Pitch describes how steep a roof is. It changes the roof area, which coverings you can use, how fast water runs off, how safe the roof is to work on, and how much the job costs. It's also one of the most confusing numbers on site, because it's written in three different ways depending on who you ask.
Three ways of saying the same thing
- Rise in 12 (x/12): how many inches the roof rises for every 12 inches of horizontal run. It's the standard in the US and on most American product data.
- Degrees: the angle from horizontal. It's the standard in Australia, the UK, Europe and in tile makers' data everywhere.
- Percent: rise ÷ run × 100. It's used for low-slope roofs, gutters and drainage.
There is also the older idea of a "1/3 pitch" or "half pitch", based on rise over span rather than run. It still turns up in old documents but causes confusion, so convert it before using it.
Conversion table
| x/12 | Degrees | Percent | Area factor |
|---|---|---|---|
| 2/12 | 9.5° | 16.7% | 1.014 |
| 3/12 | 14.0° | 25% | 1.031 |
| 4/12 | 18.4° | 33.3% | 1.054 |
| 5/12 | 22.6° | 41.7% | 1.083 |
| 6/12 | 26.6° | 50% | 1.118 |
| 7/12 | 30.3° | 58.3% | 1.158 |
| 8/12 | 33.7° | 66.7% | 1.202 |
| 9/12 | 36.9° | 75% | 1.250 |
| 10/12 | 39.8° | 83.3% | 1.302 |
| 12/12 | 45° | 100% | 1.414 |
The formulas
- Degrees = atan(rise ÷ run)
- x/12 = tan(degrees) × 12
- Percent = rise ÷ run × 100
- Area factor = √(1 + (rise ÷ run)²)
The area factor turns a plan area into true roof area. It's the single most useful number in roof estimating.
How to measure an existing roof
- Hold a level horizontally with one end touching the roof surface.
- Measure 1 metre along the level from that end.
- Measure straight down from that mark to the roof. That's the rise per metre.
From inside the roof space, a digital angle finder on the underside of a rafter gives degrees directly. From the ground, phone apps can estimate pitch from a photo, but treat that as a rough check. On gable ends you can also measure the height from wall plate to ridge and divide by half the span.
Why pitch matters
Materials
Every roof covering has a minimum pitch. Corrugated steel needs at least 5°, Trimdek 2° and Klip-Lok 1°. Concrete and terracotta tiles usually need 15° or more, depending on the profile and the sarking. Below the minimum, choose a different covering or a membrane roof.
Cost and safety
Steeper roofs mean more area, more material and slower work, often with extra edge protection or scaffolding. Many roofers add a steep-roof charge above about 35°. Low roofs are quicker to walk but less forgiving of poor detailing, because water moves slowly.
Roof space and appearance
Pitch decides headroom in the roof space, which matters for attic conversions and storage, and it shapes how the house looks. Matching the existing pitch on an extension is usually a planning or design requirement.
Using pitch in other calculations
Pitch feeds rafter length (run ÷ cos(pitch)), hip and valley lengths, gable areas, solar panel angles and gutter sizing. The roof pitch calculator converts any format to all the others, and the rafter calculator turns span and pitch into cut lengths and angles.
Common questions
Is an 18° roof "low"?
Not for metal: 18° is well above the minimum for corrugated or Trimdek. For tiles it's near the bottom of the range, so check the tile maker's minimum pitch and sarking requirements.
What pitch can I walk on safely?
Most roofers walk roofs up to about 30° in dry conditions with good footwear. Steeper than that needs roof brackets, harnesses or scaffolding. Wet, mossy or frosty roofs are dangerous at any pitch.
Do solar panels need a particular pitch?
Panels are usually fitted flush to the existing roof. Pitch changes output only modestly, and the roof's direction usually matters more: in Australia a north-facing roof is best, and tilt frames can help on flat roofs.
Pitch on drawings
Australian drawings usually give pitch in degrees on the section, for example 22.5°. Older drawings may give a ratio such as "1 in 2". Convert it with the calculator before ordering anything. When an extension has to match an existing roof, measure the existing pitch on site rather than trusting old drawings. Small differences show clearly where the two roofs meet.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.