Wood framing is the material of choice for residential construction and continues to increase its market share in non-residential construction. Wood frame construction relies on the lumber design values and allowable span tables published in building codes and standards, and industry technical publications.
Design values for Western softwood structural lumber are computed in accordance with ASTM standards based on clear-wood tests or on tests of full-size pieces in specific grades, conducted in cooperation with the USDA Forest Products Laboratory. The applicable standards are ASTM D2555 and ASTM D245 for clear wood and ASTM D1990 for full-size specimens. Spans shown in the Western Lumber Span Tables and the Product Use Manual were calculated using Western Lumber Base Values as published in WWPA’s Western Lumber Grading Rules. These spans relate to products manufactured from U.S. Western species and are not appropriate for use with products manufactured from species imported from other countries.
Maximum spans are computed using standard engineering procedures for simple spans with uniformly distributed loads. Some building codes permit reductions of certain design loads for large tributary areas and steep-pitched roofs. These reductions may vary from code to code and are not considered in the computations for spans presented in WWPA publications. Allowable roof snow load rafter spans are based on design snow loads. Ground snow loads must be converted to design snow loads for roofs in accordance with appropriate building code provisions before using the tables.