11/12 roof pitch in degrees
A 11/12 roof pitch is 42.5104°. That is a 91.67% grade, a slope ratio of 1 : 1.0909, and a rafter factor of 1.356568, and the angle is irrational so it carries the ≈.
Change the pitch
42.5104°≈
Multiply your run by 1.356568 for the rafter length. The cuts and the birdsmouth are in the app.
Rafter length at 11/12, 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' 6-13/16" | 13.566 ft |
| 24 ft | 12 ft | 16' 3-3/8" | 16.279 ft |
| 28 ft | 14 ft | 18' 11-7/8" | 18.992 ft |
| 32 ft | 16 ft | 21' 8-7/16" | 21.705 ft |
| 36 ft | 18 ft | 24' 5" | 24.418 ft |
Every row is the run multiplied by 1.356568, the rafter factor for 11/12. The same figure for every pitch is on the full roof pitch chart.
What 11/12 means on the roof
Eleven in twelve is the last pitch where the run is still longer than the rise. At 12/12 the two are equal, and past that each half of the roof is taller than it is wide. At 42.5104° it is close enough to 45 that a framing square set on 11 and 12 reads square by eye and is not.
The working is the arctangent of the rise over the run: atan(11 ÷ 12) = 42.5104°. The rafter factor is the hypotenuse of the same triangle over its base, √(12² + 11²) ÷ 12, which is why multiplying any run by 1.356568 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.
