How to Work Out Roof Pitch: Degrees, Ratios and Percent Explained

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ToolTally — angle finder with roof pitch, grade and ratio readouts, a free Android app for tradespeople

Roof pitch is how steep a roof is, and it gets written three different ways depending on where you are and who you ask: in degrees, as a ratio such as “6 in 12”, or as a percentage. Here is what each one means, how to convert between them, how to measure a roof safely, and a quick reference for the common pitches.

What roof pitch means

Pitch compares how far a roof rises with how far it runs horizontally. The run is the horizontal distance, usually half the width of the building for a simple gable roof. The rise is how much the roof climbs over that distance.

  • Degrees are the angle between the roof and horizontal. This is the usual way in New Zealand, Australia and the UK.
  • “X in 12” is the rise in inches for every 12 inches of run. A 6 in 12 roof climbs 6 inches for every foot across. This is the usual way in the US and Canada.
  • Percentage is rise divided by run, times 100. A 6 in 12 roof is a 50 per cent slope.
  • Ratio is sometimes written as 1 : N, meaning one unit of rise for every N units of run.

How to convert

All of them come from the same rise and run:

  • Degrees = arctan(rise ÷ run)
  • X in 12 = 12 × tan(angle)
  • Percentage = 100 × rise ÷ run

Example: a roof that rises 1.8 m over a run of 3.6 m. Rise ÷ run = 0.5, and arctan(0.5) = 26.6°. That is a 6 in 12 pitch, or a 50 per cent slope.

Common pitches at a glance

  • 3 in 12 = 14.0° = 25%
  • 4 in 12 = 18.4° = 33%
  • 5 in 12 = 22.6° = 42%
  • 6 in 12 = 26.6° = 50%
  • 7 in 12 = 30.3° = 58%
  • 8 in 12 = 33.7° = 67%
  • 9 in 12 = 36.9° = 75%
  • 10 in 12 = 39.8° = 83%
  • 12 in 12 = 45.0° = 100%

And the other way, for roofs specified in degrees: 15° is about 3.2 in 12, 20° about 4.4 in 12, 25° about 5.6 in 12, 30° about 6.9 in 12 and 35° about 8.4 in 12.

How to measure a roof’s pitch

  • From the plans. The fastest and safest option. Pitch is usually written on the elevations or the roof plan.
  • From the roof space. Hold a level horizontally against the underside of a rafter, with one end touching it. Measure a set distance along the level, such as 300 mm or 12 inches, then measure straight up from that mark to the rafter. That is your rise over that run.
  • From the gable end. Measure the height from the top of the wall to the ridge and half the width of the building. Rise ÷ run gives you the pitch.
  • With an angle finder. Lay a digital angle finder, or a phone app, on a rafter, a barge board or a straight edge resting on the roof surface.

Measure from the ground or the roof space where you can. If you do have to work at height, use proper access and fall protection; no measurement is worth a fall.

Why it matters

  • Roofing materials each have a minimum pitch they are designed for. Check the manufacturer’s specification for the product you are using.
  • Roof area is larger than the floor area it covers. Divide the plan area by the cosine of the pitch angle: 120 m² of plan at 25° is about 132 m² of roof.
  • Drainage, snow and solar panels all depend on the angle too.

A pitch reading in your pocket

ToolTally is a free Android app for tradespeople, and its bonus toolbox includes a spirit level and angle finder that shows the angle in degrees together with the same slope as an “X in 12” roof pitch, a percentage grade and a 1 : N rise-to-run ratio, so there is nothing to convert. The toolbox also has an AR camera measure, a trade calculator for feet-inch-fractions, a unit converter and a stud detector.

The main job of the app is a private, offline record of every tool you own, with photos, serial numbers and warranty reminders, and an Insurance Claim Pack that puts it all into one file for your insurer. It runs without an account or a signal, and it is free and ad-supported.

Get ToolTally on Google Play

More on the app: the ToolTally page. Phone angle readings are a guide for rough work on site, not a certified measuring instrument. For anything structural, follow the plans, the manufacturer’s specifications and your local building code.

Frequently asked questions

How do I convert roof pitch to degrees?

Divide the rise by the run and take the arctangent. For a 6 in 12 roof, 6 ÷ 12 = 0.5, and arctan(0.5) = 26.6°.

What is a 4 in 12 roof pitch in degrees?

About 18.4°, or a 33 per cent slope.

What does a 6/12 roof pitch mean?

The roof rises 6 inches for every 12 inches of horizontal run. That is 26.6°, or a 50 per cent slope.

Can I measure roof pitch with my phone?

Yes, with an angle finder app laid on a rafter or a straight edge. It is fine for a rough check; for anything critical, confirm it with a level and a tape measure.

How do I work out roof area from the pitch?

Divide the plan area by the cosine of the pitch angle. At 30°, for example, the roof is about 15 per cent larger than the area it covers.

Get ToolTally on Google Play

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