The geometry
For a simple roof plane, sloping length = horizontal run ÷ cos(pitch). The corresponding area factor is 1 ÷ cos(pitch).
Typical mathematical factors
At 20° the factor is about 1.064; 30° about 1.155; 35° about 1.221; 40° about 1.305; 45° about 1.414. These are geometry factors, not tile quantities.
Apply the factor to the correct plan area
Use only the horizontal area represented by the roof plane being calculated. Hips, valleys, dormers and overhangs should be measured separately.
Tile coverage still comes from the product
Once roof area is known, use the tile manufacturer’s coverage or tiles-per-square-metre data.
Use the numbers in a calculator
Accuracy note
Where a figure depends on a product, supplier, installation method or project specification, use the current information for the exact material being purchased. The reference pages explain the maths and estimating logic; they are not a substitute for current technical data.