What a 1/12 pitch means
A 1/12 roof rises 1 inches for every 12 inches it runs horizontally, which is an angle of 4.76° from level and a 8.3% slope. It is a flat / very low slope roof, walkable without roof jacks.
Roofing for a 1/12 roof
Too low for shingles or tiles — use a low-slope membrane (EPDM, TPO, modified bitumen) or standing-seam metal rated for the pitch.
Rise and rafter length at 1/12
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' 3/16" |
| 6 ft | 12 ft | 6" | 6' 1/4" |
| 8 ft | 16 ft | 8" | 8' 5/16" |
| 10 ft | 20 ft | 10" | 10' 7/16" |
| 12 ft | 24 ft | 1' | 12' 1/2" |
| 14 ft | 28 ft | 1' 2" | 14' 9/16" |
| 16 ft | 32 ft | 1' 4" | 16' 11/16" |
| 20 ft | 40 ft | 1' 8" | 20' 13/16" |
Roof area at 1/12
Multiply the building footprint (including the overhangs) by 1.003 to get the roof surface. A 30 × 40 ft footprint has 1204 sq ft of roof at 1/12 — about 38 sheets of 4 × 8 sheathing before waste.