Roof calculator

Forget the pitch and it prices flat — slope factor 1, silently, with no warning in the maths itself. It is the mistake most likely to bite, so the result says so on screen when it happens; but know that an unset or zero pitch under-orders every material and nothing stops you.

One pitch corrects one area. A mansard, a dormer, or hips running at different slopes each need their own figure. This scales a single plane; it is not roof geometry.

If you are reading the pitch off a drawing rather than knowing it, a section cut obliquely reads shallow — an off-square cut under-reports the angle, and nothing flags it. Take the pitch off a true section, square to the slope.

The correction applies to roof areas, not to sloping lines. Hips, valleys and rafters drawn on a plan are foreshortened; they are not raked. Measure those true off a section, or enter them by hand.

Implausible pitches are not rejected. A silly angle gives a silly, internally-consistent answer — the maths does not know a roof from a wall.

The tool corrects an area and divides by the coverage figure you give it — it does no course, ridge, verge or eave logic. Slate and batten figures fold in gauge and lap; felt is entered as the roll size, so felt lap is not accounted for — carry it in your wastage. Ridge tiles, verges, eaves and flashings are yours to add.

It is an accurate estimate to verify on site, not a guaranteed cutting list. It gives you the numbers and the working behind them; the final call is yours.