What does roof pitch per foot mean?
Per foot of run, not per foot of rafter
The foot in pitch per foot is measured level, across the attic floor, not along the sloped roof surface. A 6/12 roof rises 6 inches over 12 inches of level distance, but the sloped surface over that same foot is about 13.4 inches long. Confusing the two makes measured pitches come out too low.
Using rise per foot
- Ridge height: rise per foot × run in feet. A 5/12 roof over a 15 foot run rises 75 inches, or 6 feet 3 inches.
- Finding pitch: measure ridge height in inches and divide by the run in feet. A ridge 84 inches above the plate over a 12 foot run is 7 inches per foot, a 7/12.
- Slope length per foot: 4/12 is about 12.6 inches of rafter per foot of run, 8/12 about 14.4 inches, and 12/12 about 17 inches.
Low slope roofs use smaller units
On porch roofs and flat roofs, slope is often given in fractions of an inch per foot, such as 1/4 inch per foot. That is a very different number from a 4/12 pitch: 1/4 inch per foot is barely enough to drain and is meant for membrane roofs, not shingles.
Why it matters on your house
Rise per foot decides what roofing is allowed. Asphalt shingles need at least 2 inches per foot, and below 4 inches per foot the code adds extra underlayment. Above about 7 or 8 inches per foot, crews add safety equipment, which shows up in labor. See architectural vs 3-tab shingles and how roofers measure a roof.