Roof Pitch in Degrees: 5° to 45° Roofs Converted to X/12

Degree to pitch conversion chart

Every number in the table comes from one right triangle. Rise ÷ run is the tangent of the angle, so pitch per 12 is 12 × tan(angle) and slope percent is 100 × tan(angle). The rafter is the hypotenuse, so its length per unit of run — the area multiplier — is 1 ÷ cos(angle).

10° and 15° roofs

A 10 degree roof is about 2.12/12. That is just above the 2/12 floor for asphalt shingles under the IRC, so shingles are legal only with two layers of underlayment. Standing-seam metal, membranes and many corrugated or trapezoidal sheet profiles are the usual choice. In the UK and Australia, 10° is a common pitch for lean-to extensions, carports and skillion roofs.

A 15 degree roof is 3.22/12. It opens up wood shingles (3/12 minimum) and most exposed-fastener metal panels, and it is the lowest pitch many interlocking concrete tiles are rated for. Check the tile manufacturer's table: low-pitch tiles often need a fully supported underlay and extra headlap at this angle.

20°, 25° and 30° roofs

20 degrees is 4.37/12 — just past the 4/12 line where shingles need only one underlayment layer. It is a very common pitch for Australian hip roofs and project homes. 25 degrees is 5.6/12, close to the 5/12 and 6/12 pitches that dominate US houses, and is a typical minimum for natural slate in the UK.

30 degrees is 6.93/12, which framers almost always round to 7/12. It is a favourite with solar designers because it is near the best fixed tilt for much of the US, southern Europe and Australia's south-east. It is also the angle above which most crews stop walking the roof and set up roof jacks.

35°, 40° and 45° roofs

35 degrees is 8.4/12 and 40 degrees is 10.07/12. Both are steep-slope roofs: expect staging, fall protection and a labour premium. They shed snow quickly and give tall, usable attics. 45 degrees is exactly 12/12 — rise equals run, the rafter is √2 or 1.4142 times the run, and the roof has 41 percent more surface than the footprint.

How to measure a roof angle

1. In the attic, hold a digital angle finder or a phone level app against the underside of a rafter and read the angle directly.
2. From outside, photograph the gable end square-on and measure the angle in any photo editor or with an on-screen protractor.
3. With a level and tape, measure the rise over a 12-inch run and convert with arctan(rise ÷ 12).
4. Take readings on at least two rafters — older roofs often sag, and additions are frequently built at a different pitch.

How different countries describe roof pitch

In the US and Canada, pitch is almost always written as rise per 12 inches of run, so plans say 6/12 or 6:12. Roofing material instructions also use this form, and the IRC sets minimums such as 2/12 for asphalt shingles and 4/12 for slate.

In the UK, Ireland, Australia, New Zealand and South Africa, roof pitch is given in degrees on drawings, in building regulations and on tile manufacturers' charts. Builders may also talk about a ratio such as 1 in 4 or a fall in millimetres per metre for low-slope roofs. Converting between these systems is just a matter of the tangent: degrees to X/12 is 12 × tan(angle), and a 1 in X ratio is arctan(1 ÷ X).

Minimum roof angle by material

These are typical published values. Exposure to wind-driven rain, rafter length and the underlay used can all raise the minimum for a particular roof, so treat the manufacturer's chart for your location as the deciding figure.

Roof angle and solar panels

For fixed solar panels the best year-round tilt is roughly equal to your latitude, reduced by 5 to 15 degrees in many regions because summer production is higher. In practice anything from about 15 to 40 degrees produces within a few percent of the optimum, which is why most pitched roofs suit flush-mounted panels.

Very low roofs (under 10 degrees) lose some output and stay dirtier because rain does not wash the panels as well; installers often add tilt frames. Very steep roofs (over 45 degrees) lose some summer output but shed snow quickly, which can help in northern climates.

Worked conversions

• 15° to X/12: tan(15°) = 0.2679, × 12 = 3.22/12.
• 22.5° to a ratio: tan(22.5°) = 0.4142, so 1 : (1 ÷ 0.4142) = 1 : 2.41.
• 5/12 to degrees: arctan(5 ÷ 12) = arctan(0.4167) = 22.62°.
• A 1 in 6 fall to degrees: arctan(1 ÷ 6) = 9.46°, the same as 2/12.
• Roof area at 30° on a 120 m² footprint: 120 ÷ cos(30°) = 120 × 1.1547 = 138.6 m².

Most phones have a built-in level or measure app that reads angles directly, and the degrees-to-pitch calculator on this site does the tangent step for you, so you can go from a reading on site to an X/12 pitch, a ratio and a material quantity in a few seconds.

Common mistakes when working in degrees

• Treating slope percent as degrees — a 30 percent slope is 16.7°, not 30°.
• Rounding the angle before converting; 30° rounded from 29.6° changes the pitch by 0.1/12 and the area by about half a percent.
• Using a manufacturer's minimum in degrees as if it were X/12, or the other way round.
• Measuring the angle on top of thick tiles or a sagging rafter rather than on a straight section of the structure.

Frequently asked questions