12/12 Roof Pitch
A 12 in 12 roof rises 12″ for every 12″ of horizontal run. A 45° roof: Gothic, A-frame and steep Victorian gables; rafter run equals rise.
Angle
45°
- Percent grade
- 100%
- Rafter factor
- 1.4142
- Rafter per foot of run
- 16.97″
- Hip/valley per foot
- 20.78″
- Plumb cut
- 45°
- Seat cut
- 45°
On a speed square, pivot to 12 on the "common" scale for the plumb cut. On a framing square, set 12″ on the tongue and 12″ on the body.
Very steep: not walkable without roof jacks, harnesses and anchors; expect steep-slope labor pricing.
12/12 rafter length by building span
Gable roof, 1½″ ridge board, measured along the top edge from the ridge to the outside of the wall. Add the overhang: 17" of rafter per foot of level overhang.
| Building span | Rafter run | Rise | Rafter length | With 12″ overhang |
|---|---|---|---|---|
| 12 ft | 5′ 11 1/4" | 5′ 11 1/4" | 8′ 4 3/4" | 9′ 9 3/4" |
| 16 ft | 7′ 11 1/4" | 7′ 11 1/4" | 11′ 2 11/16" | 12′ 7 11/16" |
| 20 ft | 9′ 11 1/4" | 9′ 11 1/4" | 14′ 5/8" | 15′ 5 5/8" |
| 24 ft | 11′ 11 1/4" | 11′ 11 1/4" | 16′ 10 9/16" | 18′ 3 9/16" |
| 28 ft | 13′ 11 1/4" | 13′ 11 1/4" | 19′ 8 1/2" | 21′ 1 1/2" |
| 30 ft | 14′ 11 1/4" | 14′ 11 1/4" | 21′ 1 1/2" | 22′ 6 7/16" |
| 32 ft | 15′ 11 1/4" | 15′ 11 1/4" | 22′ 6 7/16" | 23′ 11 7/16" |
| 36 ft | 17′ 11 1/4" | 17′ 11 1/4" | 25′ 4 7/16" | 26′ 9 3/8" |
| 40 ft | 19′ 11 1/4" | 19′ 11 1/4" | 28′ 2 3/8" | 29′ 7 5/16" |
Different span, ridge or overhang? The rafter calculator works out the birdsmouth, HAP and ridge height for your roof.
Roof area at 12/12
Roofing materials allowed at 12/12
- Asphalt shingles
Allowed; single underlayment layer (R905.2.2). - Clay and concrete tile
Allowed (R905.3.2). - Wood shingles
Allowed from 3/12 (R905.7.2). - Wood shakes
Allowed from 3/12 (R905.8.2). - Slate
Allowed from 4/12 (R905.6.2). - Metal panels, lapped without sealant
Allowed from 3/12 (R905.10.2). Standing-seam and sealed-lap panels go lower. - Low-slope membrane (EPDM, TPO, modified bitumen)
Allowed, but usually reserved for slopes under 2/12.
Minimum slopes from the 2021 International Residential Code, Section R905. Manufacturers and local amendments can be stricter.