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 Figuring out Rafter Lengths Here's the big question, how do I figure out roof rafter lengths? If you are building a simple gable roof, this will not be that difficult. When I say it won't be that difficult, I understand it is not that difficult for me because I have been doing it for so many years. I will be able to explain this to you, as simply as it was explained to me. Remember this is a formula for figuring out simple roof rafters and could be a little bit more complicated if you are working on a building that is a polygon or four sided structure.   We will start with a simple formula: A squared plus B squared equals C squared. There it's that simple, well maybe I'll give you a little more detail. Now for example: side C is going to be the roof rafter length that we're looking for. Now in order to figure that out we are going to need the length of side A and side B. Let's say that side A is 10 feet and side B is 4 feet. We can't simply find sites C using this simple arithmetic problem. 10 x 10 + 4 x 4 = C x C Side A Side B Side C 10 x 10 equals 100 4 x 4 equal 16..... we now had 100 plus 16 equals 116 Now that we have 116, we need to find the square root of that number. On your calculator you should have a symbol that looks like this Square Root Symbol   Simply enter 116 into your calculator and push the square or symbol, this should give you 10.770 which will give you your rafter length and feet. Your rafter length will be 10 feet .770 feet long or in other words we need to convert .770 to inches. Simply enter .77 zero and dear calculator and multiply it by 12. The number 12 represents how many inches are in the foot. The number .770 is the decimal, that is left over in our original calculation. .770 x 12 = 9.24 or 9 and 1/4 inches. Now we know that side C will be 10 feet 9 1/4" long. There are other ways to figure out the roof rafter lengths and I will go into a little more detail throughout the site.