How to calculate your roof surface area
By the Toiture Wash team · Updated on 20 October 2026 · 3 min read
Short answer. Multiply the ground area covered by the roof, overhangs included, by the coefficient for its pitch: 1.15 for a roof at 30°, 1.22 at 35°, 1.41 at 45°. A house of 10 m by 8 m on the ground, with a roof at 35°, therefore has about 98 m² of roof, and about 116 m² with 40 cm overhangs. It is this surface, not the house's, that is used to calculate the price of a moss removal.
Why the roof is bigger than the house
A sloping roof is longer than the space it covers: seen from above, a roof slope looks shorter than it is. The steeper the pitch, the greater the difference. On a flat roof, the roof surface equals the ground area; on a roof at 45°, it is 41% larger.
The coefficient table
| Pitch | Coefficient | A ground area of 100 m² gives |
|---|---|---|
| 15° | 1.04 | 104 m² of roof |
| 20° | 1.06 | 106 m² |
| 25° | 1.10 | 110 m² |
| 30° | 1.15 | 115 m² |
| 35° | 1.22 | 122 m² |
| 40° | 1.31 | 131 m² |
| 45° | 1.41 | 141 m² |
| 50° | 1.56 | 156 m² |
The coefficient is the reciprocal of the cosine of the pitch angle (1 ÷ cos). The rule applies to a roof with two slopes as well as a roof with four slopes, as long as all the slopes have the same pitch: it is the ground area that counts, not the number of slopes.
The method, in four steps
- Measure the ground area covered by the roof. Length times width of the house, or of each part if the house has an extension. Without a tape measure: the "Measure distance" tool in Google Maps, on a computer, gives the area of a closed shape drawn on the aerial view; drawn around the outline of the roof, it already includes the overhangs (then skip step 2).
- Add the overhangs. The roof often projects 30 to 50 cm beyond the walls. Add the length of the walls concerned multiplied by the width of the overhang.
- Find the pitch. On the plans of the house, or in the loft with a phone inclinometer app placed on a rafter.
- Multiply the ground area, overhangs included, by the coefficient.
Dormers, extensions with a different pitch and flat roofs are calculated separately, then added together.
Three examples
| House | On the ground | Pitch | Calculation | Roof |
|---|---|---|---|---|
| Terraced house, 6 m frontage by 10 m | 60 m² | 45° | 60 × 1.41 | about 85 m², before overhangs |
| Detached house, 12 m by 9 m, 40 cm overhangs | 108 m², plus 17 m² of overhangs | 30° | 125 × 1.15 | about 144 m² |
| Bungalow, 15 m by 10 m | 150 m² | 25° | 150 × 1.10 | about 165 m², before overhangs |
You have neither a plan nor a tape measure?
No problem: the Toiture Wash estimator only asks you for the ground area of the house, with guides if you do not know it (terraced, semi-detached, detached). It uses it to give a price range. For the written quote, we measure your roof ourselves, on Google Earth and without a site visit. On the day of the job, the team checks the measurements on site; if there is a difference, the invoice is based on the measurements taken on site.
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Frequently asked questions
Is the price of a moss removal calculated on the ground area?
At Toiture Wash, on the roof surface: €10 per m² excl. VAT. The pitch does not change this price per m², it adds surface. Beware of a very low price per m² applied to the ground area: at 45°, the difference reaches 41%.
How can you find the pitch without going up on the roof?
From the plans of the house, or in the loft, with a phone inclinometer app placed on a rafter.
Do solar panels count towards the surface area?
They cover part of the roof, which is not cleared of moss underneath; their cleaning is charged per panel. Mention them in your request.