With the purlin/bracing installed, the long rafter span would be broken up into two shorter spans, thus allowing the rafter to carry additional load.īefore you make your final decision on lumber sizing and span, consult your local building code officials to determine design loads for your area. 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 to vertical supports. The #1 grade material is more expensive than the #2 grade, but if you can use fewer or smaller #1 grade joists or rafters in your design, youĭepending on the availability and cost of various size framing lumber, it might be more economical to use a larger size joist or rafter 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 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 Beam Width can be either a single piece of LVL or built up from individual plies of LVL that are nailed and/or bolted together. Deeper beams or an additional ply will increase beam stiffness and reduce deflection. Contact your local building code officials to determine the building code for your area. A deflection criteria of L/240 indicates the maximum deflection allowed for a 10'-0' span beam is 10 x 12 / 240 1/2'. Some other regions have special wind and seizmic requirements that should be considered when designing wood structures. For example, some regions get large amounts of snow, whereas others do not get any. ![]() ![]() ![]() Before you can determine the maximum spans for joists and rafters, you must know the load requirements for your given area.
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