What is my roof pitch?

Roof pitch is the rise over a 12-inch run. Hold a level against the roof or a rafter, measure 12 inches along it from the point of contact, then measure straight down from that mark to the roof surface. If that drop is 6 inches, the pitch is 6/12. If you measured over a run other than 12 inches, divide the rise by the run and multiply by 12.

About 26.6°. The angle is the arctangent of rise ÷ run, so a 6/12 pitch is atan(6 ÷ 12) = 26.57° from horizontal. A 4/12 is 18.4°, a 9/12 is 36.9°, and a 12/12 is exactly 45°.

The pitch factor (or roof multiplier) converts the footprint your roof covers into the actual surface area you have to cover with material. It is the hypotenuse over the run: √(rise² + run²) ÷ run. A 6/12 roof has a factor of 1.118, so it has about 12% more surface than its footprint; a 12/12 has a factor of 1.414, about 41% more. Ignore it and you under-order material on every steep roof.

Multiply the horizontal run by the pitch factor. The factor is the hypotenuse of the pitch triangle over its run, so at 6/12 it is 1.118: a rafter spanning 12 ft of run measures 12 × 1.118 = 13.42 ft along the top edge. That is the theoretical line length — you still deduct half the ridge thickness, add the tail for the overhang, and cut the birdsmouth, none of which the geometry knows about.

They describe the same geometry in different units. Pitch is written as rise over a 12-inch run (6/12); slope is usually the same ratio as a percentage (6/12 = 50%). Architects and engineers tend to use degrees or percent, roofers use X/12.

As a general rule most asphalt shingle manufacturers require a low-slope application with additional underlayment between about 2/12 and 4/12, and do not permit shingles below roughly 2/12, where a membrane system is used instead. These are manufacturer requirements rather than a single universal code figure, so check the installation instructions for the specific product and the roofing provisions that apply locally before you quote.