A pitched roof has a larger surface than its horizontal projection. Using only building length and width can therefore underestimate tile quantities.
GuideBest practicesIntermediate
How many roof tiles do you need? Area, pitch and waste
Estimate roof-tile quantities from the true roof area, pitch, effective tile coverage and an appropriate waste allowance.
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Horizontal versus sloped area
Suppose one roof plane projects 6 m horizontally from eaves to ridge and is 10 m long. Its projection is 60 m². At a 30° pitch:
sloped length = horizontal length ÷ cos(angle)
6 ÷ cos(30°) ≈ 6.93 m
The actual plane is therefore about 69.3 m². The roof tile calculator helps organize this estimate.
If the rafter/sloped length is already known reliably, simply use roof-plane area = sloped length × roof length.
Percentage and degrees are different
Pitch percentage is vertical rise ÷ horizontal run × 100. A 50% pitch is not 50°. Use the slope calculator when converting representations.
Use manufacturer coverage
Physical tile dimensions do not equal effective coverage because overlaps and gauge matter. Prefer the manufacturer’s tiles-per-m² figure or effective coverage.
For 138.6 m² at 10.5 tiles/m²:
138.6 × 10.5 = 1,455.3
Round up to at least 1,456 tiles before allowance and packaging.
With 5% allowance:
1,456 × 1.05 = 1,528.8, or at least 1,529 tiles.
Full two-plane example
For two identical planes, a 9 m roof length, 5 m horizontal run and 35° pitch:
sloped length = 5 ÷ cos(35°) ≈ 6.10 m
One plane is about 54.9 m²; two are about 109.8 m². At 11 tiles/m² the theoretical need is about 1,208 tiles. With 6% allowance it becomes about 1,281 tiles before packaging.
Different tiles, different counts
For the same 110 m² roof, a model requiring 10 tiles/m² needs 1,100 units, while one requiring 13 tiles/m² needs 1,430. The roof did not change; effective product coverage did.
Ridge, verge and other accessories may be measured by length or unit rather than area, so the field-tile count is not a complete roofing bill of materials.
What this calculation does not do
A quantity estimate does not size the structure or determine fastening, ventilation, waterproofing or regulatory compliance. Complex valleys, hips, dormers and penetrations also deserve a detailed takeoff rather than a generous percentage applied to a rough area.
Use safe, reliable dimensions rather than taking unnecessary risks to measure an existing roof. For the general workflow of measurements, allowances and packaging, read how to plan material quantities.