It is a low, easy roof: common on ranch houses, garages, porches and additions, and the point from which asphalt shingles need only the standard single layer of underlayment. The calculator is set to a 4/12 gable roof; enter your building's width and overhang for the rafter length.
How to use this calculator
- Enter the building width, wall to wall, across the direction the rafters run. The calculator halves it for a gable with the ridge in the middle.
- Keep the pitch as it is, or change it to compare a steeper roof.
- Enter the overhang as the horizontal distance the eave sticks out past the wall.
- Read the rafter length and buy the next stock length up; the ridge height tells you how tall the gable ends are.
Worked examples
A single garage
Building width: 14 ft · Pitch: 4 in 12 · Overhang: 12 in
- Rafter length
- 8.43 ft
- Without overhang
- 7.38 ft
- Ridge height above the wall
- 28.0 in
- Run
- 7.00 ft
A ranch house
Building width: 28 ft · Pitch: 4 in 12 · Overhang: 18 in
- Rafter length
- 16.34 ft
- Without overhang
- 14.76 ft
- Ridge height above the wall
- 56.0 in
- Run
- 14.00 ft
A backyard shed
Building width: 10 ft · Pitch: 4 in 12 · Overhang: 6 in
- Rafter length
- 5.80 ft
- Without overhang
- 5.27 ft
- Ridge height above the wall
- 20.0 in
- Run
- 5.00 ft
Rafter length on a 4/12 roof by building width and overhang
| Building width | 0 in | 12 in | 18 in | 24 in |
|---|---|---|---|---|
| 12 ft | 6.32 | 7.38 | 7.91 | 8.43 |
| 16 ft | 8.43 | 9.49 | 10.01 | 10.54 |
| 20 ft | 10.54 | 11.60 | 12.12 | 12.65 |
| 24 ft | 12.65 | 13.70 | 14.23 | 14.76 |
| 28 ft | 14.76 | 15.81 | 16.34 | 16.87 |
| 32 ft | 16.87 | 17.92 | 18.45 | 18.97 |
| 36 ft | 18.97 | 20.03 | 20.55 | 21.08 |
Each cell: the rafter length along its top edge from the wall to the ridge plus the overhang, for a gable roof with the ridge in the middle. Buy the next stock length up.
Shingles, metal and a 4/12 roof
Asphalt shingle makers allow their shingles down to 2 in per 12 in, but below 4 in per 12 in they ask for a double layer of underlayment, because water moves slowly over a low roof and wind can push it under the tabs. A 4/12 roof sits exactly at the line, so a single layer, lapped 2 inches, is enough. Corrugated and ribbed metal panels are rated to 3 in 12 and work well here too, and the roofing calculator turns the roof area into squares and bundles.
What a 4/12 roof is like to live with
Most people can walk a 4/12 roof in soft-soled shoes, which keeps repairs and gutter cleaning cheap. The attic is shallow: on a typical house the ridge stands only a few feet above the ceiling joists, enough for insulation and ductwork but rarely for storage. In heavy snow country a low pitch holds more snow, so the framing has to be sized for it, and ice dams form more easily at the eaves without good attic ventilation.
Measuring a pitch you are not sure of
Hold a level against the underside of a rafter in the attic, measure a foot along it, and measure straight up to the rafter: four inches there means a 4/12 roof. The roof pitch calculator works it out from any rise and run you measure, and the rafter calculator does the same lengths as here for any pitch.
Common questions
What angle is a 4/12 roof pitch?
18.4 degrees, a grade of 33.3 percent.
Is a 4/12 pitch good for shingles?
Yes. Asphalt shingles need only a single layer of underlayment from 4 in per 12 in, and a 4/12 roof is exactly that steep.
Can you walk on a 4/12 roof?
Usually, yes: it is one of the easiest pitches to walk. Still use good footwear, stay off wet or frosty shingles, and use fall protection at the edge.
What is the multiplier for a 4/12 pitch?
1.054. Multiply the flat area under the roof, or a horizontal run, by it to get the area or length along the roof surface.