Clay shrinkage-swelling exposure map 2026.
© BRGM
Around 240,000 damage claims due to the shrinkage and swelling of clay-rich soil were reported between 2018 and 2022, accounting for 58% of all claims since 1989 (MRN figures). Faced with this situation, the French government asked BRGM and the Mission Risques Naturels to update the exposure map for this phenomenon. The new zoning will apply under Article 68 of the Élan Act from 1 July 2026.
What do we mean by “clay shrinkage and swelling”?
When ground contains a certain percentage of clay, it can be highly sensitive to variations in its water content. Like a sponge, it shrinks in dry periods and swells in wet ones.
This slow-onset phenomenon, known as clay shrinkage and swelling can be significant enough to damage buildings and roads. Every year, it causes considerable damage to buildings, which can be compensated as natural disasters. The vast majority of claims concern single-family homes.
An initial clay shrinkage and swelling exposure map was published in 2019. Since 2020, as part of the Élan Act, this exposure map has been used as a framework for implementing appropriate building recommendations in the most exposed areas (medium and high exposure zones).
Comparison between the 2019 and 2026 exposure maps
Clay shrinkage-swelling exposure map 2019 (left) and 2026 (right).
© BRGM
55% of mainland France is now affected by clay shrinkage and swelling
Since the publication of this first zoning, a large number of claims have been made, particularly following the drought of 2022. One of the reasons why the French government wanted to update the risk exposure map was to take account of this high level of damage claims.
This initiative is part of the national plan to adapt to climate change, known as PNACC-3, one of the aims of which is to anticipate damage to buildings caused by the shrinkage and swelling of clay.
The new map, enacted by a decree published on 9 January 2026, will be appended to promises of sale or deeds of sale for undeveloped building land and to contracts for the construction of single-family homes signed as of 1 July 2026.
55% of France is now affected by medium or high exposure, compared with 48% previously.
How is exposure to clay shrinkage and swelling assessed?
To define the level of exposure of a given area to clay shrinkage and swelling, scientists take two factors into account:
- Predisposition of ground. To produce a map on a national scale, we need to study the nature of the ground: the presence of clay and marl formations, and the degree to which these formations are susceptible to shrinkage and swelling.
- Claims related to building damage,based on Cat Nat [Natural Catastrophe] data recorded by the Natural Hazards Mission (MRN)
In practical terms, the exposure map is produced by cross-referencing geological, lithological (nature of the rock), mineralogical and geotechnical data, on the one hand, with damage claims data, on the other, i.e. claims for incidents that have actually occurred.
