How is a roof measured for a quote?

The usual mistake is to take the width and depth of the house and treat the answer as the roof. A pitched roof is bigger than the ground beneath it, because the slopes lean outwards and upwards. The steeper the pitch, the greater the difference, so two houses with the same footprint can have noticeably different roof areas.

The roof also overhangs the walls at the eaves and, usually, at the verges. Add the area under bays, porches, dormers and extensions, and the total moves further from the floor plan. Hips and valleys alter the arithmetic again, since cut tiles along those lines are used up in a different way.

So the first step is always to measure each slope as its own shape, whether a rectangle, a trapezium or a triangle, and add them up. Our page on surveying the roof of a period property shows why older buildings, with their uneven slopes and later additions, rarely behave like a tidy rectangle.

If you have architect's drawings, planning documents or a set of building plans, a roofer can calculate a lot from the office. Elevations give the pitch and the heights, and the floor plans give the span. It is a quick route and it suits a new extension that does not exist yet.

The weakness is accuracy over time. Drawings show what was designed, not necessarily what was built, and old plans for older houses are often missing or rough. Tiles and slates may have been changed, bays added, and chimneys moved since the paper was drawn. A quote from drawings alone is therefore a starting figure and is best checked on site before anyone orders material.

Plans also say little about condition. They cannot show a sagging ridge, rotted battens or a covering that has been repaired repeatedly, all of which shape the work needed and in turn the price.

The traditional method is a tape, a laser meter or both, used from a ladder at the eaves, from a scaffold or from the roof itself. The roofer measures the length along the eaves, the rafter length from eaves to ridge, and the dimensions of each hip, valley and abutment. It gives a figure for the real roof, as it now stands.

It also lets the person doing the measuring see the covering up close. The same visit shows loose tiles, rusting fixings, crumbling pointing and the state of the flashings, which our survey notes. That is a real advantage, since the area and the condition come together.

The drawback is that some of the roof cannot be reached from a ladder. High, steep or shaded slopes are estimated, not measured, and a figure from the ground has a margin of error that a careful roofer allows for. Wet or mossy slates make walking on the roof unwise, so measuring from a ladder or tower is often the safer choice.

A drone flight can capture every slope in a single pass, and software can then work out the pitch and area from the images. That helps on large or hazardous roofs, on terraces where access is tight, and where a roofer wants to see the whole roof at once. Photographs taken in the same flight double as a record of the roof's condition.

It is not magic. Image-based measurement relies on good light, a clear view and a sensible scale reference, and trees or shadows can distort the result. Hidden areas, such as the underside of overhangs or valleys deep between slopes, may need confirming by hand. Our guide to drone roof surveys sets out where a flight helps and where it does not.

For most houses, a mix works best: drone or ladder photographs for the whole picture, and a tape on the details where accuracy counts.

When one roofer says the roof is one size and another gives a different one, the cause is rarely dishonesty. The methods may differ, with one person taking figures from plans and the other from a ladder. One may include the overhangs and the porch while the other ignores them. One may add an allowance for cutting and waste, and another may quote the net area and add the waste later.

Pitch is another source. Tiles overlap, and the number needed depends on the gauge and the lap, which in turn depend on the type of covering and the roof's slope. A quote for plain clay tiles and one for large interlocking concrete tiles will not use the same count, even on the same roof.

So the useful step is to ask each roofer how the area was worked out and what it includes. VAT and scaffolding deserve a look too, because leaving either out is a common reason one figure sits well below another. Our page on checking a roof before an auction covers the same questions for buyers who have little time.

On our surveys we take the dimensions we need from the roof itself where it can be reached safely, check the rest against pictures and make allowances for anything hidden. You receive those pictures with a written total price once the survey is done. If the area looks odd to you, ask where the figure comes from; the images and the measurements should line up with it.

For certain jobs, such as a mortgage or lender request, the measurements may not be the main issue at all. Our page on what a mortgage roof report is explains what lenders tend to ask for. For the role of pictures in the process, see why roof inspection photos matter.

Size the work to the fault, not to the biggest figure on the table: that is how we approach every roof, and work we carry out has a 10-year workmanship guarantee, in writing. To book a measured survey, ring 01793 626096 or send a WhatsApp, and see the full range under roof inspections and surveys.