Clay shrink–swell (RGA): 38 French municipalities at very high risk of building cracks

Graham Whitfield • August 12, 2026 15:27

This is a deeply troubling issue for the people affected.

It is not a problem to shrug off: cracks appearing on building façades could end up making life extremely difficult for homeowners and property professionals alike. For residents, these fissures feel like a ticking time bomb, with the constant worry that the building will deteriorate further-or even ultimately collapse. Here is what you need to know about this phenomenon.

38 French municipalities at very high risk

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Last year, the website L’Internaute analysed data on municipalities in which at least 90% of the land area is exposed to a high risk of cracking. These findings back up a projection published in 2021 by France Assureurs, a federation representing 247 companies in the sector.

What clay shrink–swell (RGA) is

The cracking seen on many buildings is more precisely referred to as “clay shrink–swell” (RGA). It occurs when clay-rich soils take up water-particularly during the wet autumn and winter months-and then shrink during dry spells, especially in summer. These repeated changes in volume cause ground movement that weakens house foundations and can lead to cracks that are sometimes substantial.

Where the RGA risk is concentrated

At present, thirty-eight municipalities are especially affected by the RGA phenomenon. More than half are located in Haute-Garonne and neighbouring departments, while others are found on the Côte d’Azur. Additional areas are also affected, though to a lesser extent. Altogether, more than 515,000 residents are exposed to this risk.

Here is the list of the 38 municipalities particularly affected by this RGA:

Aniche: 91.72% of the municipal area exposed to level 3 RGA (the highest stage)
Apt: 93.52%
Auch: 99.99%
Balma: 99.99%
Bon-Encontre: 90.08%
Castanet-Tolosan: 94.39%
Castelginest: 100.00%
Caussade: 99.26%
Condom: 99.51%
Coubron: 92.44%
Escalquens: 95.94%
Esvres: 95.90%
Fontenay-sous-Bois: 91.33%
Foulayronnes: 98.01%
Gardanne: 94.83%
Gignac-la-Nerthe: 99.50%
La Verrière: 99.87%
Le Pré-Saint-Gervais: 92.60%
Les Ulis: 99.74%
L’Isle-Jourdain: 99.75%
Montfermeil: 98.98%
Nandy: 97.62%
Noisy-le-Sec: 90.82%
Ormesson-sur-Marne: 99.99%
Port-de-Bouc: 90.01%
Pulnoy: 96.66%
Quint-Fonsegrives: 98.26%
Ramonville-Saint-Agne: 99.37%
Revel: 93.11%
Rognac: 97.17%
Rousset: 99.10%
Saint-André-de-Cubzac: 90.85%
Saint-Jean: 99.99%
Saint-Orens-de-Gameville: 98.18%
Sceaux: 94.07%
Seichamps: 94.88%
Vaujours: 90.52%
Velaux: 97.09%

What France Assureurs reported in 2021

In its 2021 report, France Assureurs also highlighted this drought-related threat:

“The drought peril concerns the phenomenon of subsidence, meaning damage caused to buildings (almost exclusively detached houses) by the effect of shrinkage, then swelling, of subsoils linked to an initial lack of rainfall combined with high temperatures.”

More information is available on the organisation’s dedicated page.