Roofing Calculator

True roof area from the building footprint and pitch, plus tile count.

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About this calculator

A roof is bigger than the building beneath it: the pitch stretches the footprint into a larger sloped surface. This calculator applies the pitch factor to your footprint, adds waste, and multiplies by your tile's coverage rate to give a materials count.

The formula

Sloped area = Footprint ÷ cos(Pitch°) With waste = Sloped area × (1 + Waste% ÷ 100) Tiles = With waste × Tiles per m²

Worked example

A 9m × 6m footprint at 35°: 54 ÷ cos35° = 65.9 m², +10% = 72.5 m² → 725 large-format tiles at 10/m².

Common mistakes

Using the footprint area for materials — a 45° roof is over 40% bigger than its footprint.
Forgetting ridge, hip and verge tiles, which are bought separately by the linear metre.
Working on a roof without proper access equipment — leave steep roofs to professionals.

Frequently asked questions

Measure the rise over a 1m horizontal run in the loft: pitch = atan(rise). A smartphone inclinometer app on a rafter also works.

Large-format interlocking concrete ≈ 10/m²; plain clay tiles ≈ 60/m²; natural slate ≈ 13–20/m² depending on size. Check the manufacturer's datasheet.

It models a dual-pitch roof over a rectangular footprint. Hips, valleys, dormers and L-shapes add area — split them into rectangles and add 5% extra waste.

Assumptions & limitations

Symmetrical dual-pitch roof; battens, membrane, ridge and fixings not itemised.

Sources & verification

NFRC — National Federation of Roofing Contractors — Trade body guidance; tile coverage per m² depends on tile format and lap — confirm with your tile manufacturer

All sources last accessed 24 July 2026.

Formula last verified: 24 July 2026

Maintained by: Steve Davis, Founder & Editor — see our editorial standards

Spotted an error or an out-of-date figure? Report it — corrections are prioritised over all other work (see our corrections policy).

Related guide

How much waste to allow for building materials