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. 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 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. BOXSPAN® Residential Span Tables Non Cyclonic up to N3 Wind Class Joists I Bearers I Rafters I Lintels I Roof beams Residential Guide.indd 1 4:59 pm. ![]() 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 typical on-center spacing provided in the International Residential Code (IRC) 2021 includes spacings of 12, 16, 19.2, and 24. The span values (displayed above) are from the American Softwood Lumber standard sizes. Rafter spacing is the distance between the centers of two consecutive parallel rafters. 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. 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. Under no circumstances should there be a join unsupported. Tutorial for Understanding Loads and Using Span Tables. ![]() 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. Consult your local building code authority to determine the snow load in your area. SPANS The tables in this publication refer to three types of spans. Note: Snow load factors can be specific to the regional location of a structure. 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. Additionally, compres-sion perpendicular to grain design values (F c ) are included as a consideration for selection of joists and rafters. Note also that you can break up the span of a rafter by adding a purlin and bracing to the underside. These span tables for joists and rafters are calcu-lated on the basis of a series of modulus of elasticity (E) and bending design values (F b). When calculating the maximum span of a rafter, use the horizontal distance between two vertical supports.
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