Roof Pitch Calculator

How roof pitch is written

American roofers state pitch as rise per 12 inches of run, written x:12. A 6:12 roof climbs 6 inches for every foot travelled horizontally. The 12 never changes — only the first number does, and that number is the whole description of the roof.

The run is measured on the horizontal, from the outside face of the wall in to the centre of the ridge. That is half the building span on a symmetrical gable roof — not the full width, and not the distance up the slope. Getting this wrong is the single most expensive mistake on this page, because it doubles every rafter you cut.

The same roof can be described three other ways, and this calculator returns all of them at once:

• Angle — degrees from horizontal, useful for setting a saw or a solar array.
• Percent grade — rise ÷ run × 100, the form drainage and civil drawings use.
• Slope as a decimal — plain rise ÷ run, what the algebra calls m.

Measuring the pitch on an existing roof

You need a 2 ft level and a tape. You do not need to get on the roof.

Measuring in the attic works just as well: put the level against the underside of a rafter and take the same 12 inch reading. On a finished roof you can also measure the ridge height above the wall plate and divide by the run.

Pitch, angle, percent and multiplier

Notice that 100% grade is 45°, not 90°. Percent grade and degrees are not the same scale, and they only agree at 0. Anything you read as a percent on a civil drawing has to be converted before it means anything to a framer — the percent grade calculator does that both ways.

Getting the rafter length

The rafter multiplier converts horizontal run into actual rafter length. It is the hypotenuse of the pitch triangle divided by its base, so it is always slightly more than 1:

The multiplier is also the quickest way to price a roof. Multiply the flat footprint area by it and you get the true sloped area — a 6:12 roof has about 12% more surface than the floor beneath it, so a 2,000 sq ft footprint needs roughly 2,236 sq ft of covering before waste.

Choosing a pitch for a new roof

• Below 2:12 — treated as a low-slope roof. Shingles are not permitted at all; you need a membrane system.
• 2:12 to 4:12 — shingles are allowed in most codes but only with a doubled underlayment. Watch for ice damming in snow country.
• 4:12 to 9:12 — the conventional range. Sheds water well, walkable for maintenance, cheapest to frame and to cover.
• Above 9:12 — sheds snow readily and gives usable attic space, but costs more in material, staging and labour. Wind uplift rises as well.

These are general ranges. Your local code, your shingle manufacturer's written instructions and your local snow and wind loads decide what is actually permitted — and the manufacturer's instructions are what a warranty claim is judged against. Check all three before ordering material.

What each material needs

Minimum pitch is set by how long water sits on the surface. The steeper the roof, the less time water has to find a lap joint, so steeper roofs can use materials with more seams.

• Asphalt shingles — 4:12 standard, 2:12 absolute minimum with doubled underlayment.
• Standing seam metal — commonly 2:12 and up; some panel profiles go lower with sealed seams.
• Exposed fastener metal panels — usually 3:12 and up.
• Clay and concrete tile — typically 2.5:12 minimum, with underlayment requirements that tighten below 4:12.
• Wood shakes and shingles — 4:12 and up.
• Slate — 4:12 and up, and heavy enough that the framing has to be checked.
• Single-ply membrane (TPO, EPDM, PVC) — anything down to 1/4:12, which is where the low-slope drainage minimum takes over.

Below about 1/4 inch per foot you are no longer designing a roof pitch, you are designing drainage — the same rules that govern a drain line.

Common mistakes

• Using the full span instead of half. The run is wall to ridge centre, not wall to wall. Using the full span doubles every rafter.
• Measuring along the rafter. The run is the horizontal projection. The sloped distance is the answer, not the input.
• Reading percent as degrees. A 100% grade is 45°, not 90°.
• Forgetting the ridge board. Subtract half its thickness from the calculated rafter or every pair sits proud at the peak.
• Ordering material off the footprint. A 6:12 roof needs about 12% more covering than the floor area under it, before waste.
• Mixing units. A rise in inches over a run in feet is out by a factor of 12. Use the unit selectors above rather than converting in your head.

Related tools: the slope calculator for the underlying rise-over-run maths, the percent grade calculator for civil drawings, and the ramp slope calculator when the slope has to meet ADA limits instead of shedding water.