calcs.homes
calcs.homes
Enter a roof's rise and run and this roof pitch calculator returns the pitch as a rise-per-12 ratio, the angle in degrees, the slope as a percent, and the slope factor that converts flat area to actual roof area.
Estimate only — measure rise over a level 12 in (or 30 cm) run held against a rafter for the truest reading.
Roof pitch is the ratio of vertical rise to horizontal run, written as a rise per 12 units of run — a “6:12” roof climbs 6 inches for every 12 inches across. We divide rise by run, multiply by 12 for the x-in-12 figure, take the arctangent for the angle in degrees, and express rise ÷ run as a percentage.
The slope factor is √(1 + (rise÷run)²). Multiply a roof's flat footprint by this factor to get the true surface area you have to cover — a 6:12 roof has about 12% more surface than its footprint, and a 12:12 roof about 41% more. That is why steeper roofs need more shingles and cost more to replace on the same size house.
A 6/12 pitch rises 6 inches for every 12 inches of horizontal run — an angle of about 26.6°. It is the most common residential slope and is still walkable for most roofers.
Hold a level horizontally against the underside of a rafter, mark 12 inches along it, then measure straight down from that mark to the rafter. That drop is your rise over a 12-inch run.
Anything below about 2:12 is low-slope and needs a membrane roof rather than shingles, because shingles rely on gravity to shed water and can leak at shallow angles.