10/12 roof pitch in degrees
A 10/12 roof pitch is 39.8056°. That is a 83.33% grade, a slope ratio of 1 : 1.2, and a rafter factor of 1.301708, and the angle is irrational so it carries the ≈.
Change the pitch
39.8056°≈
Multiply your run by 1.301708 for the rafter length. The cuts and the birdsmouth are in the app.
Rafter length at 10/12, 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 | 13' 0-3/16" | 13.017 ft |
| 24 ft | 12 ft | 15' 7-7/16" | 15.620 ft |
| 28 ft | 14 ft | 18' 2-11/16" | 18.224 ft |
| 32 ft | 16 ft | 20' 9-15/16" | 20.827 ft |
| 36 ft | 18 ft | 23' 5-3/16" | 23.431 ft |
Every row is the run multiplied by 1.301708, the rafter factor for 10/12. The same figure for every pitch is on the full roof pitch chart.
What 10/12 means on the roof
Ten in twelve is steep. It sheds snow and water readily, it is not walkable, and everything above the plate line needs staging.
The working is the arctangent of the rise over the run: atan(10 ÷ 12) = 39.8056°. The rafter factor is the hypotenuse of the same triangle over its base, √(12² + 10²) ÷ 12, which is why multiplying any run by 1.301708 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.