Roofing Calculator

How to use the roofing calculator

1. Fill in the fields — they start with a worked example.
2. Read the answer, and the arithmetic shown beneath it.
3. Copy the results, or adjust the inputs to compare scenarios.

About this tool

A roof is always larger than the building underneath it, and by how much depends entirely on the pitch. The footprint is what you can measure from the ground; multiplying it by the pitch factor gives the surface the covering has to reach across. A 6/12 roof is about 12 per cent larger than its footprint, and a 12/12 roof is 41 per cent larger — order for the footprint and you are a third short.

The pitch factor is the hypotenuse of the rise over the run: the square root of one plus the slope squared. That is the whole of the geometry, and it is why the number is unintuitive — a roof that looks twice as steep is nowhere near twice the area.

Roofing is sold by the square, which is one hundred square feet, and asphalt shingles typically run three bundles to a square. Both figures are shown alongside the metric area because suppliers in the same country will quote you each of them.

The waste allowance is not padding. Cuts, breakages, spillage and the offcuts around edges and openings are real losses on every job, and a second delivery costs far more than a spare bag or board. On a roof the allowance also covers starter and ridge courses, and it should rise with complexity: a plain gable wastes little, while hips, valleys and dormers each mean cuts, and 15 per cent is nearer the mark on a cut-up roof than 10.

Common uses

• Ordering shingles or tiles for a re-roof.
• Converting a ground footprint into actual roof area.
• Checking a roofer's quantity against your own figure.

Frequently asked questions

How do I work out roof area from the ground?

Measure the footprint the roof covers, then multiply by the pitch factor — the square root of 1 plus the slope squared. A 6/12 pitch multiplies by about 1.12.

What is a roofing square?

One hundred square feet of roof surface, which is how roofing is sold in the US. Asphalt shingles usually come three bundles to the square.

How much waste should I allow?

Around 10 per cent on a simple gable roof and 15 or more where there are hips, valleys and dormers, since every one of those means cutting.