How Do I Calculate Roof Pitch? (And Rafter Length)

Roof pitch is measured three interchangeable ways — rise-in-12, degrees and percent — and the same slope also gives you rafter lengths. Learn to convert between the three and read a pitch off an existing roof, and the rest is trigonometry the calculator handles.

Three ways to say the same slope

Rise-in-12 — the US standard — is written x:12: a 4:12 pitch means 4 inches of rise for every 12 inches of run. Degrees is the angle from horizontal; percent is rise ÷ run × 100.

Measuring an existing roof

Hold a level horizontally against the rafter, measure 12 in along it, then measure straight down to the roof surface — that vertical figure in inches is the rise, giving the pitch as rise:12. A 4 in drop over 12 in is a 4:12 pitch (18.4°). Enter the ratio and the calculator reads off the angle and percent.

Rafter length

Rafter = horizontal half-span ÷ cos(pitch angle). A building 24 ft wide (12 ft from wall plate to ridge) at a 4:12 pitch needs 12 ÷ cos 18.4°  ≈ 12.6 ft (3.85 m) rafters, and the ridge rises 12 × tan 18.4° ≈ 4 ft (1.22 m) above the plates. Add the eaves overhang and birdsmouth allowance on top.

Minimum pitch for the covering

It depends on the material: standing-seam and profiled metal commonly go down to about 1:12–2:12 (5–10°), asphalt shingles generally want 2:12 minimum (special underlayment below 4:12), and concrete tiles typically need 4:12–6:12 (17–26°). Below the covering’s minimum, wind-driven rain gets under the laps. Always use the manufacturer’s stated minimum for the exact product, not a generic figure.

Does steeper cost more?

Yes — rafters lengthen, the covering area grows (area = plan area ÷ cos  pitch), and scaffolding and labor rise with the pitch. A 12:12 roof (45°) has over 40% more surface than a flat one over the same plan. Against that, steep roofs shed water and dirt better, resist leaks and can add attic space.