Tile & slate quantity calculator
Roof area, gauge and cover width, converted to a tile count — plus a wastage allowance so the order isn't short.
Roof & tile detail
Use the roof area calculator above if you need this first.
From the manufacturer's fixing table for this tile/pitch.
Coverage once lapped against the next tile — not the full width.
For cuts, breakages and eaves/verge trims.
Tiles to order
— includes a 10% allowance for cuts and breakages.
Main field only — not a full setting-out. Excludes ridge, hip and verge tiles. Confirm gauge against the manufacturer's data sheet before ordering.
Once you know the roof area, the next question is how many tiles or slates it actually takes to cover it — and that comes down to two numbers that don't get much thought day to day: gauge and cover width. Get either one wrong and the order comes in short or long, neither of which anyone wants halfway through a job.
Gauge and cover width
Gauge is the batten spacing — the vertical distance between rows of battens, set by the tile or slate's headlap and overall length. Cover width is how much of a tile or slate is actually visible and counted once it's lapped against its neighbour, not its full physical width. Multiply gauge by cover width and you get the area one tile covers on the roof; divide 1 by that figure and you get tiles per square metre.
Why it varies by tile
Interlocking concrete or clay tiles tend to run a wider cover and a longer gauge than plain tiles, which are laid double-lap and sit much tighter together — so the tiles-per-m² figure for a plain tile roof can run two or three times higher than for a large interlocking profile. Natural slate adds another variable again, since gauge shifts with slate size and the headlap the pitch demands. There's no single "right" gauge across products — it always comes from the specific tile or slate being used.
A worked example
A tile with a 100mm gauge and a 200mm effective cover width covers 0.02m² each — 50 tiles per m². On a 100m² roof, that's 5,000 tiles before any wastage. Add a typical 10% wastage allowance for cuts, breakages, and extra tiles at eaves and verges, and the order comes to 5,500 — the number worth taking to the merchant rather than the bare theoretical count.
What this isn't
This gives a flat-field estimate for the main body of the roof. It doesn't add ridge, hip or verge tiles, account for cut tiles at abutments and dormers, or replace the manufacturer's own gauge table — gauge varies by tile profile, headlap requirement and roof pitch, and using the wrong figure throws the whole quantity off. Check the data sheet for the actual gauge before setting out battens or placing an order.
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