| Pitch | Angle | Grade | Slope factor | Character |
|---|---|---|---|---|
| 3/12 | 14.0° | 25% | 1.031 | Low-slope limit for most shingles |
| 4/12 | 18.4° | 33% | 1.054 | Common ranch / modern |
| 6/12 | 26.6° | 50% | 1.118 | The residential workhorse |
| 8/12 | 33.7° | 67% | 1.202 | Traditional, sheds snow well |
| 12/12 | 45.0° | 100% | 1.414 | Steep; strong attic volume |
Rafter length = run × √(1 + (rise ÷ 12)²). The run is half the span for a gable, the full width for a lean-to. Add the overhang, then deduct half the ridge thickness at the peak.
Gable slopes two ways to a ridge with triangular end walls; hip slopes all four sides with hip rafters and a shorter ridge; lean-to (shed) is a single plane from a high wall to a low wall.
Angle = arctan(rise ÷ 12). 4/12 ≈ 18.4°, 6/12 ≈ 26.6°, 12/12 = 45°. The tool shows angle and grade beside the rise-in-12 figure.
Its plan run is the common run × √2, so hip length = common run × √(2 + (rise ÷ 12)²), plus overhang. This tool reports it whenever the hip roof type is selected.
Most homes run 4/12–9/12. 6/12–8/12 balances shedding, walkability and material. Steeper adds attic volume and a traditional look at higher cost.