Roof pitch is the most-shorthand number in residential construction. A single notation — "6/12" or "8/12" — drives the rafter cut, the rise of the ridge, the slope of the fascia, the shingle quantity, and the labour cost of working on the roof. This guide explains how to read it, what each number means in practice, and the values that matter for code compliance.
What "6/12" actually means
A roof pitch is written as rise over run. The first number is the vertical rise in inches; the second is the horizontal run in inches. "6/12" means six inches of vertical rise for every twelve inches of horizontal run.
The 12 in the denominator is a convention. North American carpentry has always used 12 inches as the standard run because the rafter square is divided into twelfths. So pitches are always quoted as some-number over 12: 1/12, 2/12, 3/12 ... up to 12/12 (a 45-degree roof) and beyond.
The notation on a drawing is usually a small triangle in the elevation, with two numbers — the pitch — written along the slope of the triangle. The vertical leg has the rise; the horizontal leg has 12. You will see this every time on US residential drawings; on Canadian metric drawings you sometimes see degrees instead, but pitch-in-12 is still extremely common because the materials are imperial.
For the math, see the Roof Pitch Calculator — type the pitch and the run, get the rafter length, rise, and angle in degrees.
What different pitches mean on a real roof
Below 2/12 (under 9.5°): Low-slope. Cannot use standard asphalt shingles — manufacturers require 2/12 minimum with double underlayment. Low-slope roofs use membrane systems (EPDM, TPO, modified bitumen) instead. Common on flat-roofed modern homes and commercial buildings, plus on porch overhangs and certain dormer connections.
2/12 to 4/12 (9.5° to 18.5°): Shallow but shingle-compatible with extra underlayment. 2/12 is the absolute minimum for asphalt shingles (per most manufacturers), and even then it requires ice-and-water shield over the entire roof, plus two layers of underlayment. Below 4/12, you can also expect issues with wind-driven rain getting under the shingles.
4/12 to 7/12 (18.5° to 30.3°): Standard residential. Single layer of underlayment, asphalt-shingle warranty applies, walkable for trades without harness or staging. 4/12 is the most common pitch on US residential drawings; 5/12 to 7/12 is common in colder climates where steeper roofs shed snow better.
8/12 to 12/12 (33.7° to 45°): Steep. Above 8/12 the labour cost rises sharply because crews need staging or harnesses. Materials cost rises too because of the steeper-slope shingle premium some manufacturers charge. 12/12 is the famous "A-frame" pitch and is visually dramatic.
Above 12/12 (above 45°): Decorative or specialty. Steeples, mansards, decorative dormers, Victorian and Tudor styles. Above 18/12, walking is not really possible — work is done from staging hung off the roof or from the inside of the rafter cavity.
Code-minimum pitch by roofing material
The minimum pitch depends on the material. From the 2018 IRC (US) and similar Canadian provincial codes:
- Asphalt shingles: 2/12 minimum (with double underlayment + ice/water shield); 4/12 single layer.
- Wood shake or shingle: 4/12 minimum.
- Concrete or clay tile: 2.5/12 minimum (with underlayment); 4/12 typical.
- Slate: 4/12 minimum; 6/12 typical.
- Standing-seam metal: 1/4 in 12 minimum (0.25/12, basically flat) for non-residential; 3/12 for residential.
- Modified bitumen / EPDM / TPO membranes: 1/4 in 12 minimum (0.25/12) — these are the membranes used on flat roofs.
Always check the local code edition and any provincial or municipal amendments. The minimums above can be raised by the local authority.
What the pitch affects beyond the rafters
Shingle quantity. Surface area on a sloped roof is more than the floor area below it. The multiplier is the "slope factor" — for a 4/12 roof it is about 1.054, for 8/12 it is 1.202, for 12/12 it is 1.414. A 1,500 ft² footprint at 8/12 has 1,500 × 1.202 = 1,803 ft² of actual roof surface. Get the slope factor wrong and you under-order shingles.
Snow load (engineering). Steeper roofs shed snow more easily but the load that does accumulate is harder to predict because it depends on wind direction. Above 30°, building codes reduce the snow-load assumption in the structural calculation; below 10°, they may increase it. For load calculations, use the local building code's snow-load tables — but as a thumb rule, steeper roofs let you frame with smaller rafters because the design snow load is lower.
Wind uplift. Steeper roofs have more wind load on the slopes but less on the eaves; shallower roofs are the opposite. The roofing-material warranty often specifies a maximum wind speed by pitch range.
Drainage. Flat-ish roofs (below 4/12) need positive drainage to a scupper or drain. Above 4/12, gravity handles it as long as there is a gutter at the eave.
Code-required ice-and-water shield. In cold climates, code requires ice-and-water membrane in the bottom several feet of every slope. The length of the bottom membrane depends on the pitch — shallower roofs need wider membrane bands because ice dams form farther from the eave.
Reading a pitch on a real drawing
On a typical elevation, you will see the triangle notation: the small symbol with the rise-over-run numbers written along the hypotenuse. The pitch usually appears once per roof slope, at one end of the rake.
On a building section, the rafter angle is drawn at the actual pitch and labelled with the rise/run notation. Compare the labelled pitch with the drawn angle — they should match. If you measure the drawn angle and convert to pitch and the numbers do not agree, the drawing is wrong somewhere.
On a roof plan (top-down view), the pitch is often called out next to an arrow that shows the slope direction. Multiple slopes are common on hip and complex roofs; each gets its own callout.
The notation differs by convention. US drawings almost always use "in 12" notation. Canadian metric drawings sometimes use degrees ("26.57°" for the equivalent of 6/12). European drawings always use degrees or percent grade. The math is the same; only the labelling changes. See the Roof Pitch Calculator to switch between all three.
When the pitch does not match the drawing
Catch: drawings are sometimes printed at the wrong scale, and the "drawn" angle does not match the labelled pitch. Always trust the labelled pitch over the drawn angle for any layout or fabrication work. If you suspect the drawing has scaled incorrectly, measure the scale bar in the title block.
For framers laying out a roof, the labelled pitch is the only number that matters. The framing square is set to that pitch, and the rafters are cut accordingly. The drawn elevation is for the visual presentation, not for layout.
Calculators on this site
- Roof Pitch Calculator — pitch to degrees to percent grade, plus rafter length and rise for any run.
- Right Triangle Calculator — for laying out the rafter triangle from any two known values.
- Architectural Scale Converter — verify the drawing scale before measuring anything.
Sources
- IRC 2018 International Residential Code, Chapter 9 (Roof Assemblies).
- NRCA Roofing Manual (slope and material requirements by region).
- CertainTeed, Owens Corning, GAF, IKO — asphalt-shingle manufacturer technical bulletins for minimum-slope requirements.
- CMHC Residential Construction references for Canadian framing conventions.