Calculating how many roof tiles you need for a roofing project requires knowing the coverage rate of the specific tile being used, the total area of the roof slopes to be tiled, and the additional tiles needed for ridges, hips, verges, and wastage. Tile coverage rates vary considerably between tile types.

Coverage Rates by Tile Type

Plain clay and concrete tiles

Plain tiles, which are the traditional small flat rectangular tiles laid in a double-lap system, have high coverage rates because the tiles are small. Traditional clay plain tiles measuring 265mm by 165mm are typically laid at approximately 60 to 73 tiles per square metre depending on the gauge (the distance between each course of tiles) used. The gauge is set to achieve the correct exposure for the specific tile and is specified by the manufacturer based on the roof pitch and exposure conditions.

Interlocking concrete tiles

Interlocking concrete and clay tiles are much larger than plain tiles and are laid in a single-lap system. Common formats include the double Roman, the Duo and single pantile, and various proprietary profiles. Coverage rates for interlocking tiles typically range from 9 to 16 tiles per square metre depending on the specific tile format. A standard concrete interlocking tile of around 420mm by 330mm lays at approximately 10 tiles per square metre.

Natural and artificial slate

Slate tiles are laid in a double-lap system similar to plain tiles. Coverage rates depend on the slate size. A common slate size of 500mm by 250mm, laid at an appropriate gauge for the roof pitch, gives approximately 18 to 20 slates per square metre. Smaller slates of 400mm by 200mm require around 35 to 40 per square metre.

Measuring the Roof Area

To calculate tile quantities, you need the area of each roof slope in square metres. For a simple pitched roof, the slope area is the horizontal plan area multiplied by a factor that accounts for the pitch angle. For a 35-degree pitch, this factor is approximately 1.22. For a 45-degree pitch it is 1.41. Multiply the horizontal footprint of each slope by the appropriate pitch factor, then sum all slopes for the total roof area.

Adding for Ridges, Hips, and Wastage

The calculated tile quantity should be increased by around ten percent for wastage and cutting. Ridges, hips, valleys, and verges require specific tile types that are calculated separately from the main field tiles. Ridge and hip tiles are typically ordered by the linear metre rather than by area coverage.

Summary

Coverage rates per square metre depend heavily on tile type. Plain clay tiles use around 60 to 73 per sqm; interlocking concrete tiles 9 to 16 per sqm; natural slate 18 to 40 per sqm depending on size. Accurate quantity calculation requires knowing the roof pitch, measuring the slope area correctly, and using the manufacturer's gauge tables for the specific tile. Add ten percent for wastage and order ridge and hip tiles separately based on linear measurements.