3/20/2024 0 Comments Flat roof rafter span calculatorTo 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. Permissible Clear Spans for Rafters (to BS 5268-7. Rafters with ceiling not attached to rafters, ground snow load = 50 Psf, dead load = 20 Psf, deflection limit L/180 Visit the roof framing page for more information on cutting roof rafters, and visit the roof pitch calculator for determining rafter lengths based on rise and run. 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. Measure the span by measuring the width of the building using a tape measure, then add the overhang on each side to the measurement. You’ll also need to account for the overhang beyond the building on either side. Note: Snow load factors can be specific to the regional location of a structure. The roof’s span is the total length of the roof. Span Tables for Flat Roof Joists (to BS 5268-7.2) Imposed loads of 0.75 kN/m² for maintenance and snow load are applicable where there is no permanent access (no fixed ladder or staircase) and most areas where the altitude does not exceed 100 metres (refer to BS 6399-3) Imposed loads of 1 kN/m² for maintenance and snow load are applicable. 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, 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. Resources provide a simplified system to determine allowable joist and rafter spans in one- and two-family dwellings, and the companion supplements for tabulating allowable bending and modulus of elasticity design values for visually graded and mechanically graded dimension lumber. 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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