3/12 Roof Pitch: Degrees, Multiplier and What It Means
A 3/12 roof rises 3 inches for every 12 inches of run. That is 14.04°, a pitch multiplier of 1.031 and a slope of 25%. Here is what those numbers do to your roof area, your materials and your quote.
3/12 at a glance
Ranch houses, porch roofs, shed dormers and room additions.
Nearby pitches
3 in rise per 12 in run
atan(3 ÷ 12) = 14.04°
√(3² + 12²) ÷ 12 = 1.031
3 ÷ 12 × 100 = 25%
Measured something different? The roof pitch calculator takes any rise and run, or an angle, and gives you all four numbers with the full reference chart.
Turn this pitch into a roof size
Measure the roof surface at this pitch, then price the whole job.
What a 3/12 roof pitch means
A 3/12 roof rises 3 inches for every 12 inches it runs horizontally. That is a low slope: shallow enough that it reads as almost flat from the street, but still steep enough to shed water rather than pond it. You see it most on single-storey ranch houses, porch roofs, shed dormers and room additions built onto a taller house.
3/12 in degrees
The angle is the arctangent of rise over run:
atan(3 ÷ 12) = atan(0.2500) = 14.04°
That is the angle between the roof surface and horizontal. It is the number a smartphone level app gives you if you hold it flat against the slope, and the number to enter if a supplier asks for degrees rather than a ratio.
The 3/12 pitch multiplier
The pitch multiplier turns the flat footprint of a roof into its real sloped area. It is the hypotenuse of the rise-and-run triangle divided by the run:
√(3² + 12²) ÷ 12 = √153 ÷ 12 = 1.031
So a 2,000 square foot footprint on a 3/12 roof is about 2,060 square feet of actual roof surface, or 20.6 roofing squares before any waste allowance. Use the roof area calculator to do this for a footprint that is not a simple rectangle, and the roofing square calculator to add a waste factor and convert to squares and bundles.
How 3/12 compares with the 6:12 that cost tables assume
Published roofing cost tables are built on a 6:12 roof. The HomeGuide roof replacement cost guide states in the footnote to its cost-by-house-size table that the figures are based on a single-storey home with a 6:12 roof pitch. A 3/12 roof has 7.8% less surface than that, on the same footprint — so a table priced off your house size slightly overstates the material your roof needs.
| Footprint | Roof at 3:12 | Roof at 6:12 | Difference |
|---|---|---|---|
| 1,200 | 1,235 sq ft | 1,340 sq ft | -7.8% |
| 1,600 | 1,650 sq ft | 1,790 sq ft | -7.8% |
| 2,000 | 2,060 sq ft | 2,235 sq ft | -7.8% |
| 2,400 | 2,475 sq ft | 2,685 sq ft | -7.8% |
Areas are rounded to the nearest 5 square feet. These are surface-area differences only. We do not publish a percentage for the labour surcharge on steep roofs, because none of the sources this site cites puts a number on it.
Walking on a 3/12 roof
A 3/12 roof is the easiest kind to walk. It sits well below the point at which roofers start pricing for staging and fall-arrest equipment.
A 3/12 roof needs two layers of underlayment
This is the one thing that matters most about a 3/12 roof. Asphalt shingles are allowed — the IRC sets the minimum at 2:12 — but any slope below 4:12 has to be installed over two layers of underlayment rather than one. A quote for a 3/12 roof that prices single-layer underlayment is not a code-compliant quote. Ask for it in writing.
For what the finished roof costs once you know your square count, see the roof replacement cost guide, or price a specific material with the roof replacement cost calculator.
Sources
- 2024 International Residential Code, Chapter 9 — minimum slopes for asphalt shingles and the underlayment requirement below 4:12 (R905.2.2).
- HomeGuide roof replacement cost guide — the cost-by-house-size table and its 6:12 footnote, and the note that slopes over 6:12 need extra safety precautions and more labour.
- Angle, multiplier and slope percentage are computed from the pitch, not taken from a source. The arithmetic is shown above so you can check it.
Every source on this site is listed on the methodology page. Nobody involved in this site installs roofs.
Related calculators and guides
3/12 roof pitch FAQ
Is a 3/12 pitch too flat for shingles?
No. The IRC allows asphalt shingles down to 2:12, so a 3/12 roof is comfortably above the minimum. What changes at 3/12 is the underlayment: every slope below 4:12 needs two layers instead of one.
Is a 3/12 roof considered flat?
No. A flat or low-slope roof in the membrane sense is under 2:12, where shingles are no longer allowed and you need TPO, EPDM, modified bitumen or standing-seam metal. A 3/12 roof is a shingled pitched roof that simply sits at the shallow end.
Can you walk on a 3/12 roof?
Yes, more easily than on any other common residential pitch. At 14.04 degrees there is very little to slide down.
How much roof is on a 3/12 house?
Multiply the footprint by 1.031. A 2,000 square foot footprint gives about 2,060 square feet of roof surface, which is roughly 8 percent less than the same house at 6:12.