Roof pitch reference
3/12 roof pitch
A 3/12 roof rises 3 inches per 12 inches of run: 14°, a 25% grade, slope factor 1.0308. Each foot of run is 12.37 inches along the slope.
√(1 + (3 ÷ 12)²) = 1.0308 · between 2/12 (+1.4%) and 4/12 (+5.4%).
Roof area at 3/12
| Footprint | Roof at 3/12 | Squares |
|---|---|---|
| 1,200 sq ft | 1,237 sq ft | 12.4 |
| 1,600 sq ft | 1,649 sq ft | 16.5 |
| 2,000 sq ft | 2,062 sq ft | 20.6 |
| 2,600 sq ft | 2,680 sq ft | 26.8 |
Rafter length at 3/12
| Run (half span) | Rafter, no overhang |
|---|---|
| 10 ft | 10 ft 3.75 in (10.31 ft) |
| 12 ft | 12 ft 4.5 in (12.37 ft) |
| 14 ft | 14 ft 5.25 in (14.43 ft) |
| 16 ft | 16 ft 6 in (16.49 ft) |
Ice barrier on a 3/12 roof in Ohio and Michigan
Freeze-thaw winters in both states make eave protection one of the most important details on a 3/12 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 14° slope that distance grows by 3.1% when measured up the roof:
| Overhang | Horizontal | Up the 3/12 slope | 36-in courses |
|---|---|---|---|
| 12 in | 36 in | 37.1 in | 2 |
| 18 in | 42 in | 43.3 in | 2 |
| 24 in | 48 in | 49.5 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 3/12
At 3/12 (14°) asphalt shingles are allowed, but the IRC calls for double underlayment between 2/12 and 4/12 because water moves slowly on a 25% slope.
Working on a 3/12 roof
Water on a 25% grade drains slowly, and snow on a 14° roof in an Ohio or Michigan winter tends to sit rather than slide. Flashing and underlayment details carry more of the load at 3/12. OSHA classifies roofs at 4/12 or less as low-slope, so 3/12 falls under the low-slope rules.
Frequently asked questions
What angle is a 3/12 roof?
A 3/12 roof is about 14° from horizontal, a 25% grade. Over a 10-foot run it rises 2.5 feet.
How much bigger is a 3/12 roof than the footprint?
The slope factor is 1.0308, so the roof is 3.1% larger than the flat area under it. A 1,600 sq ft footprint has about 1,649 sq ft of roof (16.5 squares).
Can you put asphalt shingles on a 3/12 roof?
At 3/12 (14°) asphalt shingles are allowed, but the IRC calls for double underlayment between 2/12 and 4/12 because water moves slowly on a 25% slope.
How much ice barrier does a 3/12 roof need at the eaves?
With a 12-inch overhang, reaching 24 inches inside the wall takes about 37.1 inches measured up a 3/12 slope, which is 2 courses of 36-inch membrane with a 3-inch lap. Your local code decides whether ice barrier is required.