3/12 roof pitch in degrees
A 3/12 roof pitch is 14.0362°. That is a 25.00% grade, a slope ratio of 1 : 4, and a rafter factor of 1.030776, and the angle is irrational so it carries the ≈.
Change the pitch
14.0362°≈
Multiply your run by 1.030776 for the rafter length. The cuts and the birdsmouth are in the app.
Rafter length at 3/12, by span
Run is half the span on a symmetrical gable. Length is along the rafter, before any birdsmouth, ridge or tail is taken off.
| Building span | Run | Rafter length | Decimal feet |
|---|---|---|---|
| 20 ft | 10 ft | 10' 3-11/16" | 10.308 ft |
| 24 ft | 12 ft | 12' 4-7/16" | 12.369 ft |
| 28 ft | 14 ft | 14' 5-3/16" | 14.431 ft |
| 32 ft | 16 ft | 16' 5-15/16" | 16.492 ft |
| 36 ft | 18 ft | 18' 6-5/8" | 18.554 ft |
Every row is the run multiplied by 1.030776, the rafter factor for 3/12. The same figure for every pitch is on the full roof pitch chart.
What 3/12 means on the roof
Three in twelve sits inside the IRC low-slope band, between 2/12 and 4/12, where asphalt shingles still need that doubled underlayment. It is the pitch at which a lot of builds give up on shingles and switch to standing seam metal.
The working is the arctangent of the rise over the run: atan(3 ÷ 12) = 14.0362°. The rafter factor is the hypotenuse of the same triangle over its base, √(12² + 3²) ÷ 12, which is why multiplying any run by 1.030776 gives the rafter. That figure is the roof pitch multiplier, and it is the same number whatever the size of the roof.
Shingle slope limits quoted here are from the IRC, the United States model code. Your state, province or county may amend them, and several do.