How to Measure Roof Pitch (Safely, From the Attic)
What pitch means
Roof pitch is expressed as rise over a 12 in run: a "6/12 roof" climbs 6 in vertically for every 12 in horizontally. It is a ratio, so it does not matter how big the roof is. Converting to degrees is one arctangent: 6/12 is 26.57°, 4/12 is 18.43°, and 12/12 is exactly 45°.
Method 1: level and tape in the attic (best)
- Take a 12 in level (or a longer one marked at 12 in) and a tape measure into the attic.
- Hold one end of the level against the underside of a rafter and bring the bubble to centre — the level must be truly horizontal.
- Measure vertically from the 12 in mark on the level up to the underside of the rafter.
- That measurement, in inches, is the pitch. Six inches means 6/12.
This is the method to prefer: you are inside, on a joist, not on a slope. Measure against a rafter rather than the sheathing, and take it in two places to catch a rafter that has sagged.
Method 2: smartphone inclinometer
An inclinometer or bubble-level app reads the angle directly when the phone is laid flat against a rafter. It gives degrees, which the roof pitch calculator converts to rise over 12 and to a slope factor. Calibrate the app on a known-level surface first, and keep the phone flat against the wood rather than against a nail plate or a joint.
Method 3: from the gable end, outside
If the attic is inaccessible, you can measure at the gable end from a ladder: hold the level horizontally against the rake board or the roof edge and measure up to the roof line the same way. It is less accurate because the rake trim may not follow the roof plane exactly — and it involves a ladder, so it is the last resort rather than the first.
Pitch, degrees and slope factor
| Pitch | Degrees | Slope factor | What it means |
|---|---|---|---|
| 1/12 | 4.76° | 1.0035 | Membrane roofing only |
| 2/12 | 9.46° | 1.0138 | Shingle minimum |
| 3/12 | 14.04° | 1.0308 | Double underlayment |
| 4/12 | 18.43° | 1.0541 | Common, walkable |
| 5/12 | 22.62° | 1.0833 | Common |
| 6/12 | 26.57° | 1.1180 | Very common |
| 7/12 | 30.26° | 1.1577 | Upper limit of walkable |
| 8/12 | 33.69° | 1.2019 | Roof jacks needed |
| 10/12 | 39.81° | 1.3017 | Steep |
| 12/12 | 45.00° | 1.4142 | Very steep |
Why you need the number
- Material quantity. The slope factor converts the building footprint into real roof area. A 1,200 sq ft footprint at 6/12 is 1,342 sq ft of roof — order off the footprint and you are short by more than a square.
- What roofing is allowed. Asphalt shingles are generally limited to 2/12 and above, with doubled underlayment required below 4/12. Under 2/12 you need a membrane system.
- Access and cost. Above about 7/12 the work needs roof jacks or harnesses, which shows up in labour quotes.
- Rafter and truss lengths. Rafter length equals run × slope factor, so the pitch sets your cut list.
Enter whichever value you measured — pitch, degrees, or a raw rise and run — into the roof pitch calculator and it returns the other two, plus slope factor, roof area, squares and bundle count.
Convert between rise-over-run and degrees, then turn a footprint into real roof area — with the slope factor, rafter length, squares and shingle bundles you need to order.
Frequently asked questions
What is a 6/12 pitch in degrees?
26.57 degrees. The angle is the arctangent of the rise divided by 12, so 6 over 12 gives 26.57. A 12/12 pitch is exactly 45 degrees.
Can I measure roof pitch with a phone?
Yes. An inclinometer app laid flat against a rafter reads the angle directly, which converts to rise over 12. Calibrate it on a surface you know is level first, and measure against solid wood rather than a plate or joint.
What is the steepest roof you can walk on?
Around 7/12 is the practical limit for most people in normal footwear, and even that is unsafe when wet or mossy. From 8/12 up, roof jacks, staging or a harness are required rather than optional.
- How Many Shingles Do I Need? Squares, Bundles and the Slope Factor
- Hip Roof vs Gable Roof: Calculating the Area
- How Much Underlayment and Ice Shield Do You Need?
- What Is a Good Roof Pitch?