Each piece of lumber is examined and assigned a grade depending on its physical characteristics. In addition to visually graded lumber, machine-stress-rated (MSR) lumber is available in Canada. MSR lumber is identified in grade stamps by its structural properties and, for most wood-frame construction purposes, is independent of species.
Many softwood lumber species in Canada are harvested, milled and marketed together. Those having similar properties that enables them to be used together easily are combined into a single-species combination and marketed under a group designation. The Canadian commercial species combinations and their characteristics are shown in Table 9.
The top grade is Select Structural, which is used only where high strength, stiffness and good appearance are required. Lumber marked No. 1 grade often contains some percentage of Select Structural material, but permitted knots are slightly larger.
Tests have shown that No. i and No. 2 grades of lumber have the same strength. These grades are popular for most general construction uses. No. 3 grade lumber is used in general construction where appearance is not a factor.
2 x 4 in. (38 x 89 mm) and 2x6 in. (38 x 140 mm) lumber are available as Stud grades. Stud grade is stiff, straight lumber suitable for vertical wall members. 2 x 4 in. (38 x 89 mm) is also available as Construction, Standard, Utility and Economy grades. Construction and Standard grade lumber are used for structural purposes. Construction grade lumber has similar strength properties as No. 3 grade, and Standard grade is lower. Utility and Economy grades are not used for structural purposes. Economy is the lowest grade.
Minimum grades for various uses of lumber in wood-frame construction, such as stud wall framing, plank-frame construction, posts and beams, sheathing and subflooring are set forth in the National Building Code of Canada. Tables giving maximum allowable spans for visually graded lumber and for MSR lumber when used as joists and rafters are available from the Canadian Wood Council. In this publication, a number of tables have been provided which present the maximum allowable spans for structural components using various grades of lumber.
Metric sizes of softwood lumber are the same as those in use in Canada under the imperial system of measure ment, but their sizes are expressed in millimetres of actual thickness and width after surfacing. The concept of "nominal size" is not used. Table 10 relates the current metric dimensions to the imperial equivalents actual and nominal.
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