By Sam Alex, civil engineer · About the author
Roof pitch comes up everywhere: choosing roofing, ordering sheets, getting a solar quote, working out rafter lengths or checking whether a skylight can be fitted. The confusing part is that pitch is described three different ways, and they are easy to mix up.
Three ways to describe the same slope
Take a roof that rises 1.2 m over a horizontal run of 4.8 m: - Ratio (rise over run): 1.2 : 4.8 = 1 : 4, or "3 in 12" in US plans - Percent slope: 1.2 ÷ 4.8 × 100 = 25% - Degrees: arctan(1.2 ÷ 4.8) = 14.0°
All three describe the same roof. The trap is that 25% is not 25°. A 25° roof is much steeper, at about 46.6% slope. Mixing them up can mean buying the wrong product or getting a solar or tile quote badly wrong.
Conversion table
| Degrees | Percent slope | Ratio (in 12) |
|---|---|---|
| 2° | 3.5% | 0.4 in 12 |
| 5° | 8.7% | 1.05 in 12 |
| 10° | 17.6% | 2.1 in 12 |
| 15° | 26.8% | 3.2 in 12 |
| 22.5° | 41.4% | 5 in 12 |
| 30° | 57.7% | 6.9 in 12 |
| 45° | 100% | 12 in 12 |
The roof pitch calculator gives the percent slope from rise and run. Use arctan to convert to degrees.
Why pitch matters
Minimum pitch. Every roofing product has a minimum pitch below which water may not drain fast enough, and wind-driven rain can get back up under the laps. Some metal sheet profiles are rated down to 1–3°. Concrete and terracotta tiles generally need much steeper roofs, often around 15° or more, depending on the tile and the batten spacing. Always check the manufacturer's installation guide. Roofing below its rated pitch can void the warranty and cause leaks.
Roof area. A sloping roof has more surface area than its footprint. Multiply the plan area by 1 ÷ cos(pitch): about 1.08 at 22.5° and 1.15 at 30°. Our roofing calculator guide explains how to use this.
Rafter length. Rafter length is √(run² + rise²). For a 3.6 m run and 1.5 m rise, that's 3.9 m before the overhang. See the rafter length calculator.
Solar. Panels on a pitched roof usually follow its slope. In Australia, a north-facing roof at around 20–30° is close to ideal for annual output, but flatter roofs still work well.
Safety. Steeper roofs need edge protection, harnesses and often scaffolding. Working at heights is heavily regulated in Australia. Many roof jobs legally require fall protection regardless of pitch.
How to measure an existing roof safely
You don't need to get on the roof: 1. Go into the roof space and find a rafter or the top chord of a truss. 2. Hold a spirit level horizontally against its underside. 3. Measure 1,000 mm along the level from where it touches the rafter. 4. Measure vertically from that point up to the rafter. 5. That vertical distance ÷ 1,000 × 100 is the slope in percent. For example, 400 mm gives 40%, which is about 21.8°.
Other options: a phone inclinometer app held against a rafter, or the original building plans, which usually state the pitch.
Typical pitches in Australian homes
- Tiled hip and gable roofs: commonly around 20–25°
- Metal skillion roofs: often 3–10°
- Verandas and carports: often 2–10°
- Steep feature roofs (A-frames, some heritage styles): 30–45° and above
Frequently asked questions
What is the minimum pitch for a metal roof?
It depends on the profile. Some are rated to 1°, others need 3° or more. Check the manufacturer's specification.
How do I convert roof pitch in degrees to percent?
Percent = tan(degrees) × 100. For example, tan(20°) × 100 = 36.4%.
What pitch do roof tiles need?
Commonly around 15° or more, but it varies by tile profile and exposure. Check the tile maker's installation guide.