Roof pitch angles: chart, conversions and minimum slopes

Roof pitch tells you how steep a roof is, and Americans and Britons write it differently. US builders give it as rise in 12 (4:12, 6:12), while UK tile makers and drawings use degrees. This guide converts every whole pitch from 1:12 to 24:12 and shows how to measure a real roof. It also covers rafter lengths and the minimum slope that codes and manufacturers allow for each roof covering.

To convert a pitch of your own, use the roof pitch calculator. It runs the same formulas as the chart below and works out rafter length for any run in meters, feet, inches or millimeters.

What is roof pitch?

Pitch compares the vertical rise of a roof with its horizontal run. The run is measured from the outside of the wall to the point directly under the ridge, which is half the span on a symmetrical gable. The sloping line between those two points is the rafter.

The same steepness can be written four ways:

• x:12 (US): rise for every 12 units of run, so 6:12 means 6 in of rise per 12 in (1 ft) of run. The units cancel, so it works just as well as 500 mm per 1,000 mm.
• Degrees (UK and most of Europe): the angle between the rafter and the horizontal, tan⁻¹(rise ÷ run).
• Percent: rise ÷ run × 100. A 6:12 roof is a 50% slope.
• Rise : run ratio: 6:12 reduces to 1 : 2.

How do you convert roof pitch to degrees?

Divide the rise by the run, then take the arctangent (the tan⁻¹ or “inverse tan” key on a calculator). For a 9:12 roof, 9 ÷ 12 = 0.75 and tan⁻¹(0.75) = 36.87°.

To go the other way, from degrees to x:12, multiply the tangent of the angle by 12. A 35° roof is tan(35°) × 12 = 8.40, so about 8.4:12.

Don’t scale degrees in proportion to x:12. An 8:12 roof is not twice as steep in degrees as a 4:12 roof: it is 33.69° against 18.43°. Because the arctangent curve flattens as the pitch rises, 24:12 is only 63.43°.

Roof pitch chart: 1:12 to 24:12

We calculated every row from the formulas above rather than copying them from another chart. Degrees are rounded to two decimals, percent to one and the rafter factor to four, the same rounding the roof pitch calculator uses. “Rise per 1 m run” is the metric version of the same pitch, rounded to the nearest millimeter.

Common angles in degrees, converted to x:12

UK product literature quotes pitches in degrees. This table runs the conversion the other way for angles that turn up often.

How do you measure roof pitch?

You need two measurements: a horizontal distance and the vertical rise over that distance. Any of these methods will give you both.

1. On the roof surface. Rest one end of a level on the roof and hold it horizontal. Measure straight down from the 12 in mark to the roof surface. That number in inches is your x in x:12. With a 1 m (3 ft 3 in) level, the drop in millimeters is the rise per 1 m run in the chart above. Only do this on a roof you can stand on safely.
2. From the attic. Do the same against the underside of a rafter. This is safer and gives the same angle, since the rafter and the roof surface are parallel.
3. From the gable end or the drawings. Measure the height from the top of the wall plate to the ridge, then divide by half the building’s width. A 2 m (6 ft 7 in) rise over a 4 m (13 ft 1 in) run is 0.5, or 6:12.
4. With an angle finder or a phone level app. Lay it on a rafter or on the roof surface and read off the angle in degrees, then convert it with the table above or the calculator.

Take readings at more than one spot. Old rafters sag, and a 1 in (25 mm) error over a 12 in (305 mm) level changes the result by a whole step on the chart.

How do you calculate rafter length from pitch?

Multiply the horizontal run by the rafter factor, √(1 + (rise ÷ run)²). The factor is also called the pitch multiplier. Then add the overhang, measured along the slope.

• Metric example: an 8 m (26 ft 3 in) wide house with a symmetrical 6:12 gable has a 4 m run. 4 × 1.1180 = 4.472 m, so the rafter is about 4.47 m (14 ft 8 in) from wall line to ridge line before overhang. The ridge sits 2 m (6 ft 7 in) above the wall plate.
• Imperial example: a 24 ft (7.32 m) span at 8:12 has a 12 ft run. 12 × 1.2019 = 14.42 ft, or about 14 ft 5 in (4.40 m), before overhang. The rise is 8 ft (2.44 m).

These are line lengths to the ridge line. A real rafter is cut shorter at the top to allow for half the ridge board’s thickness. It also gets a seat cut (birdsmouth) where it sits on the wall, so take the final cut lengths from your framing drawings.

What counts as a low-slope or steep-slope roof?

There is no single legal definition. One widely cited line comes from the U.S. EPA’s ENERGY STAR roof product requirements (Version 2.1), which define low-slope roofs as 2:12 or less and steep-slope roofs as anything greater than 2:12. The same document lists single-ply membranes, built-up roofing and modified bitumen as typical low-slope products. Composite shingles, clay, concrete or fiber-cement tile, slate, metal panels and metal shingles are listed as steep-slope products.

A “flat” roof is never truly flat. The City of Lincoln, Nebraska, tells re-roofers that the minimum slope for a roof is ¼ in per 12 in (1.19°, or 2.1%) to allow for drainage.

What is the minimum roof pitch for shingles, tile and metal?

Minimum slopes for each roof covering are set by the building code and by the manufacturer’s installation instructions, whichever is stricter. In the US, the International Residential Code (IRC) gives them in Section R905. The table below comes from the 2018 IRC as summarized in MyBuildingPermit.com’s re-roof checklist for Washington jurisdictions, which states that it reflects the 2018 IRC with Washington amendments. The City of Hopkins, Minnesota, gives the same asphalt-shingle rule under section R905.2, and the City of Lincoln, Nebraska, says it enforces the 2018 IRC. We have not checked these figures against the 2021 or 2024 editions of the IRC, so confirm which edition, and which local amendments, your jurisdiction uses.

For asphalt shingles, the Hopkins guidance says shingles “may only be used on roof slopes of 2 units vertical in 12 units horizontal (2:12) or greater,” with double underlayment from 2:12 to 4:12. A shingle roof between those pitches is allowed, but it needs the extra layer.

How is roof pitch written in the UK?

In the UK, pitch is almost always given in degrees: a “35-degree roof,” not an 8.4:12 roof. For tiled and slated roofs, the working minimum comes from the product maker’s data, with the tiles fixed to the British Standard code of practice for slating and tiling (BS 5534). That minimum depends on the headlap (how far each course overlaps the one below) and on rafter length.

Two published examples show how much this varies by product:

• Marley says its Mendip 12.5 tile can be used down to 12.5° (about 2.66:12), and its Lincoln, Wessex and standard Mendip tiles down to 15° (about 3.22:12). Both come with limits on rafter length. The company’s advice is not to use a tile below the pitch it was designed for.
• Wienerberger’s datasheet for its Modula clay tile gives a minimum pitch of 17.5° at 100 mm (4 in) headlap, rising to 22.5° at 75 mm (3 in) headlap. A smaller overlap between courses needs a steeper roof to shed water.

Whatever the product, check its datasheet for the minimum pitch, and confirm with building control.

Why roof pitch matters beyond the numbers

Pitch affects how much usable loft space a house has, which coverings it can take, and how it looks from the street. Roof shape and steepness are among the quickest clues when you are trying to place a house in our house styles guide or the wider types of architecture overview. Our guide to Georgian architecture is one place where the roof line is part of the story.

If you are pricing a loft conversion, pitch sets the headroom, so work out the pitch first. Then check the stair with the stair calculator and the floor area with the room area calculator. For any roof pitch not in the chart, the roof pitch calculator converts x:12, degrees and percent instantly. More measured guides are in the technical section.