The pitch (the angle at which the rafter is rising) The rise (distance the rafter will rise above the plate) There are three important components that make up that triangle In the rafter example that corner is the corner where the rise and the run meet. When calculating the length of a rafter you are actually finding the length of the hypotenuse (long, slope side) of a right triangle.Ī right triangle is any triangle that has one corner that is exactly 90 degrees. I don't know if this helps, but I figure it's was worth a shot. Then you just multiply that x your run dim to get your slope/rafter dim.įor me it's easier to visualize as ratios instead of trig values. If using a basic math calculator, you just do somthing like this: It is also equal to:ġ/cosine of the slope angle. It's the hypotenuse divided by the adjacent side (or in carpenter's use it's the run). Now you just divide the slope dimension by the run to get your multiplier.ġ.20 is then multiplied x the needed run, after taking out your ridge. Using Dave's example of 8/12 pitch or slope, to get multiplier, you get: It usually helps to visualize if you draw out two triangles with the knowns, & unknowns 1st. Use your known pitch/slope to get that value. The multiplier wil just be the slope divided by the run. All you need is the Pythagorean theorum to get multiplier. If you have the roof pitch in rise/run, you can get a multiplier to get the slope dimension. Dave k's post decribes it well, but it might help to break it down to smaller packets of info.īasically, it's all about ratios.
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