Roofing Calculator
Enter your building's footprint dimensions and roof pitch to find the actual roof surface area and the material area needed, including waste.
How roof area is calculated
A sloped roof has more surface area than the building's flat footprint, because the slope stretches that area out. The roof surface area is footprint area ÷ cos(pitch angle) — as the pitch gets steeper, the roof surface grows faster than the footprint.
A waste allowance (commonly 10-15%) is added on top of the roof surface area to account for cutting, overlaps, ridge caps, and edge trim when ordering roofing material.
Frequently asked questions
Why is the roof area larger than the footprint area?
Because the roof is tilted, its actual surface is longer than the flat distance it covers when viewed from above — the steeper the slope, the bigger the difference.
What's a typical roof pitch?
Roof pitch varies widely by style and climate — often expressed as a ratio (like 4:12) or an angle; a 4:12 pitch is roughly 18.4°, while steeper decorative roofs can exceed 45°.
Does this account for roof shape complexity (hips, valleys, dormers)?
No — this calculator assumes a simple single-plane roof over a rectangular footprint. Complex roof shapes with multiple planes, hips, and valleys need additional material beyond this basic estimate.