1/12 roof pitch: 4.76 degrees
A 1/12 roof pitch is 4.7636° to four places. Its multiplier, or rafter factor, is 1.003466, its grade is 8.33% and its slope ratio is 1 : 12.
Change the pitch
4.7636°≈
Multiply your run by 1.003466 for the rafter length. Hips and valleys at this pitch 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' 7/16" | 10.035 ft |
| 24 ft | 12 ft | 12' 1/2" | 12.042 ft |
| 28 ft | 14 ft | 14' 9/16" | 14.049 ft |
| 32 ft | 16 ft | 16' 11/16" | 16.055 ft |
| 36 ft | 18 ft | 18' 3/4" | 18.062 ft |
Every row is the run multiplied by 1.003466, the rafter factor for 1/12. The multiplier for every pitch is on the roof pitch multiplier table.
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.
A hip or valley where both roofs are 1/12 runs at 45° across the plan, so it is longer than the common rafter beside it: 1.4167 ft of hip for every foot of common run, which is 14' 2" over a 10 ft common run against 10' 7/16" for the rafter.
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.
Other pitches: 2/12 · 3/12 · 4/12 · 4.5/12 · 5/12 · 6/12 · 6.5/12 · 7/12 · 8/12 · 9/12 · 10/12 · 11/12 · 12/12. Every pitch from 1/12 to 24/12 is on the roof pitch to degrees chart.
