Roof Area Calculator: measuring notes and method
Plan area is different from roof surface area
Enter pitch as rise per 12 units of horizontal run, not as degrees or a percentage. If the roof contains different pitches, calculate their footprints separately and add the sloping areas.
Area is not roof-sheet linear metres
This result is m², not LM. For simple full-length sheets, divide the width across the slope by effective sheet cover and round up, then multiply the sheet count by the measured sheet length. Hips, valleys and multiple sheet lengths need a sheet schedule.
CHECK A PRACTICAL EXAMPLE
Footprint versus sloping surface
A 10 × 8 m footprint includes the measured eaves. The comparison uses a uniform pitch and no waste.
3 in 12 pitch
- Roof area with waste
- 82.462 m²
- Sloping roof area
- 82.462 m²
6 in 12 pitch
- Roof area with waste
- 89.443 m²
- Sloping roof area
- 89.443 m²
The footprint is unchanged at 80 m², but the roof surface grows with pitch. A surface-area result still needs effective sheet cover and individual sheet lengths to become roof-sheet linear metres.
Fixed comparison in metric units. Values are illustrative unless linked to a recorded project; changing the calculator above does not change this example.
Formula
Sloping area = footprint × √[1 + (pitch rise ÷ 12)²]. Apply waste. One roofing square = 100 ft².
Worked examples in metric and imperial units
Illustrative values match Reset example and stay fixed when you edit the calculator. Both columns describe the same calculation; converted inputs are rounded.
Metric worked example
- Length
- 6 m
- Width
- 4 m
- Pitch rise per 12 units of run
- 6
- Waste allowance
- 10 %
Roof area with waste: 29.5161 m²
Imperial worked example
- Length
- 19.685 ft
- Width
- 13.1234 ft
- Pitch rise per 12 units of run
- 6
- Waste allowance
- 10 %
Roof area with waste: 317.7086 ft²
Follow the calculation
Lengths in this working are in metres.
- Horizontal area = 6 × 4 = 24 m².
- Pitch multiplier = √(1 + (6 ÷ 12)²) = 1.118034.
- Multiply horizontal area by the pitch multiplier, then by 1.1 for waste.
Reviewed 19 October 2026. How calculations are checked · Report a correction