What a 13/12 pitch means
A 13/12 roof rises 13 inches for every 12 inches it runs horizontally, which is an angle of 47.29° from level and a 108.3% slope. It is a steep roof, too steep to walk safely without roof jacks or a harness.
Roofing for a 13/12 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 13/12
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' 4" | 5' 10 3/4" |
| 6 ft | 12 ft | 6' 6" | 8' 10 1/8" |
| 8 ft | 16 ft | 8' 8" | 11' 9 9/16" |
| 10 ft | 20 ft | 10' 10" | 14' 8 15/16" |
| 12 ft | 24 ft | 13' | 17' 8 5/16" |
| 14 ft | 28 ft | 15' 2" | 20' 7 11/16" |
| 16 ft | 32 ft | 17' 4" | 23' 7 1/16" |
| 20 ft | 40 ft | 21' 8" | 29' 5 13/16" |
Roof area at 13/12
Multiply the building footprint (including the overhangs) by 1.474 to get the roof surface. A 30 × 40 ft footprint has 1769 sq ft of roof at 13/12 — about 56 sheets of 4 × 8 sheathing before waste.