1/12 roof pitch in degrees
A 1/12 roof pitch is 4.7636°. That is a 8.33% grade, a slope ratio of 1 : 12, and a rafter factor of 1.003466, and the angle is irrational so it carries the ≈.
Change the pitch
4.7636°≈
Multiply your run by 1.003466 for the rafter length. The cuts and the birdsmouth are in the app.
Rafter length at 1/12, 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' 0-7/16" | 10.035 ft |
| 24 ft | 12 ft | 12' 0-1/2" | 12.042 ft |
| 28 ft | 14 ft | 14' 0-9/16" | 14.049 ft |
| 32 ft | 16 ft | 16' 0-11/16" | 16.055 ft |
| 36 ft | 18 ft | 18' 0-3/4" | 18.062 ft |
Every row is the run multiplied by 1.003466, the rafter factor for 1/12. The same figure for every pitch is on the full roof pitch chart.
What 1/12 means on the roof
One in twelve is below anything the IRC will let you put shingles on: 2/12 is the floor and this is half of it. A roof at this slope is a membrane or a standing seam, and the slope is there to drain the deck rather than to shed weather off a covering.
The working is the arctangent of the rise over the run: atan(1 ÷ 12) = 4.7636°. The rafter factor is the hypotenuse of the same triangle over its base, √(12² + 1²) ÷ 12, which is why multiplying any run by 1.003466 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.
