What a 21/12 pitch means
A 21/12 roof rises 21 inches for every 12 inches it runs horizontally, which is an angle of 60.26° from level and a 175% slope. It is a steep roof, too steep to walk safely without roof jacks or a harness.
Roofing for a 21/12 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 21/12
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 | 7' | 8' 3/4" |
| 6 ft | 12 ft | 10' 6" | 12' 1 1/8" |
| 8 ft | 16 ft | 14' | 16' 1 1/2" |
| 10 ft | 20 ft | 17' 6" | 20' 1 7/8" |
| 12 ft | 24 ft | 21' | 24' 2 1/4" |
| 14 ft | 28 ft | 24' 6" | 28' 2 5/8" |
| 16 ft | 32 ft | 28' | 32' 3" |
| 20 ft | 40 ft | 35' | 40' 3 3/4" |
Roof area at 21/12
Multiply the building footprint (including the overhangs) by 2.016 to get the roof surface. A 30 × 40 ft footprint has 2419 sq ft of roof at 21/12 — about 76 sheets of 4 × 8 sheathing before waste.