The formula
true area = plan area × √(1 + pitch²)
Where it comes from
The mistake that ruins the order is measuring the roof off the plan. A pitched roof always measures more than its shadow on the ground, and the steeper it is, the more: at a 30 % pitch the difference is 4 %, and at 80 % it is already over 28 %. On a hundred square metres of plan, that is nearly nine hundred barrel tiles of difference. So the first step is correcting the area with the pitch factor, and only then counting pieces.
How to work it out by hand
- Measure the roof area on plan
- Correct it with the pitch factor for the true area
- Multiply by the tiles each square metre takes for that model
- Add an allowance for breakage, cuts and trims
What is worth knowing
Tiles per square metre vary a lot by model, and the figure comes from the manufacturer: barrel tile runs around thirty, pantile around twelve, and a large concrete panel around two and a half. It is worth checking the data sheet before ordering, because there are variations within a single type. Fittings are counted separately, by the linear metre rather than by area: ridge tiles — around three per metre — hips, verge trims and vent tiles. And one piece of site advice: order a bit more than comes out and keep what is left over. In ten years, when a falling branch breaks two of them, that exact model will no longer be made.