![]() ![]() These factors include the building span, the timber grade used to make the rafter, and the load the roof carries. Knowing the size of rafters suitable for your flat roof depends on certain factors. 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. There are four main rafter sizes which are 2×4, 2×6, 2×10, and 2×12. ![]() To calculate maximum joist spans using different design criteria (loading, spacing, grade, etc.) for these common lumber species, see the International Residential Code (IRC). The above spans are from the American Softwood Lumber standard sizes. Uninhabitable attics with limited storage, a live load of 20 Psf, a dead load of 10 Psf, and a deflection limit of L/240 For this, use the tables in Design Values for Joists and Rafters. hese span tables for joists and rafters are calculated T on the basis of a series of modulus of elasticity (E) and bending design values (F b). ** Span is greater than 26 feet in length. Ceiling Joist Span Tables Uninhabitable attics without storage, a live load of 10 Psf, a dead load of 5 Psf, and a deflection limit of L/240 Based on the following design requirements: an uninhabitable attic with limited storage has a live load of 20 Psf, a dead load of 10 Psf, and a deflection limit of L/240 (span in inches divided by 240). Example: 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. Example: The highlighted cell (below) shows that a 2" x 6" Douglas Fir joist, with a grade of #2, spaced 24" apart, can have a maximum span of 10 feet - 8 inches (10-8). ![]()
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