Roofing calculators

Roofing has one arithmetic trap and almost everyone falls into it once. You measure the building on the ground, because that is where you can stand, but you cover the roof on the slope — and a pitched plane is always larger than the ground beneath it.

How much larger depends on the pitch, and it climbs faster than intuition suggests. A shallow 4/12 roof is only five percent bigger than its footprint. A 9/12 is a quarter bigger. Order for the footprint on a steep roof and you are a full plane short before you start.

So every tool here asks for the pitch and shows the multiplier it applied, rather than hiding it inside the answer. If the number looks wrong you can check the working — and on a material where running short means a second delivery in a possibly different colour batch, that is worth more than a tidier result.