Question answered
How do you measure a roof's area?
The short answer
To measure a roof's area, work out the plan area (the footprint including the overhang), then multiply it by a pitch factor. The factor is 1 divided by the cosine of the pitch angle, so a 30 degree roof is about 1.155 times its footprint. Measure from the ground or from plans, never by walking the roof, and add the roof's extras last.
A roof is bigger than the house beneath it, because every slope leans away from flat. The reliable method is to measure the plan area, then correct it for the pitch.
Plan area against sloping area
The plan area is what you would see looking straight down from above: the footprint of the roof, including the overhang at the eaves and the verges. The sloping area is the actual surface of the tiles, which is larger because the surface leans. A roof pitched at 30° has the same footprint as a flat one but more covering.
That is why a roofer asks for the pitch and not just the floor area of the house. A bungalow with 80 square metres of footprint might look like 80 square metres of roof, but the sloping area is noticeably more. The steeper the pitch, the bigger the gap between the two numbers.
Roof coverings, battens, underlay and labour are all priced against the sloping area, which is the number a quote relies on. For costs by area, see the typical UK ranges in our other guides, and remember that the written quote follows the survey, once the real surface is known.
The pitch factor and how to use it
The pitch factor converts plan area to sloping area. It is 1 divided by the cosine of the pitch angle, and a calculator or a printed table will give it. Some common values: 1.04 at 15°, 1.06 at 20°, 1.10 at 25°, 1.155 at 30°, 1.22 at 35°, 1.31 at 40° and 1.41 at 45°.
Take a simple example. A gable roof with a footprint of 60 square metres, pitched at 35°, has a sloping area of about 60 multiplied by 1.22, which is 73 square metres. Measure the footprint in metres, multiply by the factor and note the result, rounded sensibly.
Find the pitch from a drawing, from the tile maker's data or with a pitch finder or a phone inclinometer app held against a rafter in the loft. If your roof has two different pitches, such as a main roof and a lean-to, work each part separately and add them. Never use one factor for the whole house when the slopes differ.
Measuring the footprint safely
No climbing is needed. Measure the outside of the house at ground level, including any part that sits under the roof, then add the projection at the eaves and the gables. A tape measure, a laser measure or a measured plan will do. Count bays, porches, extensions and garages as separate roofs, each with its own pitch.
For a simple gable roof, a quicker route is to measure the length of the ridge and the slope length from ridge to eaves, which you can find from the loft or a drawing, then multiply and double for the two slopes. For a hipped roof, divide it into rectangles and triangles and add them.
If the property is high or the roof is awkward, a roofer can measure from the ground with a laser, a drone or a pole camera. Our guide to the pole camera roof inspection shows what that kind of reach can and cannot capture. Do not go onto the roof to measure it.
Hips, valleys, ridges and the awkward bits
A hip is the outward corner where two slopes meet, and a valley is the inward one. Both add cutting, waste and labour that a plain rectangle does not. Ridges and hips are measured in linear metres rather than in area, because they take ridge tiles and fixings by length.
Chimneys, roof windows and dormers come out of the covering area but add flashings, soakers and lead. A soaker is a small lead sheet that is worked in between tiles at a wall or chimney. Write the sizes down separately for the roofer rather than hiding them in one big number.
Allow for the extra material the cuts need. The amount varies with the roof's shape and the covering, and the roofer works it out from the plan, so do not try to guess it. Our should I pay for a roof inspection page explains when it makes sense to have this done properly.
Where the number gets used
You will be asked for the roof area when comparing quotes for replacement or cleaning, when planning solar panels and, often, when an insurer wants to know what is being covered. The insurer roof survey guide explains how the size and type of covering feed into a report.
Compare like with like. Check whether a figure is plan area or sloping area, and whether it includes the overhang, ridges, hips and valleys. A quote built on the wrong area will be wrong all the way through, and a difference of ten per cent in area is a lot of tiles.
Ask the roofer how they measured, and ask to see the figure on the written quote and in the survey. After a few years, roofs get re-measured whenever work is planned, which is a good moment to see what is up there. The full list of our inspection guides is under roof inspections and surveys.
Roof measuring: two quick questions
Can I use the floor area of my house to estimate the roof?
Only as a rough start. The roof is larger than the floor area because of the overhang and the slope, and rooms under the roof, porches and extensions change it again. Use the outside footprint and the pitch factor for a closer figure.
Is a flat roof measured the same way?
Almost. A flat roof has only a slight fall, so the plan area is close to the surface area, and you add upstands, parapet linings and edge trims by length. Check each fall and outlet, since those drive the detail.
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