![]() ![]() If you have this figure, you can use it plus the run, to get the rafter length. Pitch can be used to help calculate the rise. Table showing the angle in degrees for various roof pitches. You can also find the angle for your roof pitch in the table below. The angle in degrees is equal to the inverse tangent of the pitch of the roof. But, in order to calculate the rafter length, we need to calculate the pitch in degrees. With a scientific calculator (calc set to degrees):Ĭonvert roof pitch to degrees- Degree measurement= inverse tangent of (rise/run)ĭiagonal measurement = Span/cosine of roof pitch in degreesĮnter all lengths in inches for easy mathĮnter pitch in degrees or inches, and press "pitch"Ģ0 "feet" - 1.5 "inch"/ 2= 9 feet 11 1/4 inchĢ0 "feet" - 1.The roof pitch is the angle of the roof and can be measured in several ways, but is most commonly expressed in rise over a standard 12-inch run. Total rise= (rise/run)*Span (measures from bottom of birds mouth plumb cut to bottom of ridge plumb cut)ĭiagonal measurement=Square root of (span^2+total rise^2) Span= (total roof width-ridge thickness)/2 ![]() With a regular basic function calculator (should work with most basic cell phone calculators): Rule of thumb if the side of the triangle you know falls in the denominator divide by the appropriate trig function if in the numerator multiply by the appropriate trig function.įor a simple gable (example 5/12" pitch, 20' outside of wall to outside of wall, 1 1/2" thick ridge). Lets say we have a 30° rafter with a run of 60"ġ)Identify what trig function to use based on the picture above and what we know about our roof,in this case we need to use the cosine function.Ģ)Make cos(30) a fraction by putting a 1 under it turning it into a improper fraction.ģ)Use the rules of proportion (cross multiply and divide) to solve for the length of our rafter.Ĥ)Cross Multiply-60*1=60 (I know its silly to include multiplying by 1 but its just for those who may not understand proportion) What confuses a lot of people about trig is that they don't understand when to multiply and divide. Its just basic right angle trigonometry,you would need to use the cosine function to find the length of a rafter knowing only the angle of the roof and the span of the rafter. ![]() :thumbup:īasic high school math is a very powerful tool.:thumbsup: Even a cabinetmaker uses this on a nearly daily basis. Once that concept takes root, you can figure out the most complex layout equations that are common to the construction industry. The beauty of the construction calculators is that you only need two of those three components to find the length of that rafter. The rise and run are the two short legs of that triangle and the pitch is the angle the third side is bisecting those two legs. 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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