What a 20/12 pitch means
A 20/12 roof rises 20 inches for every 12 inches it runs horizontally, which is an angle of 59.04° from level and a 166.7% slope. It is a steep roof, too steep to walk safely without roof jacks or a harness.
Roofing for a 20/12 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 20/12
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 | 6' 8" | 7' 9 5/16" |
| 6 ft | 12 ft | 10' | 11' 7 15/16" |
| 8 ft | 16 ft | 13' 4" | 15' 6 9/16" |
| 10 ft | 20 ft | 16' 8" | 19' 5 1/4" |
| 12 ft | 24 ft | 20' | 23' 3 7/8" |
| 14 ft | 28 ft | 23' 4" | 27' 2 9/16" |
| 16 ft | 32 ft | 26' 8" | 31' 1 3/16" |
| 20 ft | 40 ft | 33' 4" | 38' 10 1/2" |
Roof area at 20/12
Multiply the building footprint (including the overhangs) by 1.944 to get the roof surface. A 30 × 40 ft footprint has 2332 sq ft of roof at 20/12 — about 73 sheets of 4 × 8 sheathing before waste.