How the calculation works
The formulas
Enter horizontal plan dimensions. Each overhang applies on both opposite sides and is measured horizontally. For pitched roofs, divide projected plan area by the cosine of pitch from horizontal.
Plan area = (length + 2 × end overhang) × (width + 2 × side overhang)
Roof area = plan area ÷ cos(pitch)
Material area = roof area × (1 + allowance/100)
Assumptions to check
- Gable planes have equal pitch; hips, valleys, dormers, rooflights, and unequal pitches are excluded.
- This is geometry and a material allowance, not structural capacity, drainage, or a roofing specification.
A worked example
A 10 × 8 m plan with no overhang and 30° pitch has 80 ÷ cos(30°) ≈ 92.38 m² of roof surface.