistance values obtained using the CSA O86 standard. Housing and other small buildings can be built without a full structural design using the prescriptive requirements outlined in Part 9 ‘Housing and Small Buildings’ of the NBC.
For further information, refer to the following resources:
As identified in the design philosophy of the CSA S-6, safety is the overriding concern in the design of highway bridges in Canada. For wood products, the CSA S-6 addresses design criteria associated with ultimate limit states and serviceability limit states (primarily deflection, cracking, and vibration). Fatigue limit states are also required to be consider for steel connection components in wood bridges. The structure design life in the CSA S-6 has been established at 75 years for all bridge types, including wood bridges.
The CSA S-6 applies to the types of wood structures and components likely to be required for highways, including; glued-laminated timber, sawn lumber, structural composite lumber (SCL), nail-laminated decks, laminated wood-concrete composite decks, prestressed laminated decks, trusses, wood piles, wood cribs and wood trestles. The standard does not apply to falsework or formwork.
CSA S-6 considers design of wood members under flexure, shear, compression and bearing. In addition, the standard provides guidance and requirements related to the camber and curvature of wood members. Further information on durability, drainage and preservative treatment of wood in bridges is also discussed.
Article by Len Garis and Karin Mark. When assistant deputy fire chief Ray Bryant heard about construction of the tallest wood building in the world in Vancouver, his reaction...
Western Larch
Amabilis Fir 
Engleman Spruce 
Red Spruce
Black Spruce 
Lodgepole Pine 
Balsam Fir 
Alpine Fir 
Eastern White Pine 
Largetooth Aspen
Balsam Poplar 
