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Lumber grading is overseen by the National Lumber Grades Authority, each mill will have their mill identification on the grade stamp along with the Grading Agency who oversaw the grading.
A lumber grade is the minimum characteristics that a piece of lumber will exhibit. If the piece of lumber is part of a bundle, the grade of the bundle guarantees the minimum standard of any single piece.
Lumber is either graded visually or mechanically to determine the quality of the lumber. Visual criteria such as knot size, grain characteristics, and defects are used as indications of the lumber’s structural properties. Mechanical properties are also used to grade, such as density that can be determined through non-destructive tests and compared to known data. Lumber that you buy at a building centre will typically be visually graded, while lumber that is used by truss manufacturers is all mechanically graded.
Lumber is most frequently grouped into a “Common Grade Mix” which combines grades and species with similar structural properties. This is done to save sorting costs while guaranteeing a minimum quality of lumber. A common grade mix often used in framing is ‘spruce-pine-fir No.2 and better’, which could contain wood from grades No.1 and No.2 of any spruce, pine, or fir species.
If building under section 9 of the building code, Table 188.8.131.52 specifies minimum lumber grades for specific end uses.
OBC 184.108.40.206. says that joist, rafter, lintel and beams can be undersized up to 5% of the Canadian standard sizes, if the allowable span is decreased by 5% from the values in the span tables (Found in OBC section 9) for full size members.
Treated wood is required by code in several locations, including structural wood elements within 15cm of the finished grade, and on retaining walls or cribbing greater than 1.2m in height (which wooden frames for bunker silo walls). A full list of treatment categories and their applications can be found in OBC 220.127.116.11, the CSA document. Micro pro- sienna is a leading brand of pressure treatment in Ontario and breaks their products into two application groups- above ground and ground contact rated. The use categories that are covered by each application group can be found in the micro-pro sienna technical data document. For other brands it is advisable to find the technical information that specifies the use categories. If the lumber is used as intended, most pressure treating companies offer warranties against early failure of their products.
OBC 93.2.5. says that moisture content of lumber shall not be more than 19% at the time of installation. The Canadian Wood Council says that wood will most dramatically shrink or swell between 19% moisture (maximum to be considered dry wood) and 28% moisture (point where wood will absorb no more moisture.) Lumber stamped with S-DRY (surface dried) or KD (kiln dried) has been dried to below 19% moisture and will not have dramatic shrinking or swelling as it finds an equilibrium with the environment. It is advisable to check the moisture content of lumber before a building is enclosed, if it is found that the moisture is higher than 19% let the building air dry until the moisture is acceptable.
Agricultural buildings under 600m2, of low occupancy and no more than 1 storey in height are uniquely allowed to use ungraded lumber in their construction. The conditions set out in the Ontario Building Code Supplementary Standard SB-11 are:
“1.2.2. Ungraded Lumber
- Ungraded lumber may be used for wood posts, joists, rafters, lintels, beams, and wall studs in a farm building of low human occupancy of of not more than one storey in building height.
- Ungraded lumber means lumber that has not been-grade-stamped to indicate its grade, as determined by the NLGA “Standard Grading Rules for Canadian Lumber (interpretation Included)”, but that meets the following visual attributes:
- It is rough sawn to full nominal size,
- It has no evidence of decay
- It has no tight knots that exceed 25 percent of the cross section and that are spaced closer than 150mm on centres,
- It has no loose knots that exceed 25 percent of the cross section and that are spaced closer than 600mm on centres
- It has the slope of grain not exceeding 1 (vertical) in 4 (horizontal) and
- It is free of excessive warp.