Four ways of saying one angle
A roof pitch turns up in four different costumes depending on who wrote the document, and they trip people up because three of them are numbers between about 1 and 100.
- 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
You need a spirit level and a tape. 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 at that point. That drop is your rise for a 1200 run.
A phone inclinometer laid on a rafter gets you within about half a degree, which is plenty for ordering materials. Don't take it off the roof surface if the sheeting is ribbed — 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
Now put it to work. That roof is 12m × 9m in plan, so 108 m² of footprint. True roof surface: 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 for common pitches
| Degrees | Percentage | Ratio | Area multiplier |
|---|---|---|---|
| 5° | 8.75% | 1 in 11.43 | 1.0038 |
| 10° | 17.63% | 1 in 5.67 | 1.0154 |
| 15° | 26.79% | 1 in 3.73 | 1.0353 |
| 20° | 36.40% | 1 in 2.75 | 1.0642 |
| 22.5° | 41.42% | 1 in 2.41 | 1.0824 |
| 25° | 46.63% | 1 in 2.14 | 1.1034 |
| 30° | 57.74% | 1 in 1.73 | 1.1547 |
| 35° | 70.02% | 1 in 1.43 | 1.2208 |
| 45° | 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.
How low can the roof go?
Pitch isn't a free choice — the covering decides the floor. These are the typical minimums published by Australian manufacturers, and they move with laps, sarking, sheet length and exposure, so always confirm against the literature for the exact product.
| Covering | Typical minimum pitch |
|---|---|
| Concealed-fix steel, continuous lengths | from about 1° |
| Trapezoidal steel sheet | from about 2° |
| Corrugated steel sheet | from about 5°, more if end-lapped |
| Concrete and terracotta tiles | usually 15° and up, profile dependent |
| Slate and shingles | 25° and up |
| Membrane and torch-on | designed as a fall, commonly around 1 in 100 minimum |
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 — 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 — 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 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 — 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 pitch for a corrugated roof?
Australian manufacturers generally publish corrugated steel from about 5° for continuous sheets, and higher again where sheets are end-lapped. Trapezoidal profiles go lower, around 2°. Confirm against the literature for the profile you are buying.
What is the minimum roof pitch for tiles?
Usually 15° and up for concrete and terracotta tiles, though it varies by profile and some allow less with sarking and extra laps. Tiles rely on gravity and overlap, so they are far less forgiving at low pitch than sheet steel.
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 typical published Australian manufacturer guidance and vary by product, sheet length, lap detail and exposure — 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.