Roof pitch shows up in nearly every roofing decision — material choice, material quantity, whether you need scaffolding, whether the shingle warranty is even valid. It's a simple number, and measuring it is easier and safer than most people assume.
What the numbers mean
Pitch is expressed as rise : run, and the run is always 12. A 6:12 roof rises 6 inches over 12 inches of horizontal distance. A 4:12 is shallower; a 12:12 is a true 45 degrees.
The run being fixed at 12 is a convention, not a measurement — it exists so any two roofs can be compared directly. That's why you never see "6:10" even though it would describe a real slope.
| Pitch | Angle | Character |
|---|---|---|
| 1:12 – 3:12 | 5° – 14° | Low slope — needs special roofing |
| 4:12 – 6:12 | 18° – 27° | Conventional — walkable, most common |
| 7:12 – 9:12 | 30° – 37° | Steep — needs roof jacks or staging |
| 10:12+ | 40°+ | Very steep — scaffolding, sheds snow well |
How to measure it — from the attic
Do this inside. Getting on a roof to measure its slope is how people fall off roofs, and the attic gives you the same number more accurately.
Take a 24-inch level and a tape measure. Hold the level horizontally against the underside of a rafter, get the bubble centred, then measure straight up from the 12-inch mark on the level to the rafter. That vertical distance is your rise. If it's 6 inches, you have a 6:12 roof.
No attic access? Hold the level against the gable end from a ladder — same method, just be sensible about the ladder.
Why pitch decides your roofing material
This is the part that matters most, and it's not a style preference — it's physics. Shallow roofs drain slowly, so wind can drive rain up under the overlaps. Every roofing product has a minimum pitch below which it will leak and the warranty is void.
| Material | Minimum pitch |
|---|---|
| Asphalt shingles | 2:12 (with double underlayment below 4:12) |
| Standing seam metal | 1:12 – 3:12 |
| Clay / concrete tile | 2.5:12 – 4:12 |
| Slate | 4:12 |
| Membrane (EPDM, TPO) | 0.25:12 — for flat roofs |
Check the manufacturer's spec for your exact product. "It's basically flat, shingles will be fine" is the reasoning behind a lot of leaking roofs.
Pitch changes how much material you buy
Here's the trap. Your roof's footprint is not its surface area. A steeper roof covers the same building with more material.
For a 6:12 roof spanning 12 ft horizontally: √(6² + 12²) = √180 ≈ 13.4 ft. So 12 feet of building needs 13.4 feet of rafter — about 12% more than the footprint suggests.
At 12:12 that multiplier climbs to roughly 41%. Estimating a steep roof from its floor plan will leave you badly short. Our saltbox roof calculator handles this properly — it takes your building dimensions and wall heights and returns roof area, rafter lengths, ridge height and attic volume, so the slope is built into the numbers rather than something you correct for afterwards.
Choosing a pitch for a new build
Snow country: steeper. Snow slides off rather than accumulating, and roof collapse is a real risk with heavy wet snow on a shallow roof.
High wind: shallower. A steep roof is a sail. There's a genuine tension here if you get both weather types, and local convention usually reflects the right compromise.
Attic space: steeper. Pitch is what creates usable headroom — the difference between storage and a room.
Budget: shallower. Less material, less labour, no staging.
If your area has an obvious dominant roof pitch, that's usually a century of accumulated local knowledge about the local weather. It's worth taking seriously.
Once you know your pitch, our lumber calculator and framing calculator handle rafter and framing quantities.
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Frequently Asked Questions
What does 6:12 roof pitch mean?
The roof rises 6 inches for every 12 inches of horizontal run, which works out to about 27 degrees. The run is always expressed as 12 so any two roofs can be compared.
How do I measure roof pitch?
From inside the attic. Hold a 24-inch level against a rafter, level the bubble, then measure vertically from the 12-inch mark up to the rafter. That rise is your pitch.
What is the minimum roof pitch for shingles?
Asphalt shingles generally need at least 2:12, and below 4:12 they require a double layer of underlayment. Going below the minimum voids most warranties.
Does roof pitch affect how much material I need?
Yes, significantly. A 6:12 roof needs about 12% more material than its footprint suggests, and a 12:12 roof about 41% more, because surface area grows with slope.