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). Example: In the rafter span table below, the highlighted cell (13-0), pointing is 2' x 8' Douglas Tannenbaum 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. The span values (displayed above) are from the American Softwood Lumber standard sizes. Tutorial for Understanding Loads and Using Span Tables. 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. In making your choice, consider how you will insulate your insulate your roof. Another option are 2 X 8 rafters spaced 24 inches o.c. The table states that 2 X 6 rafters spaced 16 inches on center (o.c) can span a maximum distance of 13 feet 5 inches. Consult your local building code authority to determine the snow load in your area. Looking through the center of the table, we can find a couple of options. Note: Snow load factors can be specific to the regional location of a structure. Note: The 140 × 70 mm rafter size permitted to span 2.9 m could also have been used (since 2.9 × 1.1 multipler 3.19 m), but this is not a stock size so was not chosen. This indicates a 190 × 45 mm rafter size. Rafter Span Tables Rafters with ceiling not attached to rafters, live load = 20 Psf, dead load = 20 Psf, deflection limit L/180 Read the rafter size in the left column of Table 10.1 for the 3.3 m span chosen. 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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