| 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 equals the horizontal run multiplied by the slope factor, where the slope factor is the square root of 1 plus (rise over 12) squared. For a gable roof the run is half the span; a 6/12 pitch has a slope factor of about 1.118, so a 12-foot run gives a rafter of roughly 13 feet 5 inches before the overhang and ridge deductions.
A gable roof has two sloping planes meeting at a ridge with a vertical triangular wall at each end. A hip roof slopes on all four sides with no gable ends, adding four hip rafters and a shorter ridge. A lean-to, or shed roof, is a single plane sloping from a high wall to a low wall: one run across the full width.
The roof angle is the arctangent of rise divided by run. A 4/12 pitch is about 18.4 degrees, 6/12 is 26.6 degrees, and 12/12 is 45 degrees. This calculator shows the angle and the grade percentage alongside the standard rise-in-12 figure.
A hip rafter runs diagonally over the corner, so its plan run is the common run times the square root of 2. Its length is the common run times the square root of (2 plus the rise-over-12 squared). For a 6/12 hip on a 24-foot-wide building the hip rafter is about 17 feet 8 inches before overhang.
Most residential roofs fall between 4/12 and 9/12. Below 3/12 is considered low-slope and needs membrane rather than shingles; 6/12 to 8/12 sheds water and snow well and walks safely. Steeper pitches read more traditional and gain attic volume but use more material.