Roof pitch calculator

Roof pitch is the rise in inches over 12 inches of level run, written as x/12. Divide the rise by the run and multiply by 12. A roof that rises 6 inches over 12 inches of run is a 6/12 pitch, which is 26.6 degrees.

The angle is the arctangent of the rise divided by 12. A 4/12 pitch is 18.4 degrees, 6/12 is 26.6 degrees, 8/12 is 33.7 degrees and 12/12 is 45 degrees. The pitch chart under the calculator lists the common pitches.

From the attic, hold a level against a rafter, mark 12 inches along the level, and measure straight up from that mark to the underside of the rafter. That measurement is the rise. On a phone you can also choose Measure with phone and lay the phone on a rafter.

Multiply the roof footprint (including overhangs) by the pitch factor to get the sloped area, add 10 to 15 percent for waste, and divide by 100. One roofing square covers 100 square feet, and most three-tab and architectural shingles come three bundles to a square.

Asphalt shingles need a pitch of at least 2/12. Between 2/12 and 4/12, building codes require a double layer of underlayment. Flatter roofs need a low-slope system such as membrane or modified bitumen.