Roof pitch reference
15/12 roof pitch
A 15/12 roof rises 15 inches per 12 inches of run: 51.3°, a 125% grade, slope factor 1.6008. Each foot of run is 19.21 inches along the slope.
√(1 + (15 ÷ 12)²) = 1.6008 · between 14/12 (+53.7%) and 16/12 (+66.7%).
Roof area at 15/12
| Footprint | Roof at 15/12 | Squares |
|---|---|---|
| 1,200 sq ft | 1,921 sq ft | 19.2 |
| 1,600 sq ft | 2,561 sq ft | 25.6 |
| 2,000 sq ft | 3,202 sq ft | 32 |
| 2,600 sq ft | 4,162 sq ft | 41.6 |
Rafter length at 15/12
| Run (half span) | Rafter, no overhang |
|---|---|
| 10 ft | 16 ft 0 in (16.01 ft) |
| 12 ft | 19 ft 2.5 in (19.21 ft) |
| 14 ft | 22 ft 5 in (22.41 ft) |
| 16 ft | 25 ft 7.25 in (25.61 ft) |
Ice barrier on a 15/12 roof in Ohio and Michigan
Freeze-thaw winters in both states make eave protection one of the most important details on a 15/12 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 51.3° slope that distance grows by 60.1% when measured up the roof:
| Overhang | Horizontal | Up the 15/12 slope | 36-in courses |
|---|---|---|---|
| 12 in | 36 in | 57.6 in | 2 |
| 18 in | 42 in | 67.2 in | 3 |
| 24 in | 48 in | 76.8 in | 3 |
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 15/12
At 15/12 (51.3°, 125% grade) asphalt shingles install with standard underlayment under the IRC, plus ice barrier where the local code requires it.
Working on a 15/12 roof
A 51.3° slope behaves almost like a wall. A 15/12 roof needs full staging and fall protection, and shingle makers publish separate steep-slope application steps, such as extra nails and hand-sealing, for pitches like 15/12. OSHA treats anything steeper than 4/12 as a steep roof, so 15/12 falls under steep-roof fall protection rules.
Frequently asked questions
What angle is a 15/12 roof?
A 15/12 roof is about 51.3° from horizontal, a 125% grade. Over a 10-foot run it rises 12.5 feet.
How much bigger is a 15/12 roof than the footprint?
The slope factor is 1.6008, so the roof is 60.1% larger than the flat area under it. A 1,600 sq ft footprint has about 2,561 sq ft of roof (25.6 squares).
Can you put asphalt shingles on a 15/12 roof?
At 15/12 (51.3°, 125% grade) asphalt shingles install with standard underlayment under the IRC, plus ice barrier where the local code requires it.
How much ice barrier does a 15/12 roof need at the eaves?
With a 12-inch overhang, reaching 24 inches inside the wall takes about 57.6 inches measured up a 15/12 slope, which is 2 courses of 36-inch membrane with a 3-inch lap. Your local code decides whether ice barrier is required.