What a 12/12 pitch means
A 12/12 roof rises 12 inches for every 12 inches it runs horizontally, which is an angle of 45° from level and a 100% slope. It is a steep roof, too steep to walk safely without roof jacks or a harness.
Roofing for a 12/12 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 12/12
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 | 4' | 5' 7 7/8" |
| 6 ft | 12 ft | 6' | 8' 5 13/16" |
| 8 ft | 16 ft | 8' | 11' 3 3/4" |
| 10 ft | 20 ft | 10' | 14' 1 11/16" |
| 12 ft | 24 ft | 12' | 16' 11 5/8" |
| 14 ft | 28 ft | 14' | 19' 9 9/16" |
| 16 ft | 32 ft | 16' | 22' 7 1/2" |
| 20 ft | 40 ft | 20' | 28' 3 7/16" |
Roof area at 12/12
Multiply the building footprint (including the overhangs) by 1.414 to get the roof surface. A 30 × 40 ft footprint has 1697 sq ft of roof at 12/12 — about 54 sheets of 4 × 8 sheathing before waste.