4/12 roof pitch: 18.43 degrees
A 4/12 roof pitch is 18.4349° to four places. Its multiplier, or rafter factor, is 1.054093, its grade is 33.33% and its slope ratio is 1 : 3.
Change the pitch
18.4349°≈
Multiply your run by 1.054093 for the rafter length. Hips and valleys at this pitch are in the app.
Rafter length at 4/12, 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' 6-1/2" | 10.541 ft |
| 24 ft | 12 ft | 12' 7-13/16" | 12.649 ft |
| 28 ft | 14 ft | 14' 9-1/16" | 14.757 ft |
| 32 ft | 16 ft | 16' 10-3/8" | 16.865 ft |
| 36 ft | 18 ft | 18' 11-11/16" | 18.974 ft |
Every row is the run multiplied by 1.054093, the rafter factor for 4/12. The multiplier for every pitch is on the roof pitch multiplier table.
What 4/12 means on the roof
Four in twelve is where the low-slope rule stops. At 4/12 and above the IRC lets asphalt shingles go down over a single underlayment, which is why so many sheds, porches and garages are drawn at exactly this pitch and not one twelfth less.
The working is the arctangent of the rise over the run: atan(4 ÷ 12) = 18.4349°. The rafter factor is the hypotenuse of the same triangle over its base, √(12² + 4²) ÷ 12, which is why multiplying any run by 1.054093 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 4/12 runs at 45° across the plan, so it is longer than the common rafter beside it: 1.4530 ft of hip for every foot of common run, which is 14' 6-3/8" over a 10 ft common run against 10' 6-1/2" 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: 1/12 · 2/12 · 3/12 · 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.
