What a 3/12 pitch means
A 3/12 roof rises 3 inches for every 12 inches it runs horizontally, which is an angle of 14.04° from level and a 25% slope. It is a low slope roof, walkable without roof jacks.
Roofing for a 3/12 roof
Asphalt shingles are allowed from 2/12 with double underlayment (IRC R905.2.2); standing-seam metal also works. Clay and concrete tile need 4/12 or more with standard underlayment.
Rise and rafter length at 3/12
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 | 1' | 4' 1 1/2" |
| 6 ft | 12 ft | 1' 6" | 6' 2 3/16" |
| 8 ft | 16 ft | 2' | 8' 2 15/16" |
| 10 ft | 20 ft | 2' 6" | 10' 3 11/16" |
| 12 ft | 24 ft | 3' | 12' 4 7/16" |
| 14 ft | 28 ft | 3' 6" | 14' 5 3/16" |
| 16 ft | 32 ft | 4' | 16' 5 15/16" |
| 20 ft | 40 ft | 5' | 20' 7 3/8" |
Roof area at 3/12
Multiply the building footprint (including the overhangs) by 1.031 to get the roof surface. A 30 × 40 ft footprint has 1237 sq ft of roof at 3/12 — about 39 sheets of 4 × 8 sheathing before waste.