Calculate Roof Pitch
Pitch Results
1.000
Pitch Factor
multiplier
IRC R903.4: Minimum roof slope for asphalt shingles is 2:12 (9.5°). Low-slope applications (2:12 to 4:12) require additional underlayment and proper flashing.
Rafter Length Calculator
Rafter Dimensions
0' 0"
Rafter Length
total
0 m
0"
Overhang Rafter
length
Note: Add 6-12" to calculated length for ridge and tail cuts. HAP (Height Above Plate) should match rafter depth minus birdsmouth seat cut.
Roof Pitch Reference (IRC 2021)
| Pitch |
Degrees |
Factor |
Category |
Roofing Suitability |
| 1/12 | 4.8° | 1.003 | Flat | Built-up, membrane only |
| 2/12 | 9.5° | 1.014 | Low | Min for asphalt shingles* |
| 3/12 | 14.0° | 1.031 | Low | Low-slope shingles OK |
| 4/12 | 18.4° | 1.054 | Standard | All shingles, walkable |
| 6/12 | 26.6° | 1.118 | Standard | Ideal for most materials |
| 8/12 | 33.7° | 1.202 | Steep | Use caution walking |
| 10/12 | 39.8° | 1.302 | Steep | Roof jacks required |
| 12/12 | 45.0° | 1.414 | Very Steep | Scaffolding recommended |
*IRC R905.2.2: Asphalt shingles on slopes 2:12 to 4:12 require double underlayment or self-adhering polymer modified bitumen sheet.