How to calculate roof pitch
Measure the rise (how far the roof goes up) and the run (how far it goes across, level). Divide the rise by the run, then take the inverse tangent to get degrees. On a gable roof the run is half the span; on a lean-to it is the full span.
A worked example. A 300mm rise over a 1,000mm run is 0.3. The inverse tangent of 0.3 is 16.7°.
Roof pitch in degrees, ratio and percent
Degrees to percent: tan(degrees) × 100. Degrees to x:12: tan(degrees) × 12. Degrees to 1 in N: 1 ÷ tan(degrees). So 22.5° is about 5:12 and a 41.4% gradient, and a 4:12 pitch is 18.43°.
How to measure the pitch of an existing roof
- Spirit level: hold a level horizontal from the tiles or a rafter and measure straight down from its far end. The drop over the level's length is rise over run. Choose "Measure it" above.
- Phone: lay it on a rafter or along the tiles and use "Measure with your phone". Calibrate it on something level first.
- From the loft: measure along a rafter against a level, the same way.
- From a photo: only a rough guess.
Minimum roof pitch for tiles and slates in the UK
Every covering has a minimum pitch. Below it, wind-driven rain gets under the tiles. The figures here come from the manufacturers and BS 5534; the product data sheet, your exposure zone and the rafter length can all push the figure up.
| Covering | Minimum pitch | Notes |
|---|---|---|
| Plain clay or concrete tiles | 35° | Most plain tiles. Some single-camber clay plain tiles go down to 30°. |
| Natural slate (500mm slates, 115mm headlap) | 20° | Moderate exposure, rafters up to 9m. Many specifications use 22.5° or more. |
| Marley Edgemere interlocking slate | 17.5° | Slate-effect interlocking tile. |
| Marley Lincoln clay pantile | 17.5° | Interlocking clay pantile. |
| Marley Wessex or Mendip concrete tiles | 15° | Low pitch interlocking concrete tiles. |
| Marley Mendip 12.5 or Melodie clay tiles | 12.5° | Very low pitch interlocking tiles. |
| Pressed metal roof tiles | 5° | Check the system's own minimum. |
Roof pitch chart
Every pitch in its other forms, with the multiplier for rafter length and sloped area (1 ÷ cos of the pitch).
| Degrees | x:12 | Percent | 1 in N | Multiplier |
|---|---|---|---|---|
| 1° | 0.21 | 1.7% | 57.29 | 1 |
| 2° | 0.42 | 3.5% | 28.64 | 1.001 |
| 3° | 0.63 | 5.2% | 19.08 | 1.001 |
| 4° | 0.84 | 7% | 14.3 | 1.002 |
| 5° | 1.05 | 8.7% | 11.43 | 1.004 |
| 6° | 1.26 | 10.5% | 9.51 | 1.006 |
| 7° | 1.47 | 12.3% | 8.14 | 1.008 |
| 8° | 1.69 | 14.1% | 7.12 | 1.01 |
| 9° | 1.9 | 15.8% | 6.31 | 1.012 |
| 10° | 2.12 | 17.6% | 5.67 | 1.015 |
| 11° | 2.33 | 19.4% | 5.14 | 1.019 |
| 12° | 2.55 | 21.3% | 4.7 | 1.022 |
| 13° | 2.77 | 23.1% | 4.33 | 1.026 |
| 14° | 2.99 | 24.9% | 4.01 | 1.031 |
| 15° | 3.22 | 26.8% | 3.73 | 1.035 |
| 17.5° | 3.78 | 31.5% | 3.17 | 1.049 |
| 20° | 4.37 | 36.4% | 2.75 | 1.064 |
| 22.5° | 4.97 | 41.4% | 2.41 | 1.082 |
| 25° | 5.6 | 46.6% | 2.14 | 1.103 |
| 27.5° | 6.25 | 52.1% | 1.92 | 1.127 |
| 30° | 6.93 | 57.7% | 1.73 | 1.155 |
| 32.5° | 7.64 | 63.7% | 1.57 | 1.186 |
| 35° | 8.4 | 70% | 1.43 | 1.221 |
| 37.5° | 9.21 | 76.7% | 1.3 | 1.26 |
| 40° | 10.07 | 83.9% | 1.19 | 1.305 |
| 42.5° | 11 | 91.6% | 1.09 | 1.356 |
| 45° | 12 | 100% | 1 | 1.414 |
| 47.5° | 13.1 | 109.1% | 0.92 | 1.48 |
| 50° | 14.3 | 119.2% | 0.84 | 1.556 |
| 52.5° | 15.64 | 130.3% | 0.77 | 1.643 |
| 55° | 17.14 | 142.8% | 0.7 | 1.743 |
| 57.5° | 18.84 | 157% | 0.64 | 1.861 |
| 60° | 20.78 | 173.2% | 0.58 | 2 |
Flat roof falls: 1 in 80 and the design fall
A flat roof still needs a fall to shed water. Under BS 6229:2025 the finished roof must fall at least 1 in 80 everywhere. Build in more, because deflection and settlement take some of it away: 1 in 60 is a common starting point, and the structure decides the design fall.
A 1 in 40 fall over 1 metre is 25mm. A 5° slope over 1 metre drops about 87mm.
Lean-to and shed roof pitch
On a lean-to or mono-pitch roof the run is the full span, and the rise is the difference in height between the two walls. Many lean-to roofs are low pitched, so check the minimum for the sheet, felt or tile you plan to use.
How to work out rafter length
The rafter from the wall plate to the ridge centre line is the run ÷ cos(pitch). Add the overhang the same way, and take off half the ridge board on the slope.
A worked example. A 6m span at 35° has a 3m run and a rise of 2.1m. The rafter is 3.66m, plus 0.49m for a 400mm overhang, less 0.03m for half a 50mm ridge board: 4.12m, so order 4.2m lengths. Plumb cut 35°, seat cut 55°. Building the gable ends in brick or block? The brick and block calculator counts them.
Is 5 degrees enough for a roof?
It depends on the covering. 5° is the minimum for pressed metal roof tiles, and some sheet systems go that low, but it is far too shallow for tiles and slates. Flat roof membranes are designed to their own falls.
Frequently asked questions
How can I calculate my roof pitch?
Divide the rise by the run and take the inverse tangent for degrees. On a gable roof the run is half the span. A 300mm rise over 1m is 16.7°. Enter the numbers above, or measure the roof with a spirit level or your phone.
What pitch is a 22.5 degree roof?
22.5° is about 5:12, a 41.4% gradient, or 1 in 2.41. It is shallower than most UK house roofs, which usually sit between 30° and 40°.
How to calculate roof pitch in the UK?
The same way as anywhere, rise over run, but UK roofers usually give it in degrees rather than x:12. Take the inverse tangent of rise ÷ run for degrees, then check the minimum pitch for your covering in the table below.
How much fall is 5 degrees over 1 metre?
About 87mm. The drop over a length is the length × tan(angle), so 5° over 1,000mm is 1,000 × 0.0875.
What is a 1 in 40 fall over a metre?
25mm. 1 in 40 means one unit of drop for every 40 across, so 1,000mm ÷ 40 is 25mm, about 1.43°.
Is 5 degree slope enough for a roof?
For pressed metal tiles and some sheet systems, yes. For tiles and slates, no: most need 12.5° or more, and plain tiles 35°. Flat roof membranes work to their own falls under BS 6229.
What is the most common roof pitch in the UK?
Most UK pitched roofs are between 30° and 40°. Plain tiles need at least 35°, which is why older houses often have steeper roofs.
What is a 4:12 pitch in degrees?
18.43°. Take the inverse tangent of 4 ÷ 12. An 8:12 pitch is 33.69°.
Does the calculator work in feet and inches?
Yes. Switch any length to ft/in and type it the way you would say it. The pitch comes out the same in any unit, because it is a ratio.
Can I use my phone to measure roof pitch?
Yes. Use "Measure it", then "Measure with your phone". Lay the phone on a rafter or along the tiles, pointing up the slope. Calibrate it on a level surface first. On an iPhone, allow motion access when asked.
Sources
- Marley: low pitch tiles and minimum pitches (opens in a new tab)
- Cupa Pizarras: minimum pitch for a slate roof (NHBC and BS 5534) (opens in a new tab)
- Maguire Brothers: roof pitches (opens in a new tab)
- Bauder: BS 6229:2025, the key changes for flat roofs (opens in a new tab)
- Marley: how to calculate roof pitch (opens in a new tab)
Pitch and length figures are geometry. Which covering you can use also depends on the product, the exposure zone and the rafter length: check the data sheet, and BS 5534 and BS 6229.
By Janusz Wozniak, Founder & Lead Developer
Built so a roofer or a chippy can get any pitch into any form, and a cutting list, on a phone on the scaffold.
Last updated .
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