Four ways of saying one angle
A roof pitch turns up in four costumes depending on who wrote the document, and three of them are numbers between about 1 and 100, which is exactly why they get mixed up.
- Degrees, what an architect's plan and a roofing manufacturer usually state. 22.5°.
- Percentage, rise divided by run, times a hundred. Common on civil drawings, driveways and drainage. 22.5° is 41.42%.
- 1 in N, how far across you go for every one up. Plumbers and drainers live in this one. 22.5° is 1 in 2.41.
- Rise over run, what a carpenter actually measures on site with a level and a tape.
The trap is reading a percentage as degrees. 45° is 100%, not 45%. A drainage line at "1 in 4" is 25% and about 14°. Mix those up on a fall and you're out by a factor of two or more.
Measuring a pitch on site
A spirit level and a tape does it. Hold the level against the underside of a rafter or along the roof line, run it out dead level to exactly 1200mm, then measure straight down to the roof. That drop is your rise for a 1200 run.
A phone inclinometer on a rafter gets within about half a degree, plenty for ordering materials. Don't read it off ribbed sheeting, sit it on a straight edge.
A worked example
500mm of rise over a 1200mm level run.
- Ratio, 1200 ÷ 500 = 2.4, so 1 in 2.4
- Percentage, 500 ÷ 1200 = 41.67%
- Degrees, the angle whose tangent is 0.4167 = 22.62°
- Rafter per 1200 of run, √(1200² + 500²) = √1,690,000 = 1300mm, a clean 5, 12, 13 triangle
- Multiplier, 1300 ÷ 1200 = 1.0833
That roof is 12m × 9m in plan, so 108 m² of footprint. True roof surface is 108 × 1.0833 = 117 m². Nine square metres you'd have left off the order.
And the rafters. A 9m span gives a 4.5m run per side. Rise to the ridge, 4.5 × 0.4167 = 1.875m. Rafter length, 4.5 × 1.0833 = 4.875m. Order 5.1m stock and take the birdsmouth and the plumb cut out of the extra.
Conversion table, degrees to fall in mm per metre
The fall column is the one the flat roof and patio questions are really asking. A 1° roof drops about 17.5mm every metre of run, which over a 6m patio is 105mm end to end.
| Degrees | Fall per metre | Percentage | Ratio | Area multiplier |
|---|---|---|---|---|
| 1° | 17.5 mm | 1.75% | 1 in 57.3 | 1.0002 |
| 2° | 34.9 mm | 3.49% | 1 in 28.6 | 1.0006 |
| 3° | 52.4 mm | 5.24% | 1 in 19.1 | 1.0014 |
| 5° | 87.5 mm | 8.75% | 1 in 11.43 | 1.0038 |
| 10° | 176.3 mm | 17.63% | 1 in 5.67 | 1.0154 |
| 12° | 212.6 mm | 21.26% | 1 in 4.70 | 1.0223 |
| 15° | 267.9 mm | 26.79% | 1 in 3.73 | 1.0353 |
| 20° | 364.0 mm | 36.40% | 1 in 2.75 | 1.0642 |
| 22.5° | 414.2 mm | 41.42% | 1 in 2.41 | 1.0824 |
| 25° | 466.3 mm | 46.63% | 1 in 2.14 | 1.1034 |
| 30° | 577.4 mm | 57.74% | 1 in 1.73 | 1.1547 |
| 35° | 700.2 mm | 70.02% | 1 in 1.43 | 1.2208 |
| 45° | 1000 mm | 100% | 1 in 1 | 1.4142 |
Notice how slowly the multiplier moves at the bottom and how fast it moves at the top. Under 10° you can almost ignore it. Past 30° it's the difference between a roof that gets sheeted and one that runs out.
Minimum roof pitch in Australia, by sheet profile
The covering sets the lowest pitch, not you. Lysaght publishes a minimum roof slope on the page for each of its steel profiles, and Monier publishes one for its tiles. These are their figures, read 26 October 2026.
| Covering | Profile | Minimum roof slope |
|---|---|---|
| Concealed fix steel | Lysaght Klip-Lok 406 | 1° |
| Concealed fix steel | Lysaght Klip-Lok 700 Hi-Strength | 1°, subject to design provisions being met |
| Trapezoidal steel | Lysaght Trimdek | 2° |
| Trapezoidal steel | Lysaght Spandek | 2°, subject to design provisions being met |
| Corrugated steel | Lysaght Custom Orb | 5° |
| Terracotta tile | Monier Terracotta Nouveau | 12° |
| Concrete tile | Most Monier concrete tiles | 15° |
Sources, Lysaght's product pages for Klip-Lok, Trimdek, Spandek and Custom Orb, and Monier's FAQ, all read 26 October 2026. Monier adds that rafter length, wind speed, terrain category and the tile profile all count, so the tile maker's literature for your exact tile has the last word.
Going below a covering's minimum is where low pitch roofs leak. Water backs up under a lap instead of running off it, and wind drives it further. If a design lands close to a limit, change the profile rather than hoping.
Why the multiplier is the number that costs money
Anything sold by the square metre goes on the sloped area, not the footprint. That's sheeting, tiles, sarking, insulation blanket, membrane, battens by area. Anything measured up the slope is longer by the same factor, rafters, barge capping, valley gutter, the fall on a raked ceiling.
Gutters and downpipes are the one big exception, and they're an exception for a good reason. Rain falls vertically, so a gutter only ever sees the footprint. Never apply the multiplier to a drainage catchment, and the gutter calculator explains what that mistake does to a downpipe count. Once you've got the pitch, the roof sheet calculator turns it into sheets and run lengths.
Pitch sorted. Now price the roof.
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Try Yamate free → Free to try · no card · sign in with a link, no password to rememberRoof pitch FAQ
What is a 1 degree roof pitch in mm per metre?
About 17.5mm of fall for every metre of run, tan(1°) × 1000. A 2° pitch is 34.9mm per metre and 3° is 52.4mm. Over a 6 metre patio run, 1° is 105mm of total fall, which is why low-slope roofs get set out with an instrument rather than by eye.
What is a 22.5 degree roof pitch as a percentage?
41.42%. As a ratio it is 1 in 2.41, and the roof area multiplier is 1.0824, so 100 m² of footprint is 108.24 m² of actual roof.
How do I measure roof pitch without a plan?
Hold a spirit level against a rafter or the roof line, run it out level to exactly 1200mm, and measure straight down to the roof. That drop over 1200 is your rise and run, so enter both above and the rest converts itself.
Is roof pitch percentage the same as degrees?
No, and mixing them is the classic error. Percentage is rise divided by run times a hundred, so 45° comes out as 100%. A 10% fall is only about 5.7°.
How do I work out rafter length from span?
Halve the span to get the run of one rafter, then divide by the cosine of the pitch. A 9m span at 22.62° gives a 4.5m run and a 4.875m rafter, before the birdsmouth and the plumb cut.
What is the minimum roof pitch in Australia?
It depends on the covering. On the makers' own pages, Lysaght's concealed fix Klip-Lok goes down to 1°, Trimdek and Spandek to 2°, Custom Orb corrugated to 5°, and Monier's tiles to 12° or 15° depending on the profile. Below the maker's figure, water backs up under the laps.
What is the minimum pitch for a corrugated roof?
Lysaght publishes 5° as the minimum roof slope for Custom Orb, its corrugated sheet. Trapezoidal Trimdek and Spandek go to 2°, and concealed fix Klip-Lok to 1°. Confirm against the maker's page for the profile you're buying.
What is the minimum roof pitch for tiles?
Monier says most of its concrete tiles can be used to 15° and its Terracotta Nouveau as low as 12°, and that rafter length, wind speed, terrain category and the tile profile all count. Tiles rely on gravity and overlap, so they're far less forgiving at low pitch than sheet steel.
What is a 12 degree roof pitch?
A fall of 212.6mm per metre, which is 21.26% or 1 in 4.70. The area multiplier is 1.0223, so 100 m² of footprint is 102.23 m² of roof. It's also as low as Monier's Terracotta Nouveau tile goes.
What is a 45 degree roof pitch?
Rise equals run, so it's 1 in 1 and 100%, not 45%. The area multiplier is 1.4142, so 100 m² of footprint is 141.42 m² of roof, and an 8m span puts the ridge 4m up on a 5.66m rafter.
Other calculators you'll want
General information for Australian trades, and an estimate only, not tax, financial or legal advice. It is not engineering, building or design advice either. A calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. The conversions and rafter lengths here are pure geometry and are exact for the figures you enter. Minimum pitches are the makers' published figures, read 26 October 2026, and vary by product, sheet length, lap detail and exposure, so check the literature for what you are actually installing. Rafter sizes and spans come from AS 1684 span tables and are not worked out here.