Roof Pitch Calculator

Enter rise and run in the same unit. Get the pitch, angle, grade and slope factor.

Worked example

Rise 15 cm over a run of 30 cm. Ratio = 15 / 30 = 0.5. Pitch = 0.5 x 12 = 6/12. Angle = arctan(0.5) = 26.6°. Grade = 50%. Rafter factor = sqrt(1 + 0.5²) = 1.118, so a rafter spanning 30 cm of run is 30 x 1.118 = 33.5 cm along the slope, before overhang and ridge adjustments.

How it works

Pitch (x/12) = rise / run x 12. Angle = arctan(rise / run). Grade percent = rise / run x 100. The rafter length factor (slope factor) = sqrt(1 + (rise / run)²). Multiply any horizontal run by the factor to get the true sloped length, which is also how sloped roof area is found from the flat footprint area.

FAQ

What pitch do most Canadian houses have?

Most residential roofs fall between 4/12 and 8/12. Steeper pitches shed snow better but cost more to build and to re-shingle.

Why does the rafter length not match my measurement?

The factor gives the pitch-line length only. Real rafters add the eave overhang, the ridge board deduction, and the birdsmouth cut, so order stock longer and cut to fit.

Estimates only. This calculator gives planning estimates, not professional advice. Verify measurements before cutting materials.