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
Plywood
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: APA – The Engineered Wood Association CSA O121 Douglas fir plywood, CSA O151 Canadian softwood plywood CSA O153 Poplar plywood CSA O86 Engineering design in wood CSA O80 Wood preservation ASTM D5516 Standard Test Method for Evaluating the Flexural Properties of Fire-Retardant Treated Softwood Plywood Exposed to Elevated Temperatures ASTM D6305 Standard Practice for Calculating Bending Strength Design Adjustment Factors for Fire-Retardant-Treated Plywood Roof Sheathing National Building Code of Canada Example Specifications for Plywood Plywood Grades Plywood Handling and Storage Plywood Manufacture Plywood Sizes Quality Control of Plywood
Fire-Retardant-Treated Wood
“Fire-retardant treated wood” (FRTW), as defined by the National Building Code of Canada (NBC), is ‘…wood or a wood product that has had its surface-burning characteristics, such as flame spread, rate of fuel contribution and density of smoke developed, reduced by impregnation with fire-retardant chemicals.’ FRTW must be pressure impregnated with fire-retardant chemicals in accordance with the CAN/CSA-O80 Series of Standards, Wood Preservation and when fire-tested for its surface flammability, must have a flame spread rating not more than 25. Fire-retardant chemical treatments applied to FRTW retard the spread of flame and limit smoke production from wood in fire situations. FRTW products are harder to ignite than untreated wood products and preservative treated wood products. Fire-retardant treatments applied to FRTW enhances the fire performance of the products by reducing the amount of heat released during the initial stages of fire. The treatments also reduce the amount of flammable volatiles released during fire exposure. This results in a reduction in the rate of flame spread over the surface. When the flame source is removed, FRTW ceases to char. FRTW contains different chemicals than preservative treated wood. However, the same manufacturing process is used to apply the chemicals. FRTW must be kiln-dried after treatment to a moisture content of 19% for lumber and 15% for plywood. The fire-retardant treatments used in FRTW do not generally interfere with the adhesion of surface paints and coatings unless the FRTW has an increased moisture content. The finishing characteristics of specific products should be discussed with the manufacturer. Typical interior applications of FRTW include architectural millwork, paneling, roof assemblies/trusses, beams, interior load bearing and non-load bearing partitions. Exterior-type fire retardants use different chemical formulations from those used for interior applications, since they must pass an accelerated weathering test (ASTM D2898), which exposes FRTW to regular wetting and drying cycles to represent actual long-term outdoor conditions. Generally, exterior-type fire retardants are applied to shingles and shakes. FRTW can be crosscut to length (not ripped) and drilled for holes following treatment without reducing its effectiveness. End cuts in the field, whether exposed or butt jointed, do not require treatment, since any untreated areas are relatively small compared to the total surface area and the flame spread rating remains unaffected. Plywood can be both crosscut and ripped without concern, since the chemical treatment has penetrated throughout the individual layers/plys. FRTW is not excessively corrosive to metal fasteners and other hardware, even in areas of high relative humidity. In fact, testing has demonstrated that FRTW is no more corrosive than untreated wood. Exterior use of FRTW Fire retardant coatings Fire-retardant-treated wood roof systems Flame-spread rating For more information on FRTW, visit the manufacture’s websites: Arch Wood Protection, Lonza: www.wolmanizedwood.com Viance LLC: www.treatedwood.com
Grades
Visual grading of dimension lumber In Canada, we are fortunate to have forests that are capable of producing dimension lumber that is desirable for use as structural wood products. Some primary factors that contribute to the production of lumber that is desirable for structural uses include; a favourable northern climate that is conducive to tree growth, many Canadian species contain small knots, and many of the Western Canadian species grow to heights of thirty meters or more, providing long sections of clear knot free wood and straight grain. The majority of the structural wood products are grouped within the spruce-pine-fir (S-P-F) species combination, which has the following advantages for structural applications: straight grain good workability light weight moderate strength small knots ability to hold nails and screws 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). Canadian dimension lumber is manufactured in accordance with CSA O141 Canadian Standard Lumber and must conform to the requirements of the Canadian and US lumber grading rules. Each piece of dimension lumber is inspected to determine its grade and a stamp is applied indicating the assigned grade, the mill identification number, a green (S-Grn) or dry (S-Dry) moisture content at time of surfacing, the species or species group, the grading authority having jurisdiction over the mill of origin, and the grading rule used, where applicable. Dimension lumber is generally grade stamped on one face at a distance of approximately 600 mm (2 ft) from one end of the piece, in order to ensure that the stamp will be clearly visible during construction. Specialty items, such as lumber manufactured for millwork or for decorative purposes, are seldom marked. To ensure this uniform quality of dimension lumber, Canadian mills are required to have each piece of lumber graded by lumber graders who are approved by an accredited grading agency. Grading agencies are accredited by the CLSAB. NLGA Standard Grading Rules for Canadian Lumber provide a list of the permitted characteristics within each grade of dimension lumber. The grade of a given piece of dimension lumber is based on the visual observations of certain natural characteristics of the wood. Most softwood lumber is assigned either an appearance grade or a structural grade based on a visual review performed by a lumber grader. The lumber grader is an integral part of the lumber manufacturing process. Using established correlations between appearance and strength, lumber graders are trained to assign a strength grade to dimensional lumber based on the presence or absence of certain natural characteristics. Examples of such characteristics include; the presence of wane (bark remnant on the outer edge), size and location of knots, the slope of the grain relative to the long axis and the size of shakes, splits and checks. Other characteristics are limited by the grading rules for appearance reasons only. Some of these include sap and heart stain, torn grain and planer skips. The table below shows a sample of a few of the criteria used to assess grades for 2×4 dimensional lumber that is categorized as ‘structural light framing’ or as ‘structural joist and plank’. Grades Characteristic Select Structural No.1 & No. 2 No. 3 Edge of wide face knots ¾” 1 ¼” 1 ¾” Slope of grain 1 in 12 1 in 8 1 in 4 To keep sorting cost to a minimum, grades may be grouped together. For example, there is an appearance difference between No.1 and No.2 visually graded dimension lumber, but not a difference in strength. Therefore, the grade mark ‘No.2 and better’ is commonly used where the visual appearance of No.1 grade dimensional lumber is not required, for example, in the construction of joists, rafters or trusses. Pieces of the same grade must be bundled together with the engineering properties dictated by the lowest strength grade in the bundle. Dimension lumber is aggregated into the following four grade categories: Structural light framing, Structural joists and planks, Light framing, and Stud. The table below shows the grades and uses for these categories. Grade Category Size Grades Common Grade Mix Principal Uses Structural Light Framing 38 to 89mm (2″ to 4″ nom.) thick and wide Select Structural, No.1, No.2, No.3 No.2 and Better Used for engineering applications such as for trusses, lintels, rafters, and joists in the smaller dimensions. Structural Joists and Planks 38 to 89mm (2″ to 4″ nom.) thick and 114mm (5″ nom.) or more wide Select Structural, No.1, No.2, No.3 No.2 and Better Used for engineering applications such as for trusses, lintels, rafters, and joists in the dimensions greater than 114mm (5″ nom.). Light Framing 38 to 89mm (2″ to 4″ nom.) thick and wide Construction, Standard, Utility Standard and Better (Std. & Btr.) Used for general framing where high strength values are not required such as for plates, sills, and blocking. Studs 38 to 89mm (2″ to 4″ nom.) thick and 38 to 140mm (2″ to 6″ nom.) wide and 3m (10′) or less in length Stud, Economy Stud Made principally for use in walls. Stud grade is suitable for bearing wall applications. Economy grade is suitable for temporary applications. Notes: Grades may be bundled individually or they may be individually stamped, but they must be grouped together with the engineering properties dictated by the lowest strength grade in the bundle. The common grade mix shown is the most economical blending of strength for most applications where appearance is not a factor and average strength is acceptable. Except for economy grade, all grades are stress graded, meaning specified strengths have been
Canadian Species
Canadian species of visually graded lumber 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. Click to enlarge the map. This map appears courtesy of Natural Resources Canada.
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.
Light-frame Trusses
A truss is a structural frame relying on a triangular arrangement of webs and chords to transfer loads to reaction points. This geometric arrangement of the members gives trusses high strength-to-weight ratios, which permit longer spans than conventional framing. Light-frame truss can commonly span up to 20 m (60 ft), although longer spans are also feasible. The first light-frame trusses were built on-site using nailed plywood gusset plates. These trusses offered acceptable spans but demanded considerable time to build. Originally developed in the United States in the 1950s, the metal connector plate transformed the truss industry by allowing efficient prefabrication of short and long span trusses. The light-gauge metal connector plates allow for the transfer of load between adjoining members through punched steel teeth that are embedded into the wood members. Today, light-frame wood trusses are widely used in single- and multi-family residential, institutional, agricultural, commercial and industrial construction. The shape and size of light-frame trusses is restricted only by manufacturing capabilities, shipping limitations and handling considerations. Trusses can be designed as simple or multi-span and with or without cantilevers. Economy, ease of fabrication, fast delivery and simplified erection procedures make light-frame wood trusses competitive in many roof and floor applications. Their long span capability often eliminates the need for interior load bearing walls, offering the designer flexibility in floor layouts. Roof trusses offer pitched, sloped or flat roof configurations, while also providing clearance for insulation, ventilation, electrical, plumbing, heating and air conditioning services between the chords. Light-frame wood trusses are prefabricated by pressing the protruding teeth of the steel truss plate into 38 mm (2 in) wood members, which are pre-cut and assembled in a jig. Most trusses are fabricated using 38 x 64 mm (2 x 3 in) to 38 x 184 mm (2 x 8 in) visually graded and machine stress-rated (MSR) lumber. To provide different grip values, the truss connector plates are stamped from galvanized light-gauge sheet steel of different grades and gauge thicknesses. Many sizes of truss plates are manufactured to suit any shape or size of truss or load to be carried. Light frame trusses are manufactured according to standards established by the Truss Plate Institute of Canada. The capacities for the plates vary by manufacturer and are established through testing. Truss plates must conform to the requirements of CSA O86 and must be approved by the Canadian Construction Materials Centre (CCMC). To obtain approval, the truss plates are tested in accordance with CSA S347. During design, light-frame trusses are generally engineered by the truss plate manufacturer on behalf of the truss fabricator. When light-frame trusses arrive at the job site they should be checked for any permanent damage such as cross breaks in the lumber, missing or damaged metal connector plates, excessive splits in the lumber, or any damage that could impair the structural integrity of the truss. Whenever possible, trusses should be unloaded in bundles on dry, relatively smooth ground. They should not be unloaded on rough terrain or uneven spaces that could result in undue lateral strain that could possibly distort the metal connector plates or damage parts of the trusses. Light-frame trusses can be stored horizontally or vertically. If stored in the horizontal position, trusses should be supported on blocking spaced at 2.4 to 3 m (8 to 10 ft) centres to prevent lateral bending and reduce moisture gain from the ground. When stored in the vertical position, trusses should be placed on a stable horizontal surfaced and braced to prevent toppling or tipping. If trusses need to be stored for an extended period of time measures must be taken to protect them from the elements, keeping the trusses dry and well ventilated. Light-frame trusses require temporary bracing during erection, prior to the installation of permanent bracing. Truss plates should not be used with incised lumber. Contact the truss manufacturer for further guidance on the use of light-frame trusses in corrosive environments, wet service conditions, or when treated with a fire retardant. For further information, refer to the following resources: Canadian Wood Truss Association Truss Plate Institute of Canada CSA O86 Engineering design in wood CSA S347 Method of test for evaluation of truss plates used in lumber joints Canadian Construction Materials Centre
i -Joists
Prefabricated wood I-joists are proprietary structural wood members that consist of fingerjoined solid sawn lumber or laminated veneer lumber (LVL) flanges attached to a plywood or oriented strand board (OSB) web using adhesive. Web panel joints are glued and mated by several methods such as butting of square panel ends, scarfing of the panel ends, or shaping of either a toothed or tongue and groove type joint. Exterior rated, waterproof adhesives such as phenol-formaldehyde and phenol-resorcinol are the principally used for the web to web and web to flange joints. Different combinations of flange and web materials using alternative connections between the web and the flanges are available from several manufacturers (refer to Figure 3.20, below). Wood I-joists are available in a variety of standard depths and in lengths of up to 20 m (66 ft). Each manufacturer produces I-joists with unique strength and stiffness characteristics. To ensure that proprietary products have been manufactured under a quality assurance program supervised by an independent third-party certification organization, manufacturers typically have their products evaluated and registered under the requirements and guidelines of the Canadian Construction Material Centre (CCMC). The cross-sectional “I” shape of these structural wood products provides a higher strength to weight ratio than traditional solid sawn lumber. The uniform stiffness, strength, and light weight of these prefabricated elements allow for use in longer span joist and rafter applications for both residential and commercial construction. Wood I-joists are usually manufactured using untreated flange and web material and therefore, are typically not used for exterior applications. Wood I-joist are also dimensionally stable as they are manufactured with a moisture content between 6 and 12 %. For the installation of mechanical and electrical services, many manufacturers provide requirements and guidance for the shape, size and location of openings, notches, holes and cuts. Most wood I-joist suppliers also stock standard joist hangers and other prefabricated connection hardware specially designed for use with wood I-joists. For further information on wood I-joists, refer to the following resources: APA – The Engineered Wood Association Canadian Construction Material Centre (CCMC), Institute for Research in Construction (NRC) Wood I-Joist Manufacturers Association (WIJMA) CSA O86 Engineering design in wood ASTM D5055 Standard Specification for Establishing and Monitoring Structural Capacities of Prefabricated Wood I-Joists
