[Cherryville Chuck]

If you are using a chop saw to cut the angles then you need the angle which as Lee said is 18.4 degrees. But if you aren't using a chop saw to make the cuts then the easiest way to do it is by using a framing square and a set of stair gauges. You set the square on the edge of your material so that the 4" mark on the short leg is lined up on the edge and the 12" mark on the long leg is lined up on it also. Once it is set like that you tighten the stair gauges onto the square and you are set to go. It will cut the correct angle board after board. They are called stair gauges because that's what they are the handiest for but they also cut rafter angles including what's called a bird's mouth notch where you cut out a notch that sits on the top of your wall. Marking along one leg will give you a vertical line once the rafter is hoisted into place and marking along the other leg will you a horizontal line. Here is a short vid that shows them in use but it wouldn't hurt to check a few out until you understand them. The math is actually really simple since every angle is constructed using a rise (y axis) and a run (x axis). That is what you are doing with the square. You are setting the buttons to match your rise and run.