Coverage
Number of tiles or slates for a roof, including waste
- where
- net number of tiles or slates
- sloping roof area (m²)
- tiles or slates per m²
- tiles or slates to order
- waste and breakage allowance (%)
Explain this equation
Turns a roof area and a coverage rate into the number of tiles or slates to order, with an allowance for cuts and breakages.
A is the sloping area, not the plan area, and n is how many units cover one square metre at your gauge. Their product is the net count with nothing spare. The factor 1 + w/100 adds the waste percentage, and the ceiling brackets round up to a whole tile.
Straight multiplication of area by coverage, then a percentage uplift, because you cannot buy part of a tile.
Sense check: 97.7 m² at 9.9 per m² is about 967 tiles; with 5% waste you order 1016. A simple gable roof might need 3 to 5%; lots of hips, valleys and dormers need more.
Watch out: eaves, verge and ridge courses may need special or tile-and-a-half units. Count those separately.
Quantity of tiles or slates to order: the sloping area times the coverage rate, plus a waste allowance, rounded up.
Enter your values to see the result.
Good to know
Slate suppliers recommend adding at least 5%; allow more on roofs with many hips, valleys and cuts. Eaves courses of slates and plain tiles need extra short slates or eaves tiles, and verges may need tile-and-a-half or half slates: count those separately. Fittings (ridge, hip, valley, vent tiles) are separate items.
Common mistake
Using the plan area. Coverage rates are per m² of sloping roof.
Limits
Area-based estimate. Detailed take-off by course is more accurate on small or complex roofs.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the roof covering manufacturer’s fixing specification, BS 5534, BS 8000-6, BS 6229, the Lead Sheet Training Academy guidance and Building Regulations. Wind uplift and structural design need a competent designer.