![]() ![]() If you want to add on a rafter foot (overhang) add this to your rafter length. Multiply this factor by your run which gives your rafter length. Just multiply the rise or run by the appropriate rafter length factor corresponding to your roof pitch.įor example, if your roof pitch is 30ﹾ and your run is 2.4m then reading from the table the rafter length factor is 1.15. There is a useful table below for your convenience. Rafter length = 2.4 ÷ cos(45°) = 3.39m to 2 d.p.Īny modern mobile phone will be able to perform these calculations. How do you calculate rafter size Rafter size is determined by factors like span, roof pitch, snow load, and the type of wood used. Rafter length = Run ÷ cos θ = Rise ÷ sin θįor example. These span tables are the equivalent of other industry-standard span tables and are calculated to the same standards. These spans were calculated using our rafter calculation engine. This span table is to the format and methodology given in BS 5268-7.5. Calculator generates the maximum span distance as well as the bending strength (F b ), shear strength (F v) and bearing strength (F cp) values for each Stamp selected. Click on a span to generate a full calculation (you can adjust the exact span and slope). Enter lumber size, type of use, and span requirements in the Design Criteria section. Using a bit of trigonometry you can find the rafter length using the following equations. Repeat until desired number of Stamps have been added. In other words, if there is a 1 1/2 ridge board nailed into the wall, and the width of the lean to is 96, then the run will be 94 1/2 ( 96 1 1/2 94 1/2). Method 2 - You know the roof pitch, θ, and the rise or the run. The run for a lean to shed rafter ending into a wall should account for the ridge board nailed into the wall where the tops of the rafters rest into. The eagle-eyed will have spotted this is another example of a 3-4-5 triangle. ![]()
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