![]() ![]() ![]() 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. Each common rafter span is calculated by subtracting the thickness of the ridge from the total span, then dividing the answer by two. 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. building width in feet and inches (outside sheating to outside sheating) overhang in inches (from outside sheating to sub-fascia) ridge board thickness in inches slope ratio, amount of rise per foot of run in inches Results. 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. Rafter length = Run ÷ cos θ = Rise ÷ sin θįor example. Using span tables to size joists and rafters is a straight-forward process when you understand the structural principles that govern their use. How to Calculate the Length of a Rafter Step One: Calculate the Roof Pitch The roof pitch is the angle of the roof and can be measured in several ways, but is. Using a bit of trigonometry you can find the rafter length using the following equations. Method 2 - You know the roof pitch, θ, and the rise or the run. The eagle-eyed will have spotted this is another example of a 3-4-5 triangle. ![]()
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