Roof pitch reference
13/12 roof pitch
A 13/12 roof rises 13 inches per 12 inches of run: 47.3°, a 108.3% grade, slope factor 1.4743. Each foot of run is 17.69 inches along the slope.
√(1 + (13 ÷ 12)²) = 1.4743 · between 12/12 (+41.4%) and 14/12 (+53.7%).
Roof area at 13/12
| Footprint | Roof at 13/12 | Squares |
|---|---|---|
| 1,200 sq ft | 1,769 sq ft | 17.7 |
| 1,600 sq ft | 2,359 sq ft | 23.6 |
| 2,000 sq ft | 2,949 sq ft | 29.5 |
| 2,600 sq ft | 3,833 sq ft | 38.3 |
Rafter length at 13/12
| Run (half span) | Rafter, no overhang |
|---|---|
| 10 ft | 14 ft 9 in (14.74 ft) |
| 12 ft | 17 ft 8.25 in (17.69 ft) |
| 14 ft | 20 ft 7.75 in (20.64 ft) |
| 16 ft | 23 ft 7 in (23.59 ft) |
Ice barrier on a 13/12 roof in Ohio and Michigan
Freeze-thaw winters in both states make eave protection one of the most important details on a 13/12 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 47.3° slope that distance grows by 47.4% when measured up the roof:
| Overhang | Horizontal | Up the 13/12 slope | 36-in courses |
|---|---|---|---|
| 12 in | 36 in | 53.1 in | 2 |
| 18 in | 42 in | 61.9 in | 2 |
| 24 in | 48 in | 70.8 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 13/12
At 13/12 (47.3°, 108.3% grade) asphalt shingles install with standard underlayment under the IRC, plus ice barrier where the local code requires it.
Working on a 13/12 roof
A 47.3° slope behaves almost like a wall. A 13/12 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 13/12. OSHA treats anything steeper than 4/12 as a steep roof, so 13/12 falls under steep-roof fall protection rules.
Frequently asked questions
What angle is a 13/12 roof?
A 13/12 roof is about 47.3° from horizontal, a 108.3% grade. Over a 10-foot run it rises 10.83 feet.
How much bigger is a 13/12 roof than the footprint?
The slope factor is 1.4743, so the roof is 47.4% larger than the flat area under it. A 1,600 sq ft footprint has about 2,359 sq ft of roof (23.6 squares).
Can you put asphalt shingles on a 13/12 roof?
At 13/12 (47.3°, 108.3% 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 13/12 roof need at the eaves?
With a 12-inch overhang, reaching 24 inches inside the wall takes about 53.1 inches measured up a 13/12 slope, which is 2 courses of 36-inch membrane with a 3-inch lap. Your local code decides whether ice barrier is required.