Most roof calculators just use the building footprint — this one accounts for pitch, because a sloped roof always has more surface area than the ground it covers.
A flat roof's surface area equals its footprint. Every degree of pitch beyond that increases the true surface area — the roof slopes upward, so its actual sloped surface is longer than the flat distance it spans. The relationship is true area = footprint ÷ cos(pitch).
| Pitch | Area increase vs footprint |
|---|---|
| 15° | +3.5% |
| 25° | +10.3% |
| 35° | +22.1% |
| 45° | +41.4% |
At a steep 45° pitch, the true roof area is over 40% more than the building footprint — a substantial difference if you're quoting or ordering materials off footprint alone.
Tiles-per-m² depends entirely on tile type: large interlocking concrete tiles cover roughly 10/m², while small plain clay tiles can need 60 or more per m² due to their heavy overlap. Always check your specific tile manufacturer's coverage rate rather than trusting a generic default for a real order.
A 60 m² building footprint with a 35° pitch has a true roof area of 73.25 m² — 22% more than the footprint. With a 10% waste allowance and 10 tiles/m² coverage, that's 806 tiles.