Understanding Roof Pitch: 4/12, 6/12 and Other Ratios
Reading the X-in-12 notation and what different pitches mean in practice
Reading the notation
A roof pitch of 6/12 (also written 6-in-12) means the roof rises 6 units of height for every 12 units of horizontal run. The '12' is a fixed reference, making it easy to compare different pitches directly — an 8/12 roof is steeper than a 6/12 roof, which is steeper than a 4/12 roof.
The units don't matter as long as they match: 6 inches per 12 inches, or 6 feet per 12 feet, describe the same slope. In metric regions the same slope is usually given as an angle in degrees, or occasionally as a ratio such as 1:2.
Converting pitch to degrees and slope factor
Two other numbers come up constantly once you start estimating. The angle is arctan(rise ÷ 12). The slope factor — sometimes called the pitch multiplier — is √(rise² + 12²) ÷ 12, and it tells you how much longer the sloped surface is than its horizontal run. Multiply a building's plan area by the slope factor and you have the roof surface area — the Roof Area Calculator does this for you, and the Roof Shingle Calculator turns that area into squares and bundles.
| Pitch | Angle | Slope factor | Surface vs. plan | Grade |
|---|---|---|---|---|
| 2/12 | 9.46° | 1.014 | +1.4% | 16.7% |
| 3/12 | 14.04° | 1.031 | +3.1% | 25.0% |
| 4/12 | 18.43° | 1.054 | +5.4% | 33.3% |
| 5/12 | 22.62° | 1.083 | +8.3% | 41.7% |
| 6/12 | 26.57° | 1.118 | +11.8% | 50.0% |
| 7/12 | 30.26° | 1.158 | +15.8% | 58.3% |
| 8/12 | 33.69° | 1.202 | +20.2% | 66.7% |
| 9/12 | 36.87° | 1.250 | +25.0% | 75.0% |
| 10/12 | 39.81° | 1.302 | +30.2% | 83.3% |
| 11/12 | 42.51° | 1.357 | +35.7% | 91.7% |
| 12/12 | 45.00° | 1.414 | +41.4% | 100.0% |
What common pitches mean in practice
The same few numbers decide what a pitch means for a real roof: how much bigger the surface is than the plan, which code and safety thresholds it crosses, and how it is treated in snow design.
3/12 and 4/12
A 3/12 roof is about 14.0° and a 4/12 roof about 18.4°, and their surfaces are only 3–5% larger than the plan area. Both are steep enough for asphalt shingles, but the 2018 and 2021 editions of the International Residential Code require double underlayment on shingle roofs from 2/12 up to 4/12 (Section R905.2.2). Where one of those editions is adopted, a 3/12 shingle roof needs more protection underneath than a 4/12 one; local amendments and the shingle manufacturer's instructions can be stricter.
4/12 is also the dividing line in OSHA's construction rules: a roof at 4/12 or less is a low-slope roof, and anything steeper is a steep roof. For snow, the ASCE/SEI 7-22 structural standard applies its minimum roof snow load only to slopes under 15°, which takes in 3/12 but not 4/12.
6/12
A 6/12 roof rises half a unit for every unit of run: 26.6°, a 50% grade and a slope factor of 1.118, so the surface is about 12% larger than the plan and each foot of run needs about 1.12 ft of rafter. It is comfortably above the IRC shingle minimum, and under OSHA's construction rules it is a steep roof, so construction work on it falls under OSHA's steep-roof fall-protection requirements.
8/12 to 12/12
From 8/12 (33.7°) to 12/12 (45°), the roof surface is 20–41% larger than the plan, which shows up directly in the shingle, underlayment and labor estimate. In ASCE/SEI 7-22, the design snow load on a roof with a thermal factor (Ct) of 1.0 or less and a non-slippery surface such as shingles starts to reduce only above 30°, just under 7/12; unobstructed slippery surfaces such as metal can qualify for a reduction at much lower slopes, subject to the standard's conditions. The Snow Load Calculator applies the ASCE 7-22 slope factor, but the edition your local building code references may differ.
Steeper pitches also cut deeper into the rafter where it sits on the wall: with a 3.5 in seat on a 2×8, the birdsmouth notch removes about 22% of the rafter's depth at 6/12 but about 34% at 12/12. See What Is a Birdsmouth Cut on a Rafter? for the limits.
How to measure the pitch of an existing roof
You don't need to climb onto the roof to find its pitch: it can be measured from the attic, worked out from the gable end, or read with a phone level. The guide to calculating roof pitch walks through each method with worked numbers, the Roof Pitch Calculator turns the measurements into pitch, angle and slope factor, and the Rafter Length Calculator takes the pitch on to rafter length.
Rough categories
These are trade rules of thumb rather than code definitions. OSHA's construction rules draw the line lower, treating any roof steeper than 4/12 as a steep roof.
- Low-slope: under about 3/12 — often needs a different roofing system than steep-slope shingle roofs
- Moderate/walkable: roughly 3/12 to 6/12 — common on many residential homes, generally safe to walk with care
- Steep: above roughly 6/12 — sheds water and snow well but usually requires fall-protection for roof work
Pitch, slope and angle aren't quite the same word
In everyday use, 'pitch' and 'slope' both mean rise per 12 in of run. Strictly, traditional carpentry defines pitch as rise divided by the full span, so a 6/12 slope is a '1/4 pitch' and a 12/12 slope is a '1/2 pitch'. You'll still see this in older books and some framing tables. If a drawing gives a fraction without the '/12', check which convention it uses before cutting anything.
Why pitch matters beyond looks
Pitch affects drainage speed, snow shedding, attic ventilation design, and which roofing materials are appropriate — some products have a minimum pitch requirement below which they aren't rated to perform reliably. It also directly affects the roof's total surface area via the slope factor, which changes how much material a given footprint actually requires.
Both effects are covered in How Roof Pitch Affects Roofing Materials, and the difference between plan area and surface area is explained in Roof Area vs Floor Area.
Common mistakes
- Measuring rise along the sloped surface instead of vertically.
- Mixing up a pitch ratio with an angle — 6/12 is about 26.6°, not 6° or 60°.
- Estimating shingles from the building footprint without applying the slope factor.
- Assuming both sides of a roof share the same pitch; check each plane on complex roofs.
Quick recap
Pitch is rise per 12 of run. Convert it to an angle for tools and code tables, and to a slope factor for area and rafter length. For step-by-step measurement, see the guide to calculating roof pitch.
FAQ
- What does a 6/12 roof pitch mean?
- The roof rises 6 inches vertically for every 12 inches of horizontal run. That is an angle of about 26.6° and a slope factor of about 1.118, so the roof surface is roughly 11.8% larger than the area it covers in plan.
- What is a 4/12 pitch in degrees?
- About 18.4°. The angle is the arctangent of the rise divided by 12, so arctan(4 ÷ 12) = 18.43°.
- Is an 8/12 roof walkable?
- An 8/12 roof (about 33.7°) is generally considered steep. Many people can stand on it in good conditions with proper footwear, but it is not safe to work on without fall protection, and OSHA treats anything above 4/12 as a steep roof for construction work.
- What is the lowest pitch for asphalt shingles?
- Under the 2018 and 2021 editions of the International Residential Code, asphalt shingles need a slope of at least 2/12, with double underlayment required between 2/12 and 4/12. Check which edition your area has adopted, any local amendments, and the specific manufacturer's installation instructions as well.
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Sources
IRC Chapter 9
Roof assemblies — minimum slopes for roof coverings.
IRC Section R905.2.2 (2018 and 2021 editions)
Asphalt shingles: minimum slope 2/12; double underlayment from 2/12 up to 4/12. Applies where adopted; check local amendments.
OSHA 29 CFR 1926.500
Definitions of low-slope (≤ 4 in 12) and steep roofs for fall protection.
ASCE/SEI 7-22, Chapter 7
Roof slope factor Cs (Fig. 7.4-1) and the minimum roof snow load for slopes under 15°. The edition referenced by local codes may differ.