3/12 Roof Pitch
A 3 in 12 roof rises 3″ for every 12″ of horizontal run. Low-slope sheds, mobile homes, ranch additions and metal-roofed outbuildings.
Angle
14.04°
- Percent grade
- 25%
- Rafter factor
- 1.0308
- Rafter per foot of run
- 12.37″
- Hip/valley per foot
- 17.23″
- Plumb cut
- 14°
- Seat cut
- 76°
On a speed square, pivot to 3 on the "common" scale for the plumb cut. On a framing square, set 3″ on the tongue and 12″ on the body.
Low slope: easy to walk on. Roofers price it as standard work.
3/12 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: 12 3/8" of rafter per foot of level overhang.
| Building span | Rafter run | Rise | Rafter length | With 12″ overhang |
|---|---|---|---|---|
| 12 ft | 5′ 11 1/4" | 1′ 5 13/16" | 6′ 1 7/16" | 7′ 1 13/16" |
| 16 ft | 7′ 11 1/4" | 1′ 11 13/16" | 8′ 2 3/16" | 9′ 2 9/16" |
| 20 ft | 9′ 11 1/4" | 2′ 5 13/16" | 10′ 2 15/16" | 11′ 3 5/16" |
| 24 ft | 11′ 11 1/4" | 2′ 11 13/16" | 12′ 3 11/16" | 13′ 4" |
| 28 ft | 13′ 11 1/4" | 3′ 5 13/16" | 14′ 4 3/8" | 15′ 4 3/4" |
| 30 ft | 14′ 11 1/4" | 3′ 8 13/16" | 15′ 4 3/4" | 16′ 5 1/8" |
| 32 ft | 15′ 11 1/4" | 3′ 11 13/16" | 16′ 5 1/8" | 17′ 5 1/2" |
| 36 ft | 17′ 11 1/4" | 4′ 5 13/16" | 18′ 5 7/8" | 19′ 6 1/4" |
| 40 ft | 19′ 11 1/4" | 4′ 11 13/16" | 20′ 6 5/8" | 21′ 7" |
Different span, ridge or overhang? The rafter calculator works out the birdsmouth, HAP and ridge height for your roof.
Roof area at 3/12
Roofing materials allowed at 3/12
- Asphalt shingles
Allowed from 2/12, but 2/12 to under 4/12 needs two layers of underlayment (R905.1.1). - Clay and concrete tile
Allowed from 2½/12 with double underlayment below 4/12 (R905.3.2). - Wood shingles
Allowed from 3/12 (R905.7.2). - Wood shakes
Allowed from 3/12 (R905.8.2). - Slate
Not allowed below 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.
Compare other pitches
Questions people ask
What angle is a 3/12 roof pitch?
A 3/12 roof rises 3 inches for every 12 inches of horizontal run, which is 14.04° (arctan(3/12)). As a percent grade it is 25%.
What is the rafter factor for a 3/12 pitch?
1.0308. Multiply the horizontal run by 1.0308 to get the sloped rafter length, or multiply the building footprint (with overhangs) by it to get the roof area. Each foot of run needs 12.37" of rafter.
How long is a rafter on a 3/12 roof?
For a 24 ft wide building with a 1½" ridge board, the run is 143.25", so the rafter is 12′ 3 11/16" from ridge to wall line, before the overhang. The table on this page gives lengths for other spans, and the rafter calculator handles your exact numbers including birdsmouth and tail.
Can you put shingles on a 3/12 roof?
Allowed from 2/12, but 2/12 to under 4/12 needs two layers of underlayment (R905.1.1). Always follow the shingle manufacturer's installation instructions as well.
Is a 3/12 roof walkable?
Low slope: easy to walk on. Roofers price it as standard work.
How much roof area does a 3/12 pitch add?
3.1% more than the flat footprint. A 30 × 40 ft house with 1 ft overhangs (32 × 42 = 1,344 sq ft) has about 1,385 sq ft of roof, or 13.9 roofing squares before waste.