![]() With the purlin/bracing installed, the long rafter span would be broken into two shorter spans, thus allowing the rafter to carry additional load.īefore you decide on lumber sizing and span, consult your local building code officials to determine design loads for your area. You can break up a long span of rafters by installing a purlin and bracing on the underside. When calculating the span of joists or rafters, use the horizontal distance between vertical supports. The number 1 grade material is more expensive than the #2 grade, but if you can use fewer or smaller number 1 grade joists or rafters in your design, you may actually save money.ĭepending on the availability and cost of various size framing materials, it may be more economical to use larger joists or rafters in your design because you can typically use fewer of them. For example, you can span a #1 grade (no knots) southern yellow pine joist a greater distance than a #2 grade of the same species (with knots). The lengths and sizes of joists vary depending on the species and quality of the lumber used. Under no circumstances should there be a join unsupported. For a member to be in continuous span the smallest span must be greater than 80 of the other span/s or equal to them. Rafter ties or ceiling joists support the rafters and sidewalls under gravity loads, including the dead load of the roof structure and live loads such as snow (above drawing). To calculate maximum spans of additional species of lumber, use the Span Calculator or the Span Tables for Joists and Rafters on the American Wood Council website.Ĭonsiderations when using the Rafter and Joist Span Tables SPANS The tables in this publication refer to three types of spans. Collar ties (or ridge straps) prevent the ridge from separating due to uplift pressure from wind (top drawing, right). Contact your local building code officials to determine the building code for your area. Weigh all of the elements when designing wood structures. Some areas have wind and seismic conditions. For example, some localities get large amounts of snow, whereas others get none. 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.Before determining the maximum spans for joists and rafters, you must know the load requirements for your region. 1) Refer to general notes for information that is relevant for all span tables. 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 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. ![]() Uniform load tables for APA-trademarked structural panels manufactured/qualified in accordance with APA PRP-108. Note: Snow load factors can be specific to the regional location of a structure. Span tables and design values for joists and rafters in one- and two-family dwellings. These span tables make no additional allowance for water tanks or any other additional loads. These span tables are based on the imposed loads given in BS 5268-7.1 and BS 6399-1, with an imposed uniformly distributed load of 0.25kN/m² acting together with a concentrated (point) load of 0.9 kN. ![]() Rafter Span Tables Rafters with ceiling not attached to rafters, live load = 20 Psf, dead load = 20 Psf, deflection limit L/180 Joists are to have minimum end bearing of 40mm. ![]() 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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