What a 18/12 pitch means
A 18/12 roof rises 18 inches for every 12 inches it runs horizontally, which is an angle of 56.31° from level and a 150% slope. It is a steep roof, too steep to walk safely without roof jacks or a harness.
Roofing for a 18/12 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 18/12
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' | 7' 2 9/16" |
| 6 ft | 12 ft | 9' | 10' 9 13/16" |
| 8 ft | 16 ft | 12' | 14' 5 1/16" |
| 10 ft | 20 ft | 15' | 18' 5/16" |
| 12 ft | 24 ft | 18' | 21' 7 5/8" |
| 14 ft | 28 ft | 21' | 25' 2 7/8" |
| 16 ft | 32 ft | 24' | 28' 10 1/8" |
| 20 ft | 40 ft | 30' | 36' 11/16" |
Roof area at 18/12
Multiply the building footprint (including the overhangs) by 1.803 to get the roof surface. A 30 × 40 ft footprint has 2163 sq ft of roof at 18/12 — about 68 sheets of 4 × 8 sheathing before waste.