The short answer
There is no single number. Plain clay tiles, interlocking concrete tiles and pantiles all cover different areas, so the tiles needed per square metre depend on the tile's size, the gauge at which it is laid and the lap between courses. The manufacturer's data sheet gives the count for a given tile, and the roofer adds an allowance for cuts.
Why the answer depends on the tile
Tiles come in very different sizes. A small plain clay tile is not much wider than a hand, and it is laid in overlapping rows so that each tile is covered by the tile two rows up. A large interlocking concrete tile may be several times that size and locks sideways into its neighbour. The bigger the exposed face, the fewer tiles a square metre takes.
What counts is the exposed face of each tile, the part left showing after the next row has overlapped it. That is not the same as the tile's length. A long tile with a deep overlap shows only a fraction of itself, so a tile that looks large on the stack may cover less than you expect once it is on the roof.
Because of this, a figure you read online is only a rule for one product. Quoting a count for roof tiles in general, without naming the tile, tells you very little. Our page on the types of roof tiles shows how different the profiles are, and why you cannot move a number from one to another.
Gauge, lap and the sum behind the count
Gauge is the distance from the top edge of one batten to the top edge of the next, which is the exposed height of each row of tiles. The lap, or headlap, is how far each tile overlaps the one two courses below it. Together with the tile's width, gauge fixes how much roof a single tile covers.
The sum is simple. Divide one square metre by the exposed height multiplied by the cover width of the tile, with both measured in metres. A plain tile with a short gauge and narrow width needs many tiles per square metre. An interlocking tile with a long gauge and wide cover needs few. The maker publishes the correct cover width and the permitted gauge for each product.
Gauge is not a free choice. It is set between a minimum and a maximum so that the lap is deep enough to keep water out, and the allowed lap depends partly on the pitch of the roof. Marley, for example, lists a separate minimum pitch for each tile in its range, from 12.5 degrees to 40 degrees, so the data sheet for your tile also sets what is allowed on your slope.
Why the data sheet decides
The data sheet is the maker's document for one tile. It gives the cover width, the range of gauge, the minimum pitch, the weight and usually the number of tiles per square metre at its standard gauge. It also lists the fixings the tile needs. Once you know which tile is on your roof, or which one you plan to use, that sheet answers the question exactly.
For a new roof, the roofer picks the tile first and the count follows. For a repair, the aim is to find the tile already there, so the replacements fit the existing gauge and cover. If the tile is no longer made, a reclaimed or near match may use a slightly different size, and the battens may have to move to suit.
Do not rely on a count read from a forum or an old invoice. A range of tiles may have been revised, or a pack may be sold in a different sized batch. Ask for the data sheet in writing. Our guide on interlocking roof tiles explains why the fixing and lap rules differ by profile.
Waste allowance and matching stock
No roof takes the exact number of tiles that the area suggests. Hips, valleys, the ridge, the verge and the eaves all need tiles cut or special pieces, and some tiles break in transport or on the roof. A sensible roofer therefore adds an allowance for cuts and breakages. The right amount depends on how cut-up the roof is, and a plain gable needs less than one with several valleys.
It also pays to keep a small number of spare tiles after the job. If a tile is ever cracked, a spare from the same batch matches the weathered ones, and you will not be hunting for a discontinued colour. Tile colour varies slightly between batches, so one order from one supplier at one time gives the best match.
For repairs on an older roof, matching is harder, and reclaimed tiles are often the closest fit. Our page on matching old roof tiles goes further. The full job sits within pitched roofing, where we measure the roof, photograph each slope, note the condition of the battens beneath, choose the tile and give you one written price.
