![]() You can break up a long span of rafters by installing a purlin and bracing on the underside. When calculating the span of joists or rafters, use the horizontal distance between vertical supports. The number 1 grade material is more expensive than the #2 grade, but if you can use fewer or smaller number 1 grade joists or rafters in your design, you may actually save money.ĭepending on the availability and cost of various size framing materials, it may be more economical to use larger joists or rafters in your design because you can typically use fewer of them. For example, you can span a #1 grade (no knots) southern yellow pine joist a greater distance than a #2 grade of the same species (with knots). The lengths and sizes of joists vary depending on the species and quality of the lumber used. To calculate maximum spans of additional species of lumber, use the Span Calculator or the Span Tables for Joists and Rafters on the American Wood Council website.Ĭonsiderations when using the Rafter and Joist Span Tables Contact your local building code officials to determine the building code for your area. Weigh all of the elements when designing wood structures. Some areas have wind and seismic conditions. For example, some localities get large amounts of snow, whereas others get none. Those using this Span Calculator assume all liability from its use.Before determining the maximum spans for joists and rafters, you must know the load requirements for your region. While every effort has been made to insure the accuracy of the information presented, and special effort has been made to assure that the information reflects the state-of-the-art, neither the American Wood Council nor its members assume any responsibility for any particular design prepared from this Span Calculator application. Buildings in these high snow zones should be designed in accordance with accepted engineering practice. Buildings in regions with ground snow loads greater than 70 psf should not use the prescriptive provisions of the governing building code without approval of the authority having jurisdiction.The Span Calculator does not address wind load or seismic load. Joists and rafters carry only combinations of dead load plus live load (D + L), dead load plus snow load (D + S), or dead load plus roof live load (D + Lr).Consult the local building code for specific requirements. These minimum bearing lengths are to be used in the absence of calculated bearing lengths. For joists and rafters, most building codes specify a minimum bearing length of 1.5 inches on wood or metal and not less than 3 inches on masonry or concrete. ![]() The Span Calculator does not address cantilevered joists and rafters, multiple-span joists and rafters, or spliced joists and rafters.
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