To calculate maximum spans of species not shown above, use the Span Calculator or the Span Tables for Joists and Rafters on the American Wood Council website. To calculate maximum rafter spans using different design criteria (load, snow load, spacing, grade, etc.), for these common lumber species, see the International Residential Code (IRC). The above spans are based on American Softwood Lumber standard sizes. Rafters with ceiling not attached to rafters, ground snow load = 50 psf, dead load = 20 psf, deflection limit L/180 The following span table uses a moderate snow load of 50 psf, but yours could be more or less. Consult your local building code authority to determine the snow load in your area. Note: Snow load can be very specific to the actual location of a structure. Rafter Span Tables Rafters with ceiling not attached to rafters, live load = 20 psf, dead load = 20 psf, deflection limit L/180 The braces need to be supported by a bearing wall, as shown in the diagram above.Įxample: In the rafter span table below, the highlighted cell (13-0), indicates that 2" x 8" Douglas Fir rafter, with a grade of #2, spaced 24" apart, can have a maximum span of 13 feet - 0 inches (13-0) if designed for a live load of 20 psf, and dead load of 20 psf. Note also that you can break up the span of a rafter by adding a purlin and bracing to the underside. There are rules of thumb for joist and rafter span, but they depend on the joist/rafter spacing, the wood species, and the live and dead loads. When calculating the maximum span of a rafter, use the horizontal distance between two vertical supports.
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