Enter any one measure and get the rest: degrees, the 1:x ratio, percentage, rise per metre, rafter length and the multiplier that turns a floor plan into roof area. Then check what your pitch means for Colorbond, tiles and sarking.
Roof pitch calculator
Pick what you know. Everything else is worked out as you type.
Area multiplier is roof surface per square metre of footprint (1 ÷ cos pitch). It ignores eaves overhang, so add the eaves to the footprint first. Rafter length is the sloped distance from wall plate to ridge for the run entered, before overhang.
Three ways, and they all describe the same slope. Degrees is what plans, councils and manufacturers use: a 22.5° roof is the traditional Australian tile pitch. Ratio, written 1 in 4 or 1:4, is rise over run and is what older builders and plumbers say out loud. x in 12 is the American habit, rise per 12 units of run, and it turns up in imported product sheets and YouTube. The calculator above converts between all three, plus percent grade and rise per metre for plumbers setting fall on box gutters.
| Pitch | Ratio | What it usually is | Material limits |
|---|---|---|---|
| 1° to 3° | 1:57 to 1:19 | Flat roofs, box gutter falls, skillions on extensions | Membrane, or concealed-fix metal within its rated minimum (Lysaght: Klip-Lok 1°, Trimdek 2°) |
| 5° | 1:11.4 | Minimum for corrugated Colorbond; common on carports, patios and low skillions | Corrugated ok, tiles no |
| 10° | 1:5.7 | Modern skillion and low-slung metal roofs | Metal profiles within their rated minimum, tiles no |
| 15° | 1:3.7 | Minimum for concrete and terracotta tile (check the profile) | Tiles with sarking required |
| 22.5° | 1:2.4 | The classic Australian tiled roof, most 1950s to 1990s brick homes | Tile or metal |
| 25° to 30° | 1:2.1 to 1:1.7 | Federation and older terracotta, many project homes | Tile or metal, safety setup increases |
| 35° to 45° | 1:1.4 to 1:1 | Steep heritage, some Hamptons and gable-heavy designs | Tile or metal, scaffold every job |
Minimum pitches vary by profile and manufacturer, and by lap treatment and sheet length. These are the usual numbers; the product data sheet and installation guide win on any specific roof, and we confirm both before we quote.
Because it decides what you are allowed to put on. A tiled roof at 22.5° can go to Colorbond in any profile, because every metal profile is rated well below that; the check is done on the product sheet, not the roof. The reverse does not work: a 10° metal roof cannot be tiled without reframing, which is a structural job with council approval attached. Most re-roof enquiries we get where someone wants to change material are tile to Colorbond, and pitch is never the blocker in that direction.
The other place pitch shows up is the quote. Anything over about 30° needs more safety setup, slower work and often scaffold, and that is a real line on a re-roof price. Our roof replacement calculator measures the pitch of each roof plane from satellite and allows for it in the estimate, which is the fastest way to get both numbers for your own house.
Between 15° and about 20° on a tiled roof, sarking is not optional: wind pushes rain up under the tile laps at that angle and the sarking is what gets it back to the gutter. On older Sydney roofs at low pitch we regularly find no sarking at all under the tiles, which is why the ceiling stain appears. That is a section repair, not a re-roof; see sarking installation on an existing roof.
Three ways, easiest first.
From the plans. The section drawing of any house built after the 1970s states the pitch in degrees. Type it into the Degrees tab.
In the roof space. Hold a spirit level flat against the underside of a rafter, slide it until the bubble is centred, mark 1,000 mm along the level from where it touches the rafter, then measure straight down from that mark to the rafter. That vertical gap is the rise per 1,000 mm of run. Enter 1000 as the run and the gap as the rise.
From satellite. Enter the address in the replacement calculator and it returns the pitch of each plane, along with the area. This is the one to use if you are not comfortable in the roof space, and it is the only one of the three that also gives you a price.
Do not measure pitch from the ground with a phone app held up at the gutter. The perspective error is large enough to move a roof from "tiles ok" to "tiles not ok".
Corrugated Colorbond can go as low as 5°. Concealed-fix profiles like Klip-Lok go to 1° with the right fixing. Below 5° on corrugated, water sits in the pans and finds the laps.
Concrete and terracotta tiles need at least 15°, and some profiles specify more. Between 15° and about 20° the manufacturers require sarking under the tiles.
Spirit level against a rafter in the roof space, 1,000 mm along, measure the drop, enter both numbers above. Or enter your address in the replacement calculator and it measures the pitch from satellite.
Only with new framing, which is structural and usually needs council approval. Changing material is the normal ask, and tile to metal works at any existing pitch; metal to tile usually does not.
It converts footprint to roof surface. A 180 m² footprint at 22.5° is about 195 m² of roof, before eaves. Add the eaves to the footprint first for a closer number, or let the satellite calculator measure it directly.
Related: roof replacement cost calculator · Colorbond roof colours guide · Colorbond roofing Sydney · roof replacement Sydney
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