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 In a 6-inch snowfall, an inch of snow has a design density of 1.25 psf per inch, and a real-world density closer to 1 psf per inch in 48-inch-deep snow, the. Contact your local building code officials to determine the building code for your area. If your code book says your snow load is 40 psf, then you must use the 40 psf live load rafter table. Weigh all of the elements when designing wood structures. Sample Maximum Roof Beam Span Table (feet-inches) Spruce, Pine or Fir, No.1 or No.2 with 31. The table below shows built up beams for 31.3 psf of snow loading. Some areas have wind and seismic conditions. Once again, you will need to look at your local charts and use your local snow load factor to determine the maximum span for a given wood rafter size. This application requires Javascript and WebAssembly to be enabled. Span tables for visual and mechanical grades of southern pine lumber joists and rafters, and southern pine lumber and glue-laminated timber (glulam) headers, beams and girders. ![]() ![]() For example, some localities get large amounts of snow, whereas others get none. Read the Tutorial for Understanding Loads and Using Span Tables. Uniform load tables for APA-trademarked structural panels manufactured/qualified in accordance with APA PRP-108. Before determining the maximum spans for joists and rafters, you must know the load requirements for your region. (Roof Live Load 20 pounds per square foot, Ceiling Not Attached to Rafters L/ 240).
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