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2x6 rafter span calculator3/21/2024 This is the easiest way to determine allowable dead loads, live loads and deflection limits. The size of roof rafters depends on various factors such as the span, load, and spacing. The maximum distance a 2×6 can span, according to the 2018 IRC, for a floor joist, is 12’-6, ceiling joist 20’-8, rafter 18’-0, deck board 24, deck joist 9’-11, deck beam 8’-3, and 6’-1 as for header. 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. You can also use AWC’s Span Tables for Joists and Rafters. A greater distance between two rafters allows them to span a shorter length. 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). Once the rafter spacing is determined, the number of total rafters and their span can be calculated. The span values (displayed above) are from the American Softwood Lumber standard sizes. Click for STRUCTURAL DESIGN CALCULATIONS. Surveyors and structural engineers utilise data from tables below to help calculate the correct size, strength and centres of roof timbers for the required spans and loadings. The estimated dead load from the insulation, roof shingles and sheathing, drywall ceiling, etc is 10-11 psf. I looked up the snow load for my town in MA and it’s 50 psf for ground snow load and 30 psf for min flat roof snow load. Rafters with ceiling not attached to rafters, ground snow load = 50 Psf, dead load = 20 Psf, deflection limit L/180 Surveyors’ span tables for designing roof rafters for pitched roofs, flat roofs and ceiling joists. The roof is 7/12 pitch, horizontal span just under 12, 2×6 douglas fir rafters a bit over 16 O.C. Note: The 140 × 70 mm rafter size permitted to span 2.9 m could also have been used (since 2.9 × 1.1 multipler 3.19 m), but this is not a stock size so was not chosen. This indicates a 190 × 45 mm rafter size. If you add an eaves overhang dimension, then the calculator will add the amount the rafter sticks passed the wall to the rafter length. The calculation includes results for hip/valley factor, slope factor and the roof slope in degrees. The following span table uses a moderate snow load of 50 Psf, but yours could be more or less. Read the rafter size in the left column of Table 10.1 for the 3.3 m span chosen. Calculate the length of a rafter from the roof slope ratio of inches per foot and a building width measurement. Consult your local building code authority to determine the snow load in your area. Permissible Clear Spans for Rafters (to BS 5268-7. 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. 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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