The gable roof pitch is the term used to describe the angle between the horizontal of the ceiling and wall line and the diagonal angle of the roof joists. Gable roof pitch can also be given as ration, described as the rise over the run. Below you will be given short descriptions of how to calculate these two values.
Step 1: Rise Over Run
When calculating rise over run, you will need to measure from the top of the wall to the roof peak. Then you will need to measure from the roof peak to the end of the roof gable. Once you have these two measurements, you simply need to divide the rise, or the vertical length, by the run, or the diagonal length.
Step 2: Calculate the Angle
In order to determine your gable roof pitch angle, you will still need to take measurements. The first measurement will be the vertical from the ceiling/wall line to the roof peak. Then you need to decide between measuring the length of the roof from peak to edge of rafter or measuring the wall length from the centerline to the edge of the wall.
Once you have these measurements, you need to apply the geometry to find the decimal result of dividing either the base (adjacent) by the roof line (hypotenuse) or the vertical (opposite) by the roof line (hypotenuse). If using the vertical, you will need to either input the result into a calculator and hit alt/sine, or look it up on a table. If you use the wall length over the roof line, you will need to look that result up under cosine in the table.