Lumber Grades
Each piece of construction lumber is examined and assigned a grade depending on its physical characteristics such as the size and location of knots and the slope of the grain. The assigned grade is an estimate of its strength.
Softwood lumber species having similar strength properties may be combined into a single-species combination and marketed under a group designation. The most common
group is spruce-pine-fir (SPF). The Canadian commercial species combinations and their characteristics are shown in Table 10.
Select Structural grade is used where high-strength, stiffness and good appearance are required. No. 1 grade may contain some percentage of Select Structural material, but permitted knots are slightly larger. Tests have shown that No. 1 and No. 2 grades of lumber have the same strength and therefore, No. 2 and better is a typical grade category for most general construction uses.
There are two types of machine-graded lumber: machine-stress-rated (MSR) lumber; and machine-evaluated lumber (MEL). Both MSR and MEL are produced using non-destructive stiffness measuring machines.
The grade stamp on MSR and MEL indicates strength properties. MSR and MEL are most often used in applications where strength is crucial, such as for manufacturing wood trusses.
Stud grade is 38 to 89 mm (2 to 4 in. nominal) thick and 38 to 140 mm (2 to 6 in. nominal) deep lumber used for wall studs.
Finger-jointed structural lumber manufactured to the requirements of NLGA SPS 1 is interchangeable with non-finger-jointed lumber of the same species, grade and length, and can be used for joists, rafters and other applications. Finger-jointed stud lumber manufactured to the requirements of NLGA SPS 3 can only be used as vertical, end-loaded members in compression (such as wall studs).
Construction and Standard grades are used for structural purposes. Construction grade lumber has strength properties similar to No. 3 grade, and Standard grade is lower. Utility and Economy grades are not used for structural purposes.
The National Building Code lists the minimum grades for various uses of lumber in wood-frame construction. 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.
Source : Canada Mortgage and Housing Corporation (CMHC)
