How do you calculate roof pitch?

Roof pitch is the slope expressed as rise over run, in inches of vertical rise per 12 inches of horizontal distance. A "6:12" pitch rises 6 inches for every 12 inches you move horizontally. To measure it, hold a level horizontally against the roof, mark 12 inches along the level, then measure straight down from that mark to the roof surface; that measurement is the rise.

You can also measure from inside the attic against a rafter, which avoids getting on the roof. To convert pitch to degrees, take the arctangent of rise divided by run (for example 6/12 is about 26.6 degrees).

Pitch drives material choice and cost: low slopes need sealed or membrane systems, and steep slopes cost more to install because the crew works slower and needs fall protection.