![]() ![]() To calculate the 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 span values (displayed above) are from the 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 Angle Overhang : Rafter calculator angle dimension diagram. See Rafter Run to Outer Wall Calculator below. eg: Half building width minus 1/2 ridge thickness. Allow for Ridge Thickness when determining Rafter Run to Outer Wall. The following span table uses a moderate snow load of 50 Psf, but yours could be more or less. Calculate all rafter dimensions and cut angles to frame a roof - Inch. Consult your local building code authority to determine the snow load in your area. Performs calculations for ALL species and grades of commercially available softwood and hardwood lumber as found in the NDS 2018 Supplement. Note: Snow load factors can be specific to the regional location of a structure. Span Options Calculator for Wood Joists and Rafters. If you want to learn more about the way it's calculated, check out our roof pitch calculator. You can choose the rise of the roof (which is its height) or the roof pitch (which is the slope created by the rafter). Rafter Span Tables Rafters with ceiling not attached to rafters, live load = 20 Psf, dead load = 20 Psf, deflection limit L/180 If you picked rafter length: Start by deciding what you want the calculations to be based on. The braces need to be supported by a bearing wall, shown in the diagram above.Įxample: In the rafter span table below, the highlighted cell (13-0) indicates that a 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. When calculating the maximum span of a rafter, use the horizontal distance between two vertical supports.
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