In North America, builders, inspectors, and structural engineers use grade stamps to determine quality, strength, and other factors about various lumber used in building or construction projects. Because it’s naturally sourced, timber (wood, lumber) often has defects, some detectable and others not so apparent. These defects can impact the strength of lumber and may include splits or knots that occur naturally.

There are six associations in the U.S. that establish and publish rules regarding grading, including Western Wood Products Association, abbreviated with the WWPA stamp. Others include SPIB or Southern Pine Inspection Bureau and Northeastern Lumber Manufacturers Association (NELMA), among others.

WWPA represents sawmills in Alaska and a dozen western U.S. states, its lumber grading supervised by the Association’s Quality Assurance Division. WWPA approves machine stress-rated and finger-jointed lumber, and establishes standards regarding size and quality levels in accordance with the American Softwood Lumber Standard PS 20. As one of the largest associations of lumber manufacturers in the U.S., Western Wood Products Association is approved to provide mill supervisory services under its rules and the rules of the National Lumber Grades Authority for Canadian Lumber, the Redwood Inspection Service, West Coast Lumber Inspection Bureau and Southern Pine Inspection Bureau‘s NGR segment rules.

So, how do you interpret lumber grade stamps? The information below pertains to WWPA.

Five basic elements are contained for the majority of grade stamps outside of heavy timbers or rough lumber. These include:

Mill identification. Originating mills are identified by the firm’s brand or name, or by a number assigned by the WWPA.
WWPA is a symbol that indicates Western Wood Products Association has supervised the grade of the lumber.
Species may be stamped to indicate a single or combination lumber species.
Grade of lumber is indicated by the grade abbreviation, name, or number.
S-Dry is associated with the conditioning of or moisture content of lumber at the time it is manufactured – essentially to what extent the wood is seasoned.

The following grade stamps indicate moisture content:

MC-15, KD-15 indicate a maximum moisture content of 15%

S-Dry, KD indicates the maximum moisture content may be 19%

S-GRN indicates the lumber is not seasoned, and has a moisture content of more than 19%

Essentially, lumber stamps are meant to inform regarding species of lumber, grade, moisture content, mill, and certification mark. For instance, softwoods used for framing are stamped with abbreviations that indicate the strength or weakness of the particular species of wood, such as D Fir-L, indicating Douglas fir western larch, or DougFir for Douglas fir. S-P-F indicate spruce, pine or fir.

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