# How to calculate roof pitch?

Pitch is vertical rise per 12 inches of horizontal run, written as `6:12`. Measure it inside the attic: hold a level horizontally against a rafter, mark 12 inches along it, and measure straight down to the rafter. That vertical distance in inches is your pitch. Nobody needs to be on the roof.

## How do I measure it from the attic?

You need a two-foot level, a tape measure, and a torch.

Hold the level against the underside of a rafter so the bubble reads level. Measure 12 inches out along the level from the point where it touches the rafter, and mark that spot. From the mark, measure straight down to the rafter below. That measurement in inches is the rise: 6 inches down means a 6:12 pitch.

Do it against a rafter rather than the decking, and pick one away from the ridge and the eave where framing details can distort the reading. Stand on the joists.

If your attic is inaccessible, you can take the same measurement against the gable-end trim from a ladder, or estimate from a straight-on photograph of the gable end. The [pitch calculator](/calculators/roof-pitch/) converts rise and run to pitch, degrees and the area multiplier.

## What does pitch actually decide?

Four things, and they are the reason a roofer asks before quoting anything.

**Which coverings are permitted.** Below roughly 2:12, asphalt shingles are not watertight and a low-slope membrane is the correct answer. That is physics and code, not preference — water moves too slowly on a shallow slope and wind drives it back under the courses.

**How much material the roof needs.** A steeper roof has more surface than the footprint beneath it. That is the multiplier, below.

**Where the ice barrier goes.** Under IRC R905.1.2, on roofs sloped 8:12 or steeper the ice barrier is applied not less than 36 inches measured along the slope from the eave edge, in addition to reaching 24 inches inside the exterior wall line.

**Whether anyone can walk it.** Around 6:12 a roof stops being comfortably walkable. By 8:12 it is not walkable in the ordinary sense and needs staging, which shows up in the price.

## What is the pitch multiplier?

The number that converts building footprint into actual roof surface. Roof area is always larger than the footprint underneath, and the steeper the roof the larger the gap.

The formula is `√(rise² + 12²) ÷ 12`. At 6:12 that is `√(36 + 144) ÷ 12 = 1.118` — the roof surface is about 12% bigger than the footprint. At 12:12 it is 1.414, so 41% bigger.

This matters because a bid quoted from a footprint rather than a roof surface is quoting less roof than you have. Check any bid's square count against your own figure using the [roof area calculator](/calculators/roof-area-and-squares/).

| Pitch | Multiplier | Angle |
|---|---|---|
| 2:12 | 1.014 | 9.5° |
| 4:12 | 1.054 | 18.4° |
| 6:12 | 1.118 | 26.6° |
| 8:12 | 1.202 | 33.7° |
| 10:12 | 1.302 | 39.8° |
| 12:12 | 1.414 | 45.0° |

## Is pitch the same as slope?

In everyday roofing conversation they are used interchangeably, and for practical purposes that is fine.

Strictly, in framing terminology pitch is the ratio of rise to the *total span* while slope is rise over run. Almost nobody uses the strict definition on a job site, and a roofer saying "what's the pitch" means rise over 12. If a bid or a permit drawing gives a fraction like 1/4 rather than an X:12 figure, that is the strict sense and worth clarifying.

## What if the roof has more than one pitch?

Most roofs do. Measure each plane separately.

A house with a main gable at 8:12 and a porch or an addition at 3:12 has two different roofing problems on one building: the main roof takes shingles, the porch needs a low-slope membrane, and the transition between them is a common leak point that needs deliberate detailing.

When you calculate area, apply each plane's own multiplier to its own footprint and add the results. Applying the main roof's multiplier to the whole building overstates the low-slope portion.

## Common mistakes

- **Measuring against the decking instead of a rafter.** Sheathing sags slightly between rafters and the reading drifts.
- **Measuring near the ridge or the eave.** Framing details there are not representative of the plane.
- **Applying one multiplier to a multi-pitch roof.** Measure and calculate each plane.
- **Confusing footprint with roof area.** They are never the same number, and the gap grows with pitch.
- **Putting shingles on a 1:12 porch.** Below about 2:12 they will not keep water out, however well they are installed.