Roof pitch reference
18/12 roof pitch
An 18/12 roof rises 18 inches per 12 inches of run: 56.3°, a 150% grade, slope factor 1.8028. Each foot of run is 21.63 inches along the slope.
√(1 + (18 ÷ 12)²) = 1.8028.
Roof area at 18/12
| Footprint | Roof at 18/12 | Squares |
|---|---|---|
| 1,200 sq ft | 2,163 sq ft | 21.6 |
| 1,600 sq ft | 2,884 sq ft | 28.8 |
| 2,000 sq ft | 3,606 sq ft | 36.1 |
| 2,600 sq ft | 4,687 sq ft | 46.9 |
Rafter length at 18/12
| Run (half span) | Rafter, no overhang |
|---|---|
| 10 ft | 18 ft 0.25 in (18.03 ft) |
| 12 ft | 21 ft 7.5 in (21.63 ft) |
| 14 ft | 25 ft 2.75 in (25.24 ft) |
| 16 ft | 28 ft 10.25 in (28.84 ft) |
Ice barrier on an 18/12 roof in Ohio and Michigan
Freeze-thaw winters in both states make eave protection one of the most important details on an 18/12 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 56.3° slope that distance grows by 80.3% when measured up the roof:
| Overhang | Horizontal | Up the 18/12 slope | 36-in courses |
|---|---|---|---|
| 12 in | 36 in | 64.9 in | 2 |
| 18 in | 42 in | 75.7 in | 3 |
| 24 in | 48 in | 86.5 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 18/12
At 18/12 (56.3°, 150% grade) asphalt shingles install with standard underlayment under the IRC, plus ice barrier where the local code requires it.
Working on an 18/12 roof
A 56.3° slope behaves almost like a wall. An 18/12 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 18/12. OSHA treats anything steeper than 4/12 as a steep roof, so 18/12 falls under steep-roof fall protection rules.
Frequently asked questions
What angle is an 18/12 roof?
An 18/12 roof is about 56.3° from horizontal, a 150% grade. Over a 10-foot run it rises 15 feet.
How much bigger is an 18/12 roof than the footprint?
The slope factor is 1.8028, so the roof is 80.3% larger than the flat area under it. A 1,600 sq ft footprint has about 2,884 sq ft of roof (28.8 squares).
Can you put asphalt shingles on an 18/12 roof?
At 18/12 (56.3°, 150% 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 18/12 roof need at the eaves?
With a 12-inch overhang, reaching 24 inches inside the wall takes about 64.9 inches measured up a 18/12 slope, which is 2 courses of 36-inch membrane with a 3-inch lap. Your local code decides whether ice barrier is required.