in inches per foot run The Inch Rise gives you the corresponding scale number to use on the square and is calculated as follows. For example, you've measured out a distance of 3 miles (run) with a change in altitude of 396 feet (rise).  The table below shows common roof pitches and the equivalent grade, degrees, and radians for each. Another method to find the roof pitch is to go into the attic and measure the rise for a 12″ run of the roof rafters, which allows finding the pitch without going on the roof. We also cover more about roof replacement cost in detail in our cost guide. For easier readability, numbers between .001 and 1,000 will A 50 ft reel is 2.8 inches in diameter. Set the level on the edge of the speed square as shown below then place the heel of the speed square on a rafter or gable edge of the roof.