2026 shrink-swell map: geology has not changed, risk exposure has

The new map does not say that clay formations have changed. It says that regulatory exposure has been reassessed using the available evidence.

A municipality moving from low to medium, or from medium to high, has not geologically transformed. It has been reclassified because its estimated exposure level is higher under the updated national method.

The change is significant. According to BRGM and Géorisques, 55 % of mainland France is now classified as medium or high exposure to clay shrink-swell, compared with 48 % previously.

The update notably incorporates about 240,000 shrink-swell damage claims recorded between 2018 and 2022. That represents 58 % of all shrink-swell claims recorded since 1989.

The implications are practical: information for buyers, sales of buildable land, contracts for individual house construction, soil studies, prevention, insurance and local risk perception.

The new zoning applies from 1 July 2026 to preliminary sale agreements, final sale deeds for undeveloped buildable land and individual house construction contracts.

The official map defines exposure zoning. The fiñv visualisation aggregates that zoning at municipal scale to compare the 2020 and 2026 versions.

Each municipality receives a surface-based exposure class derived from the share of its territory classified as not exposed, low, medium or high exposure. To map change, these four classes are converted into an ordinal scale: not exposed = 0, low = 1, medium = 2, high = 3. The change map represents the delta between 2026 and 2020. A +1 change corresponds to an increase in exposure and appears in orange. A +2 or greater change corresponds to a strong increase and appears in red. Conversely, -1 corresponds to a decrease in exposure shown in light blue, and -2 or lower to a strong decrease shown in dark blue. Municipalities with no change appear in white or light grey.

This is a deliberately synthetic reading. It reveals broad territorial patterns: stable municipalities, upward shifts, strong upward shifts and the much rarer cases of decrease.

But it does not describe buildings directly. A municipality may contain large high-exposure surfaces with few houses. Conversely, a smaller exposed area may concentrate a large share of the built environment.

This is the main limitation of the map, and also the starting point for the next visualisation: comparing the municipal territory, the areas classified as shrink-swell exposure zones and the buildings actually located within them.

A geological map answers one question: what is beneath our feet?

A clay shrink-swell exposure map answers a different question: where can this phenomenon damage buildings?

Clay shrink-swell depends on the presence of sensitive clay soils, but not only on that. It also depends on water, drought, vegetation, soil sealing, foundation depth, building quality and the way damage is declared, assessed, recognised and compensated.

A shrink-swell exposure map is therefore not just a map of the subsurface. It is a national regulatory zoning layer for a territory that is inhabited, built and managed.

The 2026 update does not only add recent damage records. It also changes how those records are interpreted.

The 2020 map relied in part on a density of claims per square kilometre of urbanised area. The 2026 update gives more weight to claim frequency: the number of claims is compared with the number of individual houses, using the national buildings database.

This matters because individual houses are often more vulnerable to clay shrink-swell than apartment buildings, especially when they rely on lighter foundations.

Relating claims to the number of individual houses therefore gives a better comparison between observed damage and the potentially vulnerable built asset.

The map is thus a combined reading of clay-bearing formations, climate, individual houses and observed damage.

The physical mechanism is simple in principle.

A clay-rich soil can swell when it becomes wet. It can shrink when it dries. These volume changes generate differential ground movement. A house built on such soil does not always move uniformly: foundations are stressed, walls crack, doors jam.

The phenomenon itself is not new. What is changing is the intensity and frequency of hydrological cycles: more severe droughts, repeated heatwaves and sharper alternations between dry periods and rainfall.

Climate change does not create the clays. It makes their effects more frequent, more intense and more costly when buildings are vulnerable.

A recognised damage claim is not only a geological signal. It results from a full chain: a clay soil, a water deficit, a vulnerable building, a declaration, a municipal request, an administrative procedure, an insurance assessment and sometimes a dispute.

Damage records are therefore valuable, because they show where damage has been declared, assessed and sometimes recognised. But they must be interpreted with care: they describe both a physical process and the way a society builds, declares, recognises and compensates ground-related damage.

This point deserves a dedicated article, because it explains why an exposure map can never be read as a simple soil map.

High exposure to clay shrink-swell does not automatically mean that a municipality will obtain a drought-related CatNat recognition.

Recognition depends on several conditions. The presence of sensitive clays is necessary, but not sufficient. Hydrological conditions, drought intensity, the criteria used by the State and observed damage also matter.

A municipality can therefore be highly exposed to clay shrink-swell without recent CatNat recognition. Conversely, some municipalities may show a strong insurance signal while their mapped exposure appears more limited.

This difference between geotechnical exposure, observed drought and administrative recognition is one of the key challenges in reading shrink-swell risk at national scale.

This is why an exposure map must be read alongside climate data, observed damage, CatNat criteria and the actual vulnerability of buildings.

The shrink-swell exposure map is a national regulatory map. It does not predict the future behaviour of a specific house and it does not replace a parcel-scale soil investigation.

Its reference scale is that of 1:50,000 geological maps: it indicates an exposure context, not the exact nature of the soil beneath each building.

A municipality classified as medium or high exposure is not uniformly exposed. Within the same municipality, there may be sensitive clay areas, less affected sectors, vulnerable buildings and buildings with much lower exposure.

Conversely, a municipality that appears weakly exposed at map scale may still contain local situations: small clay bodies, local weathering, clay-rich veneers or specific hydrological configurations.

The map is used to identify an exposure context, guide prevention, inform owners and buyers, and frame regulatory obligations. It does not replace local analysis.

The 2026 map update is not just a colour change on a public portal. It may have concrete effects: more municipalities affected by information obligations, more buildable land subject to preliminary geotechnical studies and greater attention to shrink-swell risk in property sales, construction contracts and territorial diagnostics.

But understanding actual risk requires more than a surface-based reading. We need to know where buildings are located, how they are built, what hydrological context they sit in and how the territory has already responded to droughts.

The next visualisation will compare three complementary readings: the total municipal area, the area classified as shrink-swell exposure zoning and the buildings located within exposed zones.

This will provide a stronger link between shrink-swell mapping, CatNat criteria, observed drought and the real vulnerability of territories.

This page is based on public resources from BRGM, Géorisques and the French State concerning the 2026 update of the clay shrink-swell exposure map, the zoning methodology, the expert dossier on shrink-swell and the natural disaster compensation system.