Roof pitch reference
11/12 roof pitch
An 11/12 roof rises 11 inches per 12 inches of run: 42.5°, a 91.7% grade, slope factor 1.3566. Each foot of run is 16.28 inches along the slope.
√(1 + (11 ÷ 12)²) = 1.3566 · between 10/12 (+30.2%) and 12/12 (+41.4%).
Roof area at 11/12
| Footprint | Roof at 11/12 | Squares |
|---|---|---|
| 1,200 sq ft | 1,628 sq ft | 16.3 |
| 1,600 sq ft | 2,171 sq ft | 21.7 |
| 2,000 sq ft | 2,713 sq ft | 27.1 |
| 2,600 sq ft | 3,527 sq ft | 35.3 |
Rafter length at 11/12
| Run (half span) | Rafter, no overhang |
|---|---|
| 10 ft | 13 ft 6.75 in (13.57 ft) |
| 12 ft | 16 ft 3.25 in (16.28 ft) |
| 14 ft | 19 ft 0 in (18.99 ft) |
| 16 ft | 21 ft 8.5 in (21.71 ft) |
Ice barrier on an 11/12 roof in Ohio and Michigan
Freeze-thaw winters in both states make eave protection one of the most important details on an 11/12 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 42.5° slope that distance grows by 35.7% when measured up the roof:
| Overhang | Horizontal | Up the 11/12 slope | 36-in courses |
|---|---|---|---|
| 12 in | 36 in | 48.8 in | 2 |
| 18 in | 42 in | 57 in | 2 |
| 24 in | 48 in | 65.1 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 11/12
At 11/12 (42.5°, 91.7% grade) asphalt shingles install with standard underlayment under the IRC, plus ice barrier where the local code requires it.
Working on an 11/12 roof
At 42.5°, crews need roof jacks, harnesses or staging, and every square takes longer. Snow tends to slide off a 11/12 roof in sheets, so guards over doors and walkways are worth asking about. OSHA treats anything steeper than 4/12 as a steep roof, so 11/12 falls under steep-roof fall protection rules.
Frequently asked questions
What angle is an 11/12 roof?
An 11/12 roof is about 42.5° from horizontal, a 91.7% grade. Over a 10-foot run it rises 9.17 feet.
How much bigger is an 11/12 roof than the footprint?
The slope factor is 1.3566, so the roof is 35.7% larger than the flat area under it. A 1,600 sq ft footprint has about 2,171 sq ft of roof (21.7 squares).
Can you put asphalt shingles on an 11/12 roof?
At 11/12 (42.5°, 91.7% grade) asphalt shingles install with standard underlayment under the IRC, plus ice barrier where the local code requires it.
How much ice barrier does an 11/12 roof need at the eaves?
With a 12-inch overhang, reaching 24 inches inside the wall takes about 48.8 inches measured up a 11/12 slope, which is 2 courses of 36-inch membrane with a 3-inch lap. Your local code decides whether ice barrier is required.