Roof pitch reference
1/12 roof pitch
A 1/12 roof rises 1 inch per 12 inches of run: 4.8°, a 8.3% grade, slope factor 1.0035. Each foot of run is 12.04 inches along the slope.
√(1 + (1 ÷ 12)²) = 1.0035.
Roof area at 1/12
| Footprint | Roof at 1/12 | Squares |
|---|---|---|
| 1,200 sq ft | 1,204 sq ft | 12 |
| 1,600 sq ft | 1,606 sq ft | 16.1 |
| 2,000 sq ft | 2,007 sq ft | 20.1 |
| 2,600 sq ft | 2,609 sq ft | 26.1 |
Rafter length at 1/12
| Run (half span) | Rafter, no overhang |
|---|---|
| 10 ft | 10 ft 0.5 in (10.03 ft) |
| 12 ft | 12 ft 0.5 in (12.04 ft) |
| 14 ft | 14 ft 0.5 in (14.05 ft) |
| 16 ft | 16 ft 0.75 in (16.06 ft) |
Ice barrier on a 1/12 roof in Ohio and Michigan
Freeze-thaw winters in both states make eave protection one of the most important details on a 1/12 roof. Where the locally adopted code requires it, the model IRC (R905.1.2) extends ice barrier from the eave to at least 24 inches inside the exterior wall. On a 4.8° slope that distance grows by 0.3% when measured up the roof:
| Overhang | Horizontal | Up the 1/12 slope | 36-in courses |
|---|---|---|---|
| 12 in | 36 in | 36.1 in | 2 |
| 18 in | 42 in | 42.1 in | 2 |
| 24 in | 48 in | 48.2 in | 2 |
Courses assume a 3-inch lap (33 inches of coverage per 36-inch roll). Wall thickness adds a few inches. Check the product and your building department.
Shingles at 1/12
Below 2/12 the model IRC (R905.2.2) does not allow asphalt shingles, so a 1/12 roof at 4.8° needs a low-slope membrane or another system rated for 8.3% slopes.
Working on a 1/12 roof
Water on a 8.3% grade drains slowly, and snow on a 4.8° roof in an Ohio or Michigan winter tends to sit rather than slide. Flashing and underlayment details carry more of the load at 1/12. OSHA classifies roofs at 4/12 or less as low-slope, so 1/12 falls under the low-slope rules.
Frequently asked questions
What angle is a 1/12 roof?
A 1/12 roof is about 4.8° from horizontal, a 8.3% grade. Over a 10-foot run it rises 0.83 feet.
How much bigger is a 1/12 roof than the footprint?
The slope factor is 1.0035, so the roof is 0.3% larger than the flat area under it. A 1,600 sq ft footprint has about 1,606 sq ft of roof (16.1 squares).
Can you put asphalt shingles on a 1/12 roof?
Below 2/12 the model IRC (R905.2.2) does not allow asphalt shingles, so a 1/12 roof at 4.8° needs a low-slope membrane or another system rated for 8.3% slopes.
How much ice barrier does a 1/12 roof need at the eaves?
With a 12-inch overhang, reaching 24 inches inside the wall takes about 36.1 inches measured up a 1/12 slope, which is 2 courses of 36-inch membrane with a 3-inch lap. Your local code decides whether ice barrier is required.