Behind the maps

National view: RGA across France

The national RGA map available on Géorisques offers a synthetic view of the phenomenon across mainland France. It reveals a highly heterogeneous distribution: certain sedimentary basins, notably in Haute-Garonne, Tarn-et-Garonne and Haute-Vienne, show extremely high hazard levels, while other regions remain only lightly exposed.

This mapping relies on geological data at a 1:50,000 scale, produced in collaboration with universities, local authorities and government agencies. It is the reference tool for policymakers and homeowners wishing to assess their situation.

These areas correspond to specific geological contexts. Geotechnical research has established a correlation between the clay minerals present, their swelling potential and the depth of influence of movements. The extent of movement can reach 10 to 15 centimeters in very-high-hazard zones, directly impacting shallow building foundations.

Time evolution: how RGA is progressing

RGA evolution is not uniform. It results from several factors: first, the regular refinement of national geological databases, which allows better identification of clay formations. Second, the observable impact of climate change: repeated and prolonged droughts intensify shrinkage phenomena, revealing previously latent damage.

According to Météo-France climate projections, forecasts for 2050 indicate an increase in the frequency and intensity of extreme events (droughts, torrential rain). These sharper water variations will amplify differential ground movement, increasing risk for buildings already exposed.

For homeowners, this evolution raises an important question: even if your region wasn't classified at risk in 2017, a later reclassification is possible. This is why vigilance remains essential, and why regular diagnoses remain important.

By department: uneven exposure

This variability reflects deep geological differences. Geological formations dating from the Jurassic and Cretaceous, rich in smectites and montmorillonites, are the most sensitive. Conversely, granitic regions (Brittany, coastal Massif Central) or pure limestone areas show limited sensitivity, since the absence of swelling clay minerals removes the shrink-swell mechanism.

Official mapping available on Géorisques includes a ranking of departments by percentage of exposure. This data helps local authorities adapt their urban planning policies and helps homeowners understand their local context. Mandatory real estate diagnoses (Alur law) systematically mention this ranking in reports.

For homeowners, the departmental index provides a first level of screening. If your department shows less than 10% of its territory at risk, your risk profile will probably be low. Conversely, a ranking above 50% justifies particular attention and an in-depth diagnosis, even before buying or selling a property.

Île-de-France: zoom on the capital region

The Île-de-France region is a textbook case of urban RGA. Unlike the southwest, where the phenomenon is concentrated but extreme, the Paris region is characterized by widespread exposure to medium hazard, affecting millions of small homeowners and older residences. Tertiary geological formations (Eocene and Oligocene) contain blue-grey clay and marl layers that swell and shrink according to hydrological cycles.

Available data shows that nearly 18,000 municipalities in the Paris region have been compensated under RGA natural disaster claims between 1989 and 2023. Observed damage is mostly facade cracks (80%), followed by slab subsidence (15%) and basement swelling (5%).

For residents of the Paris region, this area benefits from excellent data coverage. Local authorities (departmental councils, the city of Paris) offer prevention aid and grants for diagnoses. Awareness is also advanced: notaries and property managers systematically include RGA risk assessment in their reports.

Aquitaine: one of the most affected areas

The reality of RGA in Aquitaine is stark. Available geological data shows that over 80% of the Haute-Garonne department rests on Jurassic formations containing smectites with very high swelling potential. This means even the newest homes, if built on shallow foundations, risk significant damage during prolonged drought periods (2003, 2012, 2022).

According to local expert reports, ground movement observed in Haute-Garonne reached 18 centimeters at certain points during the 2003 drought. Such amplitudes can crack entire foundations. This region has the highest average repair cost in the country, between €50,000 and €150,000 for an affected single-family home.

For homeowners in Aquitaine, this extreme exposure justifies preventive measures now: regular diagnoses every 5 years, monitoring groundwater depth, strict water management around the building (gutters, drainage, controlled watering). Hydro-stabilization solutions are also gaining popularity in this region, since alternatives (micropiles) represent a huge investment incompatible with individual budgets.

What does this mapping mean for you?

RGA maps aren't fatal predictions. They are probabilistic representations of geological risk. If your home is in a high-hazard zone, this means exposure to ground movement is likely, but not certain. Several local factors play a role:

• •Your foundations: a home on deep foundations (micropiles, piles) is little affected; a home on shallow foundations runs more risk
• •Your local position: even in a high-hazard zone, some micro-areas can be spared thanks to local geology or water management
• •Your preventive action: good water management (gutters, drainage, moisture regulation) can significantly reduce risk
• •Available solutions: several approaches (hydro-stabilization, micropiles, monitoring) can limit future damage

The real usefulness of these maps? Letting you move from ignorance to action. Rather than discovering a problem during a difficult sale or after a costly claim, you can now assess your situation and put an adaptation strategy in place.

Frequently asked questions about RGA maps