Roof geometry
Roof covering area from plan
- where
- slope area (m²)
- horizontal projected area (m²)
- pitch (°)
- order area (m²)
- waste (%)
Explain this equation
Turns the plan area of a roof into the true sloping area, and adds waste for ordering coverings or membranes.
Ah is the horizontal projected area, as seen from above, including overhangs, in m². θ is the pitch. As is the actual sloping area. w is the waste percentage, and Ao the order area.
Every strip of roof is longer on the slope than on plan by a factor of 1/cos θ, while its width along the eaves stays the same. So the whole area grows by that same factor.
60 m² on plan at 30° is about 69.3 m², and about 76.2 m² with 10% waste. At 45° the factor is about 1.41.
Watch out: this only works for planes of one pitch. Split mixed pitches. Laps and product coverage are a separate step.
Calculate the area of roof planes having one uniform pitch.
Enter your values to see the result.
Good to know
Plan area must include horizontal overhang projections and exclude openings as appropriate. Split unequal pitches into separate calculations. Laps and product coverage are separate.
Sources
- Geometry / arithmetic
This is a calculation aid, not a structural design or compliance certificate. Every default is an example, not a specification for your project.