How to Measure Roof Pitch

Pitch is the rise of a roof, in inches, for every 12 inches of level run. Measuring it means finding one level line and one vertical line and reading the distance between them. The four methods below differ in where you do that and how safe it is. Pick the one that gets you closest to the roof with the least risk, then check the answer one of the ways at the end.

You need pitch to pick a roofing material (every covering has a code minimum), to cut rafters, and to work out how much roof there is. Once you have a reading, type it into theroof pitch calculator to get the angle, slope and rafter multiplier.

What you need

• A spirit level at least 12 inches long. A 24 inch level is easier to hold and averages out small bumps.
• A tape measure and a pencil.
• For method 3, a phone with an inclinometer or level app, or a digital angle finder.
• For any work on the roof itself: a properly footed ladder, soft-soled shoes, dry weather, and someone on the ground. If the roof is steep, wet or high, use a method that keeps you off it.

Method 1: from the attic (safest)

This is the method This Old House recommends. You measure off a rafter, which runs parallel to the roof surface.

1. Find a straight, exposed rafter away from any dips or repairs.
2. Place the end of the level against the bottom edge of the rafter and hold the level perfectly level, with the bubble centred.
3. Measure 12 inches along the level from the end that touches the rafter and make a pencil mark.
4. Measure straight up, vertically, from the 12 inch mark to the underside of the rafter.

That vertical distance in inches is the rise per 12, the pitch. A 7 inch reading is a 7/12 roof. If deep insulation makes the middle of the attic hard to reach, use a rafter near the gable wall or the attic hatch, wherever you can stand safely on joists.

Method 2: on the roof

Fine Homebuilding describes the same measurement made from the outside. Place the level horizontally against the roof so its end touches the roof deck, mark 12 inches along it, then measure from that mark straight down to the roof surface. The principle is identical: level run of 12, vertical rise.

You don't have to climb onto the field of the roof. From a ladder at the gable end you can often reach the rake (the sloped edge). Hold the level against the underside of the rake board or the roof edge and measure the same way. Do it near the bottom of the slope, where the roof is closest to the ladder.

Method 3: read the angle with a phone or angle finder

An inclinometer reads the angle directly. Lay the phone (or a digital angle finder) on a rafter in the attic, on the rake board, or on the roof surface, along the slope. Read the angle in degrees, then convert it: pitch = tan(angle) × 12. The roof pitch calculator's angle modedoes this for you, so 26.6° becomes 6/12.

Phones aren't calibrated instruments. Before you trust the reading, set the phone on a surface you know is level and check it reads 0°, then turn it end for end and check again. A one-degree error near 6/12 shifts the pitch by about a quarter of an inch per foot.

Method 4: from plans or a framing square

If you have the house plans, the elevation drawings usually show the pitch as a small right-angled triangle on the roof line, with 12 on the level side and the rise on the other. The US Navy's carpentry manual describes pitch the same way, as units of rise per 12-inch unit of run.

If you have a rafter you can reach and a framing square, hold the square against the rafter edge with its blade level and read where the rafter crosses the tongue at the 12 inch mark on the blade. It's method 1 with a square in place of a level.

Turning a reading into pitch

If you measured over 12 inches, the reading is the pitch. If you measured over a longer level, divide by the run and multiply by 12. For example, a 10½ inch rise over a 24 inch level is 10.5 ÷ 24 × 12 =5.25/12, or about 23.6°.

For every whole pitch to 24/12 with rafter and hip factors, see the roof pitch chart.

Worked example: a porch roof you can't get above

A back porch has no attic and a low, shallow roof. From a ladder at the end of the porch you can reach the rake board. You hold a 24 inch level against its underside, centre the bubble, and measure straight up from the 24 inch end of the level to the board: 7 inches.

1. Pitch = 7 ÷ 24 × 12 = 3.5/12.
2. Angle = arctan(7 ÷ 24) = 16.3°.
3. A second reading 3 ft along the rake gives 7⅛ inches, which is 3.56/12. The readings agree within ⅛ inch, so take 3.5/12.

That number decides what you can put on it. At 3.5/12, the 2021 IRC allows asphalt shingles but only with double underlayment, because the pitch is under 4/12. Wood shingles and shakes (3/12 minimum) are allowed, just. Slate (4/12) is not. Had you guessed "about 4/12" from the ground, you would have planned one layer of underlayment where the code requires two.

Measuring in metric

The method doesn't change in metric. Measure the rise over any level run, in the same unit. Pitch in x/12 is still rise ÷ run × 12, because it is a ratio. A 25 cm rise over a 100 cm level is 25 ÷ 100 × 12 = 3/12, or 14°. If you'd rather work in degrees, a phone inclinometer (method 3) reads the angle directly. The calculator's metric setting takes cm for rise and run and metres for rafter lengths.

Check your reading

• Measure twice, in different places. Old rafters sag and sheathing has bumps. Two readings that agree within ⅛ inch are good. If they don't, take a third.
• Cross-check with an angle. If you measured rise and run, check the angle with a phone. The calculator shows both, so they should match.
• Sanity-check the result. Hold the printed pitch chart's to-scale triangles up against the roof from the ground. If your reading and the closest-looking triangle are far apart, the level probably wasn't level.
• Check every roof section. Porches, dormers and additions are often a different pitch from the main roof. Measure each one you'll be roofing or framing.

Mistakes that give the wrong pitch

• Measuring 12 inches along the slope. The 12 must be level. Measured along the roof, the run is too short and the pitch comes out low.
• A tilted tape. The rise must be vertical. Hold the tape plumb, or use a small square against the level.
• Measuring off a sagging rafter. Pick a straight rafter, or measure near the wall plate, where rafters sag least.
• Rounding to the nearest whole pitch. A 6½/12 roof isn't a 6/12 roof. Keep the half; it changes the roof area and rafter length.

What to do with the number

With the pitch in hand you can do three things. Check it against your roofing's minimum slope: the 2021 International Residential Code allows asphalt shingles from 2/12, with double underlayment below 4/12, and slate only from 4/12. The minimum roof pitch guide has every covering. Work out rafter lengths with the rafter length calculator. And turn your footprint into squares and bundles with theroofing squares calculator.