What a 15/12 pitch means
A 15/12 roof rises 15 inches for every 12 inches it runs horizontally, which is an angle of 51.34° from level and a 125% slope. It is a steep roof, too steep to walk safely without roof jacks or a harness.
Roofing for a 15/12 roof
Suits shingles, metal, slate and tile. Steep enough that roofers need roof jacks or harnesses, which adds to labour cost.
Rise and rafter length at 15/12
For a gable roof, the run is half the building width. Rafter lengths are before the ridge deduction and overhang.
| Run | Building width | Rise | Rafter length |
|---|---|---|---|
| 4 ft | 8 ft | 5' | 6' 4 13/16" |
| 6 ft | 12 ft | 7' 6" | 9' 7 1/4" |
| 8 ft | 16 ft | 10' | 12' 9 11/16" |
| 10 ft | 20 ft | 12' 6" | 16' 1/16" |
| 12 ft | 24 ft | 15' | 19' 2 1/2" |
| 14 ft | 28 ft | 17' 6" | 22' 4 15/16" |
| 16 ft | 32 ft | 20' | 25' 7 3/8" |
| 20 ft | 40 ft | 25' | 32' 3/16" |
Roof area at 15/12
Multiply the building footprint (including the overhangs) by 1.601 to get the roof surface. A 30 × 40 ft footprint has 1921 sq ft of roof at 15/12 — about 61 sheets of 4 × 8 sheathing before waste.