Roof Pitch & Rafter Calculator

Roof pitch describes steepness as inches of vertical rise per 12 inches of horizontal run. A 6/12 pitch rises 6 inches for every 12 inches travelled horizontally. The “12” is always the run, so only the first number changes — a 4/12 roof is shallow, a 12/12 roof rises at 45 degrees.

Hold a level horizontally against the roof slope. Measure 12 inches along the level from the end touching the roof, then measure straight down from that 12-inch mark to the roof surface. That vertical distance, in inches, is the pitch over 12. A 2-foot level works too — just halve the result.

Most shingled houses fall between 4/12 and 9/12. Below 3/12 is considered low-slope and usually needs membrane roofing rather than shingles. Above 9/12 is steep: harder and less safe to walk, and it needs more roofing material because the surface area grows with the slope.

A common rafter is the diagonal of the run and the rise. Its body length is the square root of run squared plus rise squared. For a 14-foot run and 7-foot rise that is the square root of (196 + 49) ≈ 15.65 feet. Any eave overhang adds extra length along the same slope.

The slope factor converts a flat (plan) area into actual roof-surface area. It is the rafter length divided by the run — the square root of (pitch squared + 144), divided by 12. A 6/12 roof has a slope factor of about 1.118, so the roof surface is roughly 12 percent larger than the footprint it covers.

Pitch is a ratio (rise over 12); angle is that ratio expressed in degrees. The angle is the arctangent of pitch divided by 12. A 6/12 pitch is about 26.6 degrees, an 8/12 is about 33.7 degrees, and a 12/12 pitch is exactly 45 degrees.

Yes. The eave overhang you enter is measured as a horizontal projection. The calculator converts it along the roof slope using the slope factor and adds it to the rafter body length, so the rafter figure is the full length including the overhang. Order stock slightly longer for the plumb and seat cuts.