How the tile count is worked out
A tile covers its width multiplied by the batten gauge, not its full length. The gauge is the distance between batten centres, which is the tile length less the headlap, and it is what decides how much roof each tile covers.
A 330 mm wide tile at a 345 mm gauge covers 0.114 square metres, so one square metre needs 8.78 tiles. The roof area multiplied by that figure, plus waste, gives the count.
The headlap is the part of the tile covered by the course above. A steeper pitch needs less headlap and a longer gauge, so the same tile covers more roof at 45 degrees than at 20.
Worked example
A roof with 60 square metres of sloping area, tiled at 330 mm wide on a 345 mm gauge, needs 527 tiles before waste. Adding 10 per cent for hips, valleys and cuts gives 580 tiles.
Measure the sloping area, not the floor plan. A simple gable roof is the two slopes added together, and a hipped roof has four.
Gauge, headlap and pitch
The manufacturer's fixing instructions give the minimum gauge for each pitch band, and using a longer gauge than allowed lets water drive up under the tiles. Take the gauge from the tile's own instructions rather than from a rule of thumb.
Plain clay tiles at 265 by 165 mm on a 100 mm gauge need 60 per square metre, which is why they cost more per square metre than a large interlocking tile.
Verges, ridges, hips and valleys use different components, and they are not in this count. Add ridge and hip tiles separately.
Limits of this calculator
It counts field tiles only. It does not include ridge, hip, verge, valley or eaves tiles, battens, membrane, fixings or flashings.
It does not measure the roof for you. Roof area has to come from the drawing or from measuring the slopes, and a roof with dormers or a complex shape needs each plane measured separately.
It does not check the pitch against the tile's minimum, which decides whether the tile can be used at all.
For US shingles sold by the square, use the US roof tile calculator.
Inputs for the roof tile calculator
Roof area
Sloping roof area in square metres, not the floor area of the building.
Tile width
Cover width of one tile in millimetres, 330 mm for a large interlocking tile.
Batten gauge
Distance between batten centres in millimetres, which is the tile length less the headlap.
Waste allowance
Extra for hips, valleys and cuts. Ten per cent suits a hipped roof.
CALCULATOR QUESTIONS
UK roof tile calculator questions
How many tiles are in a square metre?
It depends on the gauge. A 330 mm tile at a 345 mm gauge needs 8.78 per square metre; a small plain tile needs about 60.
What is the batten gauge?
The vertical distance between battens, which is the tile length minus the headlap. It sets how much roof each tile covers.
Should I measure the floor area?
No. Use the sloping roof area, which is larger, and add each roof plane separately.
Are ridge tiles included?
No. Ridges, hips, verges and valleys are separate components with their own counts.