18/12 Roof Pitch: Angle, Slope Factor, and Rafter Lengths
By RedX Apps Team · Published 2026-07-19
An 18/12 roof pitch rises 18 inches for every 12 inches of horizontal run. That equals 56.31 degrees from horizontal, a 150% grade. Its slope factor is 1.803, meaning every foot of run requires 21-5/8 in of rafter length along the slope.
This page gives you every number you need to frame at 18/12: 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.
18/12 Pitch at a Glance
| Angle in degrees | 56.31 deg |
|---|---|
| Percent grade | 150% |
| Slope factor (pitch multiplier) | 1.803 |
| Rafter length per foot of run | 21-5/8 in |
| Hip/valley factor per foot of common run | 2.062 |
| Walkability | not walkable |
| Asphalt shingles | Allowed |
Rafter Lengths for an 18/12 Roof
Computed with the 18/12 slope factor of 1.803. 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 | 14 ft 5-1/16 in | 12 ft |
| 10 ft | 18 ft 0-3/8 in | 15 ft |
| 12 ft | 21 ft 7-5/8 in | 18 ft |
| 14 ft | 25 ft 2-7/8 in | 21 ft |
| 16 ft | 28 ft 10-3/16 in | 24 ft |
| 20 ft | 36 ft 0-3/4 in | 30 ft |
Roofing Materials for a 18/12 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.
18/12 Roof Pitch FAQ
What angle is an 18/12 roof pitch?
An 18/12 roof pitch equals 56.31 degrees from horizontal. The pitch means the roof rises 18 inches for every 12 inches of horizontal run, which works out to a 150% grade.
What is the slope factor for an 18/12 roof?
The slope factor (pitch multiplier) for 18/12 is 1.803. Multiply any horizontal run by 1.803 to get the length along the slope - so each foot of run needs 21-5/8 in of rafter. Hip and valley rafters at this pitch use a factor of 2.062 per foot of common run.
Can you walk on an 18/12 roof?
An 18/12 roof is too steep to walk safely. Staging, roof jacks, and full fall protection are required to work on it.
Can I use asphalt shingles on an 18/12 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 an 18/12 pitch roof?
Multiply the run by 1.803: a 12 foot run at 18/12 needs a rafter of about 21 ft 7-5/8 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 18/12 Roof with Confidence
The RedX roof builder draws your 18/12 roof, computes every rafter and cut angle, and exports a cut list and printable blueprint. Free on web, iOS, and Android.