4/12 Roof Pitch: Degrees, Multiplier and What It Means
A 4/12 roof rises 4 inches for every 12 inches of run. That is 18.43°, a pitch multiplier of 1.054 and a slope of 33.3%. Here is what those numbers do to your roof area, your materials and your quote.
4/12 at a glance
Tract housing, garages, and the shallow end of ordinary residential framing.
Nearby pitches
4 in rise per 12 in run
atan(4 ÷ 12) = 18.43°
√(4² + 12²) ÷ 12 = 1.054
4 ÷ 12 × 100 = 33.3%
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 4/12 roof pitch means
A 4/12 roof rises 4 inches for every 12 inches of horizontal run. It is the pitch at which a roof stops being treated as low-slope by the building code and starts being treated as an ordinary pitched roof. That makes it a genuine dividing line rather than just another number on the chart.
4/12 in degrees
The angle is the arctangent of rise over run:
atan(4 ÷ 12) = atan(0.3333) = 18.43°
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 4/12 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:
√(4² + 12²) ÷ 12 = √160 ÷ 12 = 1.054
So a 2,000 square foot footprint on a 4/12 roof is about 2,110 square feet of actual roof surface, or 21.1 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 4/12 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 4/12 roof has 5.7% 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 4:12 | Roof at 6:12 | Difference |
|---|---|---|---|
| 1,200 | 1,265 sq ft | 1,340 sq ft | -5.6% |
| 1,600 | 1,685 sq ft | 1,790 sq ft | -5.9% |
| 2,000 | 2,110 sq ft | 2,235 sq ft | -5.6% |
| 2,400 | 2,530 sq ft | 2,685 sq ft | -5.8% |
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 4/12 roof
A 4/12 roof is straightforward to walk. It is well below the slope at which the HomeGuide cost guide says extra safety precautions and extra labour start.
4/12 is where single-layer underlayment begins
The IRC requires two layers of underlayment on every slope from 2:12 up to 4:12. At 4:12 you are at the top of that range, and standard single-layer underlayment applies from here upward. Practically every shingle, metal and synthetic product on the market is rated for 4/12 with no special detailing, which is why so much tract housing is framed at exactly this pitch.
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
4/12 roof pitch FAQ
What is a 4/12 pitch in degrees?
18.43 degrees. The arithmetic is atan(4 divided by 12), which is atan(0.3333).
Is a 4/12 roof a low slope roof?
It is the boundary. The IRC’s double-underlayment rule covers slopes from 2:12 up to 4:12, so 4/12 is the point at which normal single-layer underlayment takes over. Roofers generally call anything under 4/12 low-slope.
Do you need double underlayment on a 4/12 roof?
No. Double underlayment applies below 4:12. At 4:12 and above, one layer is the code minimum, though individual manufacturers can require more to honour their warranty.
Can you put a metal roof on a 4/12 pitch?
Yes. 4/12 is above the minimum slope for both standing-seam and exposed-fastener metal panels. Check the specific product’s instructions, because minimum slopes are set by the panel profile rather than by metal in general.