![]() 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. This chapter also contains prescriptive requirements for wood-framed exterior decks and their attachment to the main building. When calculating the span of joists or rafters, use the horizontal distance between vertical supports. Allowable span tables are provided that greatly simplify the determination of joist, girder and sheathing sizes for raised floor systems of wood framing and cold-formed steel framing. 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. It is very important to select the correct table. According to Section R802.4.1, roof rafters are to be sized based on Tables R802.4.1(1) through R802.4.1(8). 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). As mentioned above, the span tables within the code list the allowable roof rafter spans for common lumber sizes based on what design load scenario is applied. The lengths and sizes of joists vary depending on the species and quality of the lumber used. The span of each rafter shall be measured along the horizontal projection of the rafter. This chapter covers wood floor framing, wood floors on the ground, cold-formed steel floor framing and concrete slabs on the ground. For other grades and species and for other loading conditions, refer to the AF&PA Span Tables for Joists and Rafters. Calculations Referenced to ICC 2003 Tables R802.5.1(3) & R802.5.1(5) Title: RAFTER SPAN TABLE Author: Donald R. 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 for rafters shall be in accordance with Tables R802.5.1(1) through R802.5.1(8). 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. Before determining the maximum spans for joists and rafters, you must know the load requirements for your region.
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