Roof Pitch Calculator Australia — Angle, Ratio & Rafter Length
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How to calculate roof pitch in Australia
Roof pitch is the angle between the horizontal (run) and the slope of the roof (rafter). Enter the rise (vertical height from wall plate to ridge) and run (horizontal distance from wall plate to centreline — half the span) in millimetres. The calculator returns the pitch in degrees, the ratio (rise:run expressed as 1:X), the rafter length (hypotenuse of the rise/run triangle) and the roof area multiplier — which tells you how much larger the sloped roof surface is than the flat ceiling area. Multiply your ceiling area by the roof area multiplier to get actual roof surface area.
Common Australian roof pitches — reference table
| Pitch | Degrees | Rise : Run | Area multiplier | Common application |
|---|---|---|---|---|
| Low | 5° | 1 : 11.4 | 1.004 | Min for Colorbond (lapped joins) |
| Low | 10° | 1 : 5.7 | 1.015 | Flat-style skillion, carport |
| Standard | 15° | 1 : 3.7 | 1.035 | Low-pitch tile (min for some tile profiles) |
| Standard | 18° | 1 : 3.1 | 1.051 | Common Colorbond / sheet metal |
| Standard | 22.5° | 1 : 2.4 | 1.082 | Very common terracotta & concrete tile |
| Standard | 25° | 1 : 2.1 | 1.103 | Popular hip and gable roof |
| Steep | 30° | 1 : 1.7 | 1.155 | Higher pitch gable, heritage |
| Steep | 35° | 1 : 1.4 | 1.221 | Steeply pitched heritage / Victorian |
| Very steep | 45° | 1 : 1.0 | 1.414 | Feature gables, some heritage styles |
Roof pitch calculator — FAQs
What is the minimum roof pitch for Colorbond in Australia?
BlueScope Steel specifies a minimum pitch of 5° for Colorbond corrugated and Trimdek profiles with lapped (overlapping) joins, and 3° for mechanically seamed standing seam profiles. Most roofing contractors recommend at least 5° to ensure adequate drainage and prevent water pooling. Check the specific profile's installation guide as minimum pitches vary — Longline 305 and similar concealed-fix profiles have different minimums to corrugated Colorbond.
What is the difference between roof pitch, rise and run?
Run is the horizontal distance from the outside wall to the ridge centreline — half the building's span. Rise is the vertical height from the top of the wall plate (or ceiling joist) to the underside of the ridge. Pitch is the angle, calculated as arctan(rise ÷ run). For a symmetrical gable roof, both sides have the same run and rise. For a skillion (single slope), run is the full horizontal distance and rise is the height difference between the two walls.
What is the roof area multiplier and how do I use it?
The roof area multiplier (also called the pitch factor or slope factor) tells you how much larger the sloped roof surface is compared to the horizontal footprint. A 22.5° pitch has a multiplier of 1.082 — meaning a house with a 150 m² floor plan has approximately 150 × 1.082 = 162 m² of roof surface. Use the multiplier to calculate how many roof sheets, tiles or sarking you need. Always measure the actual roof footprint (not the floor plan) as eaves overhangs add to the footprint.
What pitch do terracotta tiles need in Australia?
Most terracotta and concrete roof tile profiles require a minimum pitch of 15° (approximately 1:3.7 ratio) for standard installation. Flat profiles (low-pitch tiles) can be installed down to about 12°. Steeper pitches (22.5°–30°) are ideal for terracotta as they shed water faster and reduce moss and lichen growth. Always check the specific tile manufacturer's installation guide — minimum pitches vary between Monier, Bristile, Boral and other AU suppliers.
How do I work out the rafter length for a roof in Australia?
Rafter length (excluding any eave overhang) is the hypotenuse of the rise/run right-angle triangle: rafter = √(rise² + run²). For example, a rise of 1,500 mm and run of 4,000 mm gives a rafter of √(1500² + 4000²) = √(2,250,000 + 16,000,000) = √18,250,000 ≈ 4,272 mm. Add the horizontal eave overhang distance ÷ cos(pitch angle) for the full rafter length including eave. The calculator above does all this from your rise and run inputs.