Roof Pitch Calculator

It means the roof rises 6 inches vertically for every 12 inches of horizontal run. In the US the second number is essentially always 12, which is why you hear 4/12, 6/12, 9/12. A 6/12 roof works out to 26.6 degrees, a 50% slope, and a pitch multiplier of 1.1180 — so the sloped surface is about 11.8% larger than the footprint underneath it. Roofers say "six-twelve." The same roof is sometimes written as a 1:2 ratio or 26.6° outside the US, but the ratio and the angle describe exactly the same plane.

From inside the attic, hold a level against the underside of a rafter, mark 12 inches out along the level, then measure straight down from that mark to the rafter — that measurement in inches is your rise. From outside, a magnetic pitch gauge or an angle-finder app laid on a rake board reads the degrees, which you can convert above. Check two or three rafters rather than one: older framing sags, and additions are frequently framed at a different pitch than the main house.

Two separate ways. Area first: a 2,000 sq ft footprint becomes about 2,028 sq ft at 2/12, 2,236 sq ft at 6/12, and 2,828 sq ft at 12/12 — that is 41% more shingles, underlayment, and nails on the steep roof. Labor second: above roughly 7/12 a crew needs roof jacks, staging, or fall-arrest gear, and steep-slope work commonly adds 25–50% to the labor line. Pitch also decides which materials are permitted at all, which matters far more than the area math on a low roof.

Up to about 6/12 most roofers will walk the roof in soft-soled boots. From 7/12 (30.3 degrees) upward it is classified as steep-slope and normally requires roof jacks, planking, or fall-arrest gear — 8/12 is 33.7 degrees and 12/12 is a full 45. Wet, mossy, or frosted shingles drop the safe threshold several points, and worn shingles lose the granules that give traction. If you are unsure, take the measurement from the ground or the attic, price the job here, and let a contractor do the walking.

Asphalt shingles are permitted down to 2/12, but only with a doubled underlayment — two layers lapped 19 inches — and usually a self-adhered ice-and-water barrier at eaves and valleys. Below 2/12 shingles are not allowed at all; you need a membrane such as TPO, EPDM, or modified bitumen. 4/12 and above is the normal range where a single layer of underlayment is accepted. Treat these as planning figures: individual shingle warranties spell out their own low-slope requirements, and your local code amendments and inspector govern.