Building
Roof Pitch Calculator
Convert between degrees, ratio and percentage, and get rafter length.
How this calculator works
Give the calculator any one of three things: a rise and run measured off the roof, a pitch in degrees, or a ratio like 4:12. It converts between all three formats instantly, so you can talk to a roofer in whichever format they use.
Pitch in degrees is the angle whose tangent is rise divided by run. The ratio format expresses the same slope as millimetres of rise per 12 units of run, common on plans and in roofing tables.
Add a run measurement and the calculator also returns rafter length using Pythagoras, the straight line distance up the slope. Add your eaves overhang on top when ordering.
| Degrees | Ratio | Grade | Typical use |
|---|---|---|---|
| 5° | 1:12 approx | 8.7% | Minimum for most metal sheeting |
| 10° | 2.1:12 | 17.6% | Skillion roofs, patios |
| 15° | 3.2:12 | 26.8% | Common metal roof pitch |
| 22.5° | 5:12 | 41.4% | Standard tiled roofs |
| 30° | 6.9:12 | 57.7% | Steeper tiled and character homes |
| 45° | 12:12 | 100% | Steep pitched, attic conversions |
| Material | Minimum pitch |
|---|---|
| Corrugated steel | 5° |
| Trimdek profile | 2° |
| Klip-lok concealed fix | 1° to 2° |
| Concrete and terracotta tiles | 15° to 20° |
| Slate | 25° |
Always confirm with the manufacturer specification for your exact profile.
Frequently asked questions
How do I measure roof pitch from inside the roof?
In the roof space, hold a level horizontally from a rafter, measure 1,000mm along it, then measure vertically down to the rafter. That vertical measurement is your rise per 1,000mm run, and this calculator converts it from rise and run.
What does a 4:12 pitch mean?
For every 12 units of horizontal run, the roof rises 4 units, about 18.4 degrees. The x:12 convention comes from imperial framing but still appears on Australian plans and roofing charts.
What is the minimum pitch for a metal roof?
Corrugated sheeting generally needs at least 5 degrees. Concealed fix profiles like Klip-lok go as low as 1 to 2 degrees. Below the minimum, water can back up under laps and leak.
How do I calculate rafter length?
Rafter length is the square root of rise squared plus run squared. A roof rising 2,000mm over a 6,000mm run needs rafters of 6,325mm, plus your eaves overhang.
Is a steeper roof better?
Steeper roofs shed water and debris faster, suit tiles and give more attic space, but cost more in materials and framing. Most Australian homes sit between 15 and 25 degrees.
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