Preservative Treated Wood
Preservative-treated wood is surface coated or pressure impregnated with chemicals that improve the resistance to damage that can result from biological deterioration (decay) due to the action of fungi, insects, and microorganisms. Preservative treatment offers a means for improving the resistance and extending the service life of those wood species which do not have sufficient natural resistance under certain in-use conditions. It is possible to extend the service life of untreated wood products by up to ten times through the use of preservative treatment. Preservative-treated wood can be used for exterior structures that require resistance to fungal decay and termites, such as: bridges, utility poles, railway ties, docks, marinas, fences, gazebos, pergolas, playground equipment, and landscaping. Four factors are necessary to sustain life for wood destroying fungi; a suitable food supply (wood fibre), a sustained minimum wood moisture content of about 20 percent (common for exterior use conditions), exposure to air, and a favourable temperature for growth (cold temperatures inhibit, but do not eliminate fungi growth). Preservative treatment is effective because it removes the food supply by making it poisonous to the fungi and wood destroying insects such as termites. An effective wood preservative must have the ability to penetrate the wood, neutralize the food supply of fungi and insects, and be present in sufficient quantities in a non-leachable form. Effective preservatives will also kill existing fungi and insects that might already exist in the wood. There are two basic methods of treating wood; with and without pressure. Non-pressure methods include the application of preservative by brushing, spraying or dipping the piece of wood. These superficial treatments do not result in deep penetration or large absorption of preservative and are typically restricted to field treatment during construction. Deeper and more thorough penetration is achieved by driving the preservative into the wood cells with pressure. Various combinations of pressure and vacuum are used to force adequate levels of chemical into the wood. For a wood preservative to function effectively it must be applied under controlled conditions, to specifications known to ensure that the preservative-treated wood will perform under specific in-use conditions. The manufacture and application of wood preservatives are governed by the CSA O80 series of standards. CSA O80 provides information on the wood species that may be treated, the types of preservatives and the retention and penetration of preservative in the wood that must be achieved for the use category or application. To ensure that the specified degree of protection will be provided, a preservative-treated wood product may bear a stamp indicating the suitability for a specific use category. Wood preservatives in Canada are governed by the Pest Control Products Act and must be registered with the Pest Management Regulatory Agency (PMRA) of Health Canada. Common types of wood preservatives that are used in Canada include chromated copper arsenate (CCA), alkaline copper quaternary (ACQ), copper azole (CA), micronized copper azole (MCA), borates, creosote, pentachlorophenol, copper naphthenate and zinc naphthenate. Acid salts can lessen the strength of wood if they are present in large concentrations. The concentrations used in preservative-treated wood are sufficiently small so that they do not affect the strength properties under normal use conditions. In some cases, the specified strength and stiffness of wood is reduced due to incising of the wood during the pressure impregnation process (refer to CSA O86 for further information on structural design reduction factors). Hot dipped galvanized or stainless steel fasteners and connection hardware are usually required to be used in conjunction with preservative-treated wood. There may be additional materials, such as polymer or ceramic coatings, or vinyl or plastic flashings that are suitable for use with preservative-treated wood products. The manufacturer should be consulted prior to specification of fasteners and connection hardware. For further information, refer to the following resources: www.durable-wood.com Wood Preservation Canada Canadian Wood Preservation Association CSA O80 Series Wood preservation CSA O86 Engineering design in wood Pest Management Regulatory Agency of Health Canada American Wood Protection Association
Nails
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. Types of Nails For more information, refer to the following resources: International, Staple, Nail, and Tool Association (ISANTA) CSA O86 Engineering design in wood CSA B111 Wire Nails, Spikes and Staples ASTM F1667 Standard Specification for Driven Fasteners: Nails, Spikes and Staples
Screws
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: ASME B18.6.1 Wood Screws CSA O86 Engineering design in wood
Timber Joinery
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: Timber Framers Guild
Panel Products
By using roundwood that is often not be suitable for lumber production, wood-based panels make efficient use of the forest resource by providing engineered wood products with defined strength and stiffness properties. Wood-based structural panels such as plywood and oriented strand board (OSB) are widely used in residential and commercial construction. Wood-based panels are often overlaid on joists or light frame trusses and used as structural sheathing for floor, roofs and wall assemblies. These products provide rigidity to the supporting main structural members in addition to their function as a component of the building envelope. In addition, they are often an integral component of the lateral force resisting system of a wood building. In order to qualify for a particular end use, such as structural sheathing, flooring or exterior siding, wood-based panels must meet performance criteria related to three aspects: structural performance, physical properties and bond performance. For more information on performance rating and potential end uses of wood-based panel products, refer to APA – The Engineered Wood Association.
Oriented Strand Lumber
Oriented Strand Lumber (OSL) Oriented Strand Lumber (OSL) provides attributes such as high strength, high stiffness and dimensional stability. The manufacturing process of OSL enables large members to be made from relatively small trees, providing efficient utilization of forest resources. OSL is used primarily as structural framing for residential, commercial and industrial construction. Common applications of OSL in construction include headers and beams, tall wall studs, rim board, sill plates, millwork and window framing. OSL also offers good fastener-holding strength. Similar to laminated strand lumber (LSL), OSL is made from flaked wood strands that have a length-to-thickness ratio of approximately 75. The wood strands used in OSL are shorter than those in LSL. Combined with an adhesive, the strands are oriented and formed into a large mat or billet and pressed. OSL resembles oriented strand board (OSB) in appearance as they are both fabricated from the similar wood species and contain flaked wood strands, however, unlike OSB, the strands in OSL are arranged parallel to the longitudinal axis of the member. OSL is a solid, highly predictable, uniform engineered wood product due to the fact that natural defects such as knots, slope of grain and splits have been dispersed throughout the material or have been removed altogether during the manufacturing process. Like other SCL products such as LVL and PSL, OSL offers predictable strength and stiffness properties and dimensional stability that minimize twist and shrinkage. All special cutting, notching or drilling should be done in accordance with manufacturer’s recommendations. Manufacturer’s catalogues and evaluation reports are the primary sources of information for design, typical installation details and performance characteristics. As with any other wood product, OSL should be protected from the weather during jobsite storage and after installation. Wrapping of the product for shipment to the job site is important in providing moisture protection. End and edge sealing of the product will enhance its resistance to moisture penetration. OSL is a proprietary product and therefore, the specific engineering properties and sizes are unique to each manufacturer. Thus, OSL does not have a common standard of production and common design values. Design values are derived from test results analysed in accordance with CSA O86 and ASTM D5456 and the design values are reviewed and approved by the Canadian Construction Materials Centre (CCMC). Products meeting the CCMC guidelines receive an Evaluation Number and Evaluation Report that includes the specified design strengths, which are subsequently listed in CCMC’s Registry of Product Evaluations. The manufacturer’s name or product identification and the stress grade is marked on the material at various intervals, but due to end cutting it may not be present on every piece. For further information, refer to the following resources: APA – The Engineered Wood Association Canadian Construction Materials Centre (CCMC), Institute for Research in Construction CSA O86 Engineering design in wood ASTM D5456 Standard Specification for Evaluation of Structural Composite Lumber Products
Parallel Strand Lumber
Parallel Strand Lumber (PSL) Parallel Strand Lumber (PSL) provides attributes such as high strength, high stiffness and dimensional stability. The manufacturing process of OSL enables large members to be made from relatively small trees, providing efficient utilization of forest resources. In Canada, PSL is fabricated using Douglas fir. PSL is employed primarily as structural framing for residential, commercial and industrial construction. Common applications of PSL in construction include headers, beams and lintels in light-frame construction and beams and columns in post and beam construction. PSL is an attractive structural material which is suited to applications where finished appearance is important. Similar to laminated strand lumber (LSL) and oriented strand lumber (OSL), PSL is made from flaked wood strands that are arranged parallel to the longitudinal axis of the member and have a length-to-thickness ratio of approximately 300. The wood strands used in PSL are longer than those used to manufacture LSL and OSL. Combined with an exterior waterproof phenol-formaldehyde adhesive, the strands are oriented and formed into a large billet, then pressed together and cured using microwave radiation. PSL beams are available in thicknesses of 68 mm (2-11/16 in), 89 mm (3-1/2 in), 133 mm (5-1/4 in), and 178 mm (7 in) and a maximum depth of 457 mm (18 in). PSL columns are available in square or rectangular dimensions of 89 mm (3-1/2 in), 133 mm (5-1/4 in), and 178 mm (7 in). The smaller thicknesses can be used individually as single plies or can be combined for multi-ply applications. PSL can be made in long lengths but it is usually limited to 20 m (66 ft) by transportation constraints. PSL is a solid, highly predictable, uniform engineered wood product due to the fact that natural defects such as knots, slope of grain and splits have been dispersed throughout the material or have been removed altogether during the manufacturing process. Like the other SCL products (LVL, LSL and OSL), PSL offers predictable strength and stiffness properties and dimensional stability. Manufactured at a moisture content of 11 percent, PSL is less prone to shrinking, warping , cupping, bowing and splitting. All special cutting, notching or drilling should be done in accordance with manufacturer’s recommendations. Manufacturer’s catalogues and evaluation reports are the primary sources of information for design, typical installation details and performance characteristics. PSL exhibits a rich texture and retains numerous dark glue lines. PSL can be machined, stained, and finished using the techniques applicable to sawn lumber. PSL members readily accept stain to enhance the warmth and texture of the wood. All PSL is sanded at the end of the production process to ensure precise dimensions and to provide a high quality surface for appearance. As with any other wood product, PSL should be protected from the weather during jobsite storage and after installation. Wrapping of the product for shipment to the job site is important in providing moisture protection. End and edge sealing of the product will enhance its resistance to moisture penetration. PSL readily accepts preservative treatment and it is possible to obtain a high degree of preservative penetration. Treated PSL can be specified in high humidity exposures. PSL is a proprietary product and therefore, the specific engineering properties and sizes are unique to each manufacturer. Thus, PSL does not have a common standard of production and common design values. Design values are derived from test results analysed in accordance with CSA O86 and ASTM D5456 and the design values are reviewed and approved by the Canadian Construction Materials Centre (CCMC). Products meeting the CCMC guidelines receive an Evaluation Number and Evaluation Report that includes the specified design strengths, which are subsequently listed in CCMC’s Registry of Product Evaluations. The manufacturer’s name or product identification and the stress grade is marked on the material at various intervals, but due to end cutting it may not be present on every piece. The Canadian Construction Materials Centre (CCMC) has accepted PSL for use as heavy timber construction, as described under the provisions within Part 3 of the National Building Code of Canada. For further information, refer to the following resources: APA – The Engineered Wood Association Canadian Construction Materials Centre (CCMC), Institute for Research in Construction CSA O86 Engineering design in wood ASTM D5456 Standard Specification for Evaluation of Structural Composite Lumber Products
Solid-Sawn Heavy Timber
Solid-sawn heavy timber members are predominantly employed as the main structural elements in post and beam construction. The term ‘heavy timber’ is used to describe solid sawn lumber which is 140 mm (5-1/2 in) or more in its smallest cross-sectional dimension. Large dimension timbers offer increased fire resistance compared to dimensional lumber and can be used to meet the heavy timber construction requirements outlined in the Part 3 of the National Building Code of Canada. Sawn timbers are produced in accordance with CSA O141 Canadian Standard Lumber and graded in accordance with the NLGA Standard Grading Rules for Canadian Lumber. There are two categories of timbers; rectangular “Beams and Stringers” and square “Posts and Timbers”. Beams and Stringers, whose larger dimension exceeds its smaller dimension by more than 51 mm (2 in), are typically used as bending members, whereas, Posts and Timbers, whose larger dimension exceeds its smaller dimension by 51 mm (2 in) or less, are typically used as columns. Sawn timbers range in size from 140 to 394 mm (5-1/2 to 15-1/2 in). The most common sizes range from 140 x 140 mm (5-1/2 x 5-1/2 in) to 292 x 495 mm (11-1/2 x 19-1/2 in) in lengths of 5 to 9 m (16 to 30 ft). Sizes up to 394 x 394 mm (15-1/2 x 15-1/2 in) are generally available from Western Canada in the Douglas Fir-Larch and Hem-Fir species combinations. Timbers from the Spruce-Pine-Fir (S-P-F) and Northern species combinations are only available in smaller sizes. Timbers may be obtained in lengths up to 9.1 m (30 ft), but the availability of large size and long length timbers should always be confirmed with suppliers prior to specifying. A table of available timber sizes is shown below. Both categories of timbers, Beams and Stringers, and Posts and Timbers, contain three stress grades: Select Structural, No.1, and No.2, and two non-stress grades (Standard and Utility). The stress grades are assigned design values for use as structural members. Non-stress grades have not been assigned design values. No.1 or No.2 are the most common grades specified for structural purposes. No.1 may contain varying amounts of Select Structural, depending on the manufacturer. Unlike Canadian dimension lumber, there is a difference between design values for No.1 and No.2 grades for timbers. Select Structural is specified when the highest quality appearance and strength are desired. The Standard and Utility grades have not been assigned design values. Timbers of these grades are permitted for use in specific applications of building codes where high strength is not important, such as blocking or short bracing. Cross cutting can affect the grade of timber in the Beams and Stringers category because the allowable size of knot varies along the length of the piece (a larger knot is allowed near the ends than in the middle). Timbers must be regraded if cross cut. Timbers are generally not grade marked (grade stamped) and a mill certificate can be obtained to certify the grade. The large size of timbers makes kiln drying impractical due to the drying stresses which would result from differential moisture contents between the interior and exterior of the timber. For this reason, timbers are usually dressed green (moisture content above 19 percent), and the moisture content of timber upon delivery will depend on the amount of air drying which has taken place. Like dimension lumber, timber begins to shrink when its moisture content falls below about 28 percent. Timbers exposed to the outdoors usually shrink from 1.8 to 2.6 percent in width and thickness, depending on the species. Timbers used indoors, where the air is often drier, experience greater shrinkage, in the range of 2.4 to 3.0 percent in width and thickness. Length change in either case is negligible. Allowances for anticipated shrinkage should be made in the design and construction. Shrinkage should also be considered when designing connections. Minor checks on the surface of a timber are common in both wet and dry service conditions. Consideration has been made for these surface checks in the establishment of specified design strengths. Checks in columns are not of structural importance unless the check develops into a through split that will divide the column. For further information, refer to the following resources: Timber Framers Guild International Log Builders’ Association BC Log & Timber Building Industry Association
Plank Decking
Plank decking may be used to span farther and carry greater loads than panel products such as plywood and oriented strand board (OSB). Plank decking is often used where the appearance of the decking is desired as an architectural feature or where the fire performance must meet the heavy timber construction requirements outlined in Part 3 of the National Building Code of Canada. Plank decking is usually used in mass timber or post and beam structures and is laid with the flat or wide face over supports to provide a structural deck for floors and roofs. Plank decking can be used in either wet or dry service conditions and can be treated with preservatives, dependent on the wood species. Nails and deck spikes are used to fasten adjacent pieces of plank decking to one another and are also used to fasten the deck to its supports. Decking is generally available in the following species: Douglas fir (D.Fir-L species combination) Pacific coast hemlock (Hem-Fir species combination) Various species of spruce, pine and fir (S-P-F species combination) Western red cedar (Northern species combination) In order to product plank decking, sawn lumber is milled into a tongue and groove profile with special surface machining, such as a V-joint. Plank decking is normally produced in three thicknesses: 38 mm (1-1/2 in), 64 mm (2-1/2 in) and 89 mm (3-1/2 in). The 38 mm (1-1/2 in) decking has a single tongue and groove while the thicker sizes have a double tongue and groove. Thicknesses greater than 38 mm (1-1/2 in) also have 6 mm (1/4 in) diameter holes at 760 mm (2.5 ft) spacing so that each piece may be nailed to the adjacent one with deck spikes. The standard sizes and profiles are shown below. Plank decking is most readily available in random lengths of 1.8 to 6.1 m (6 to 20 ft). Decking can be ordered in specific lengths, but limited availability and extra costs should be expected. A typical specification for random lengths could require that at least 90 percent of the plank decking be 3.0 m (10 ft) and longer, and at least 40 percent be 4.9 m (16 ft) and longer. Plank decking is available in two grades: Select grade (Sel) Commercial grade (Com) Select grade has a higher quality appearance and is also stronger and stiffer than commercial grade. Plank decking is required to be manufactured in accordance with CSA O141 and graded under the NLGA Standard Grading Rules for Canadian Lumber. Since plank decking is not grade stamped like dimensional lumber, verification of the grade should be obtained in writing from the supplier or a qualified grading agency should be retained to check the supplied material. To minimize shrinkage and warping, plank decking consists of sawn lumber members that are dried to a moisture content of 19 percent or less at the time of surfacing (S-Dry). The use of green decking can result in the loosening of the tongue and groove joint over time and a reduction in structural and serviceability performance. Individual planks can span simply between supports, but are generally random lengths spanning several supports for economy and to take advantage of increased stiffness. There are three methods of installing plank decking: controlled random, simple span and two span continuous. A general design rule for controlled random plank decking is that spans should not be more than 600 mm (2 ft) longer than the length which 40 percent of the decking shipment exceeds. Both the latter methods of installation require planks of predetermined length and a consequently there could be an associated cost premium. Profiles and Sizes of Plank Decking
Wood in non-combustible buildings
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: 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: 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: Similarly, as a final
Lumber
Dimension lumber is solid sawn wood that is less than 89 mm (3.5 in) in thickness. Lumber can be referred to by its nominal size in inches, which means the actual size rounded up to the nearest inch or by its actual size in millimeters. For instance, 38 × 89 mm (1-1/2 × 3-1/2 in) material is referred to nominally as 2 × 4 lumber. Air-dried or kiln dried lumber (S-Dry), having a moisture content of 19 percent or less, is readily available in the 38 mm (1.5 in) thickness. Dimension lumber thicknesses of 64 and 89 mm (2-1/2 and 3-1/2 in) are generally available as surfaced green (S-Grn) only, i.e., moisture content is greater than 19 percent. The maximum length of dimension lumber that can be obtained is about 7 m (23 ft), but varies throughout Canada. The predominant use of dimension lumber in building construction is in framing of roofs, floors, shearwalls, diaphragms, and load bearing walls. Lumber can be used directly as framing materials or may be used to manufacture engineered structural products, such as light frame trusses or prefabricated wood I-joists. Special grade dimension lumber called lamstock (laminating stock) is manufactured exclusively for glulam. Quality assurance of Canadian lumber is achieved via a complex system of product standards, engineering design standards and building codes, involving grading oversight, technical support and a regulatory framework. Checking and splitting Checking and splitting Checking occurs when lumber is rapidly dried. The surface dries quickly, while the core remains at a higher moisture content for some time. As a result, the surface attempts to shrink but is restrained by the core. This restraint causes tensile stresses at the surface, which if large enough, can pull the fibres apart, thereby creating a check. Splits are through checks that generally occur at the end of wood members. When a wood member dries, moisture is lost very rapidly from the end of the member. At midlength, however, the wood is still at a higher moisture content. This difference in moisture content creates tensile stresses at the end of the member. When the stresses exceed the strength of the wood, a split is formed. Large dimension solid sawn timbers are susceptible to checking and splitting since they are always dressed green (S-Grn). Furthermore, due to their large size, the core dries slowly and the tensile stresses at the surface and at the ends can be large. Minor checks confined to the surface areas of a wood member very rarely have any effect on the strength of the member. Deep checks could be significant if they occur at a point of high shear stress. Checks in columns are not of structural importance, unless the check develops into a through split that will increase the slenderness ratio of the column. The specified shear strengths of dimension lumber and timbers have been developed to consider the maximum amount of checking or splitting permitted by the applicable grading rule. The possibility and severity of splitting and checking can be reduced by controlling the rate at which drying occurs. This may be done by keeping wood out of direct sunlight and away from any artificial heat sources. Furthermore, the ends may be coated with an end sealer to retard moisture loss. Other actions which will minimize dimension change and the possibility of checking or splitting are: specifying wood products that are as close as possible in moisture content to the expected equilibrium moisture content of the end use ensuring dry wood products are protected by proper storage and handling Fingerjoined lumber Fingerjoined products are manufactured by taking shorter pieces of kiln-dried lumber, machining a ‘finger’ profile in each end of the short-length pieces, adding an appropriate structural adhesive, and end-gluing the pieces together to make a longer length piece of lumber. The length of a fingerjoined lumber is not limited by the length of the log. In fact, the manufacturing process can result in the production of joists and rafters in lengths of 12 m (40 ft) or more. The process of fingerjoining is also used within the manufacturing process for several other engineered wood products, including glued-laminated timber and wood I-joists. The specific term “fingerjoined lumber” applies to dimension lumber that contains finger joints. Fingerjoining derives greater value from the forest resource by using short length pieces of lower grade lumber as input for the manufacture of a value-added engineered wood product. The fingerjoining process utilizes short off cut pieces of lumber and results in more efficient use of the harvested wood fibre. Fingerjoined lumber can be manufactured from any commercial species or species group. The most commonly used species group from which fingerjoined lumber is produced is Spruce-Pine-Fir (S-P-F). Design advantages of fingerjoined lumber Fingerjoined lumber is an engineered wood product that is desirable for several reasons: straightness dimensional stability interchangeability with non-fingerjointed lumber highly efficient use of wood fibre The design and performance advantages of this engineered wood product are its straightness and dimensional stability. The straightness and dimensional stability of fingerjoined lumber is a result of short length pieces of lumber, consisting of relatively straight grain and fewer natural defects, being combined with one another to form a longer length piece of lumber. The grain pattern along fingerjoined lumber becomes non-uniform and random by attaching many short pieces together. This results in fingerjoined lumber being less prone to warping than solid sawn lumber. The fingerjoining process also results in the reduction or removal of strength reducing defects, producing a structural wood product with less variable engineering properties than solid sawn dimensional lumber. The most common use of finger-joined lumber is as studs in shearwalls and vertical load bearing walls. The most important factor for studs is straightness. Fingerjoined studs will stay straighter than solid sawn dimensional lumber studs when subjected to changes in temperature and humidity. This feature results in significant benefits to the builder and homeowner including a superior building, the elimination of nail pops in drywall and other problems related to dimensional changes.
Mass Timber
Advancements in wood product technology and systems are driving the momentum for innovative buildings in Canada. Products such as cross-laminated timber (CLT), nailed-laminated timber (NLT), glued-laminated timber (GLT), laminated strand lumber (LSL), laminated veneer lumber (LVL) and other large-dimensioned structural composite lumber (SCL) products are part of a bigger classification known as ‘mass timber’. Although mass timber is an emerging term, traditional post-and-beam (timber frame) construction has been around for centuries. Today, mass timber products can be formed by mechanically fastening and/or bonding with adhesive smaller wood components such as dimension lumber or wood veneers, strands or fibres to form large pre-fabricated wood elements used as beams, columns, arches, walls, floors and roofs. Mass timber products have sufficient volume and cross-sectional dimensions to offer significant benefits in terms of fire, acoustics and structural performance, in addition to providing construction efficiency.
