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Roof Area Calculator
Compute roof surface area with pitch
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Result
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What this calculator does
Estimates the sloped surface area of a roof from its plan footprint and one uniform pitch.
How to use it
- Enter the footprint width and length.
- Enter the roof pitch in degrees.
- Set waste and read the surface area.
Formula
horizontalFootprint = Width × Length Area m² = horizontalFootprint / cos(Roof Pitch (degrees) × π/180) × (1 + Waste Margin (%) / 100)
Formulas are shown in metric units, exactly as the calculator computes them. Imperial entries are converted to metric first. Waste is applied as a multiplier (1 + waste/100).
Areas are in m² and volumes in m³. cos uses radians: roof degrees are multiplied by π/180.
Variables
| Variable | Unit | Default |
|---|---|---|
| Width | m | 8 |
| Length | m | 12 |
| Roof Pitch (degrees) | ° | 30 |
| Waste Margin (%) | % | 10 |
Worked example
Using the default values of this calculator (Width = 8 m, Length = 12 m, Roof Pitch (degrees) = 30 °, Waste Margin (%) = 10 %), the calculator returns:
horizontalFootprint = 8 × 12 Area m² = horizontalFootprint / cos(30 × π/180) × (1 + 10 / 100)
- Roof Surface Area: 121.936 m²
Assumptions and limitations
- Assumes a single uniform pitch over the whole footprint: hips, valleys, dormers and overhangs are not modelled.
- Width and length are the projected footprint, not rafter lengths. This is not structural or safety design.
Waste, rounding and buying
Roof coverings are sold per square meter or pack; add extra for ridge, edges and cuts. Measure on site before ordering.
Frequently asked questions
- What is the difference between footprint and roof area?
- Footprint is the plan view. A sloped roof has more surface: area = footprint / cos(pitch).
- Does the roof have overhangs?
- Not unless you add the overhang to the footprint dimensions.