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’.

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.

Download this chart as a PDF.

Design values for visually graded Canadian dimension lumber in Canada

The specified strengths and modulus of elasticity of visually graded dimension lumber are based on lumber that is graded in accordance with NLGA Standard Grading Rules for Canadian Lumber. All grades, except economy grade, are stress graded, that is, fifth percentile specified strengths are assigned to the different engineering properties such as tensile strength parallel to grain, compression strength perpendicular to grain, longitudinal shear strength, etc. The fifth percentile specified strengths and modulus of elasticity values are listed in the CSA O86 Engineering design in wood standard.

The design values are intended to be used by qualified designers and can be used in conjunction with the appropriate adjustment factors found in the CSA O86 standard. Design tables, examples and background information can be found in the CWC’s Wood Design Manual, which includes a copy of the CSA O86 standard, along with additional background information within the CSA O86 commentary.

For more information or to purchase standards from CSA Group, please visit http://shop.csa.ca/ or call 1-800-463-6727.

Design values for visually graded Canadian dimension lumber in the U.S.

Design values for visually graded dimension lumber that is manufactured in Canada, but used in the U.S., is based on ASTM standard test methods in accordance with the requirements of American Softwood Lumber Standard PS20-99 and applies to species grown within Canada.

For more information on the design provisions for Canadian dimension lumber used in the U.S., contact the American Wood Council (AWC) Helpdesk at 202-463-2766 or email info@awc.org

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 Doeasn’t 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.

Design values for visually graded Canadian dimension lumber in Canada

The specified strengths and modulus of elasticity of visually graded dimension lumber are based on lumber that is graded in accordance with NLGA Standard Grading Rules for Canadian Lumber. All grades, except economy grade, are stress graded, that is, fifth percentile specified strengths are assigned to the different engineering properties such as tensile strength parallel to grain, compression strength parallel to grain, longitudinal shear strength, etc. The fifth percentile specified strengths and modulus of elasticity values are listed in the CSA O86 Engineering design in wood standard.

The design values are intended to be used by qualified designers and can be used in conjunction with the appropriate adjustment factors found in the CSA O86 standard. Design tables, examples and background information can be found in the CWC’s Wood Design Manual, which includes a copy of the CSA O86 standard, along with additional background information within the CSA O86 commentary.

For more information or to purchase standards from CSA Group, please visit http://shop.csa.ca/ or call 1-800-463-6727.

Adhesives can also be referred to as resins.

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

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

Canadian Wood Truss Association

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

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

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

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 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