4/12 Roof Pitch: Angle, Slope Factor, and Rafter Lengths
By RedX Apps Team · Published 2026-07-19
A 4/12 roof pitch rises 4 inches for every 12 inches of horizontal run. That equals 18.43 degrees from horizontal, a 33.3% grade. Its slope factor is 1.054, meaning every foot of run requires 12-5/8 in of rafter length along the slope.
This page gives you every number you need to frame at 4/12: the angle for your saw, computed rafter lengths for common runs, the hip and valley factor, and what roofing materials the slope allows. All values are calculated from the pitch geometry - the same math the free RedX rafter calculator runs for your exact dimensions.
4/12 Pitch at a Glance
| Angle in degrees | 18.43 deg |
|---|---|
| Percent grade | 33.3% |
| Slope factor (pitch multiplier) | 1.054 |
| Rafter length per foot of run | 12-5/8 in |
| Hip/valley factor per foot of common run | 1.453 |
| Walkability | walkable |
| Asphalt shingles | Allowed |
Rafter Lengths for a 4/12 Roof
Computed with the 4/12 slope factor of 1.054. Lengths run from the ridge to the wall plate with no overhang and no ridge deduction - the free rafter calculator applies both for your exact build.
| Run | Rafter length (no overhang) | Rise at ridge |
|---|---|---|
| 8 ft | 8 ft 5-3/16 in | 2 ft 8 in |
| 10 ft | 10 ft 6-1/2 in | 3 ft 4 in |
| 12 ft | 12 ft 7-3/4 in | 4 ft |
| 14 ft | 14 ft 9-1/16 in | 4 ft 8 in |
| 16 ft | 16 ft 10-3/8 in | 5 ft 4 in |
| 20 ft | 21 ft 0-15/16 in | 6 ft 8 in |
Roofing Materials for a 4/12 Pitch
Suitable for standard asphalt shingle installation (4/12 and above) as well as metal, tile, and most other coverings.
Always confirm the manufacturer minimum slope for your specific product and your local code - underlayment requirements change at low slopes, and high-wind or heavy-snow regions add their own rules.
4/12 Roof Pitch FAQ
What angle is a 4/12 roof pitch?
A 4/12 roof pitch equals 18.43 degrees from horizontal. The pitch means the roof rises 4 inches for every 12 inches of horizontal run, which works out to a 33.3% grade.
What is the slope factor for a 4/12 roof?
The slope factor (pitch multiplier) for 4/12 is 1.054. Multiply any horizontal run by 1.054 to get the length along the slope - so each foot of run needs 12-5/8 in of rafter. Hip and valley rafters at this pitch use a factor of 1.453 per foot of common run.
Can you walk on a 4/12 roof?
A 4/12 roof is generally considered walkable for experienced workers in good conditions, though fall protection is always recommended and required on many jobsites.
Can I use asphalt shingles on a 4/12 roof?
Suitable for standard asphalt shingle installation (4/12 and above) as well as metal, tile, and most other coverings.
How long is a rafter for a 4/12 pitch roof?
Multiply the run by 1.054: a 12 foot run at 4/12 needs a rafter of about 12 ft 7-3/4 in before overhang. The table on this page lists computed lengths for common runs, and the free RedX rafter calculator adds your exact overhang, ridge deduction, and birdsmouth cuts.
All Roof Pitches
Frame Your 4/12 Roof with Confidence
The RedX roof builder draws your 4/12 roof, computes every rafter and cut angle, and exports a cut list and printable blueprint. Free on web, iOS, and Android.