What a 9/12 pitch means
A 9/12 roof rises 9 inches for every 12 inches it runs horizontally, which is an angle of 36.87° from level and a 75% slope. It is a steep roof, too steep to walk safely without roof jacks or a harness.
Roofing for a 9/12 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 9/12
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 | 3' | 5' |
| 6 ft | 12 ft | 4' 6" | 7' 6" |
| 8 ft | 16 ft | 6' | 10' |
| 10 ft | 20 ft | 7' 6" | 12' 6" |
| 12 ft | 24 ft | 9' | 15' |
| 14 ft | 28 ft | 10' 6" | 17' 6" |
| 16 ft | 32 ft | 12' | 20' |
| 20 ft | 40 ft | 15' | 25' |
Roof area at 9/12
Multiply the building footprint (including the overhangs) by 1.25 to get the roof surface. A 30 × 40 ft footprint has 1500 sq ft of roof at 9/12 — about 47 sheets of 4 × 8 sheathing before waste.