![]() ![]() ![]() 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. This article explains the basic concepts and requirements of rafter and ceiling joist framing, including span tables, design values, and connections. 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 rafter and ceiling joist framing is a critical part of any roof system, as it supports the roof loads and transfers them to the walls and foundation. 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 ** 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). (the tables permit a bearing length of up to 3.5 inches, but since 1.5 is probably the worst case that you’ll encounter for joist or rafter bearing, it’s a safe value.) You can get. Span Tables for Rafters (to BS 5268-7.5) Imposed loads of 0.75 kN/m² snow load are applicable to most areas where the altitude does not exceed 100 metres (refer to BS 6399-3) Imposed loads of 1 kN/m² snow load are applicable to most areas where the altitude exceeds 100 metres but does not exceed 200 metres, but excluding parts of Scotland. 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). Table 9.1 in Span Tables for Joists and Rafters (Figure 5) gives a required compression value of 237 psi for a span of 16 feet and bearing length of 1.5 inches. ![]()
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