Roof pitch reference
4/12 roof pitch
A 4/12 roof rises 4 inches per 12 inches of run: 18.4°, a 33.3% grade, slope factor 1.0541. Each foot of run is 12.65 inches along the slope.
√(1 + (4 ÷ 12)²) = 1.0541 · between 3/12 (+3.1%) and 5/12 (+8.3%).
Roof area at 4/12
| Footprint | Roof at 4/12 | Squares |
|---|---|---|
| 1,200 sq ft | 1,265 sq ft | 12.6 |
| 1,600 sq ft | 1,687 sq ft | 16.9 |
| 2,000 sq ft | 2,108 sq ft | 21.1 |
| 2,600 sq ft | 2,741 sq ft | 27.4 |
Rafter length at 4/12
| Run (half span) | Rafter, no overhang |
|---|---|
| 10 ft | 10 ft 6.5 in (10.54 ft) |
| 12 ft | 12 ft 7.75 in (12.65 ft) |
| 14 ft | 14 ft 9 in (14.76 ft) |
| 16 ft | 16 ft 10.5 in (16.87 ft) |
Ice barrier on a 4/12 roof in Ohio and Michigan
Freeze-thaw winters in both states make eave protection one of the most important details on a 4/12 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 18.4° slope that distance grows by 5.4% when measured up the roof:
| Overhang | Horizontal | Up the 4/12 slope | 36-in courses |
|---|---|---|---|
| 12 in | 36 in | 37.9 in | 2 |
| 18 in | 42 in | 44.3 in | 2 |
| 24 in | 48 in | 50.6 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 4/12
At 4/12 (18.4°, 33.3% grade) asphalt shingles install with standard underlayment under the IRC, plus ice barrier where the local code requires it.
Working on a 4/12 roof
Most crews can work a 18.4° roof with standard setups. At 4/12, snow usually sheds after a few sunny days, but eaves over unheated overhangs still ice up in freeze-thaw weather. OSHA classifies roofs at 4/12 or less as low-slope, so 4/12 falls under the low-slope rules.
Frequently asked questions
What angle is a 4/12 roof?
A 4/12 roof is about 18.4° from horizontal, a 33.3% grade. Over a 10-foot run it rises 3.33 feet.
How much bigger is a 4/12 roof than the footprint?
The slope factor is 1.0541, so the roof is 5.4% larger than the flat area under it. A 1,600 sq ft footprint has about 1,687 sq ft of roof (16.9 squares).
Can you put asphalt shingles on a 4/12 roof?
At 4/12 (18.4°, 33.3% 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 4/12 roof need at the eaves?
With a 12-inch overhang, reaching 24 inches inside the wall takes about 37.9 inches measured up a 4/12 slope, which is 2 courses of 36-inch membrane with a 3-inch lap. Your local code decides whether ice barrier is required.