![]() Builders can quickly choose a standard rafter size from tables that works for the building width. The tables indicate the maximum distance the rafter can safely span before additional support is needed. Let's look again at the drawing above and review some definitions of basic rafter and roof framing terms.Table R802.5.1(1) – Allowable rafter spans for common lumber species (ft-in)Įxcerpt of Rafter Span Table from 2018 IRC (Credit: Woodbin) Roof slope also as inches of rise per foot of horizontal run. The span tables have NOT considered the 1.4kN maintenance loading and propping and crawl boards are recommended when accessing the roof. The span of each rafter shall be measured along the horizontal projection of the rafter. For other grades and species and for other loading conditions, refer to the AWC STJR. 40ft x 12 480 ÷ 24 on-center 20 rafters + 1 21 rafters per side x 2 42 rafters. You’ll get the number of rafters needed for your roof. After getting the answer, add 1 for the ‘0’ rafter and multiply it by 2. Live Loads: 0.25kPa (non trafficable roof). Spans for rafters shall be in accordance with Tables R802.5.1(1) through R802.5.1(8). 40ft x 12 480 (length in inches) ÷ 24 on-center 20 rafters. Length in inches of the rafter type for the corresponding particular Rafters Table 16 22 Ridge & Intermediate Beams Table 17 23 Ceiling Joists Table 18 24 Attached Awnings & Verandahs. Inch number along the blade's upper edge. To the right of each of these rafter type line names are columns of numbers under each SIDE CUT OF HIP RAFTER OR VALLEY RAFTER. ![]()
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