Radon, cavities, earthquakes: how do you read Géorisques for an address?
For each address, Géorisques gives the commune’s radon potential (category 1 to 3), the seismic zone (1, very low, to 5, high), recorded underground cavities and known ground movements. These are ratings by commune or area, not measurements of the house. Radon category 3 or seismic zones 2 to 5 must appear in the seller’s state-of-risks report.
Radon: a rating by commune
Radon is a natural radioactive gas that comes from certain rocks, granite in particular, and can build up in poorly ventilated rooms. An order of 27 June 2018 places each commune in one of three categories:
| Category | What it means |
|---|---|
| 1 | Geological formations with the lowest uranium content |
| 2 | Low content, but faults, karst or mines can help radon rise |
| 3 | Higher content on at least part of the commune |
The category does not give the concentration in a particular house: that depends on the building, the ventilation and how it is used. Only a measurement over several weeks in the heating season gives it. In category 3, the state-of-risks report must include the radon information sheet.
Other data often matter for the same house, such as airport and road noise and clay shrink-swell. You can see how a dossier sets them out register by register in the sample dossier.
Earthquakes: five zones
France is divided into five seismic zones: 1 very low, 2 low, 3 moderate, 4 medium, 5 high. Zones 4 and 5 are mainly in the Alps, the Pyrenees and the French Caribbean; much of the Paris basin is in zone 1.
For an existing house the zone changes nothing at sale, except that the state-of-risks report must mention it, with an information sheet, from zone 2 upwards. For a new building or a large extension, earthquake-resistant building rules may apply depending on the zone and the type of building; the architect or builder knows them.
Underground cavities and ground movements
Géorisques records known cavities: old quarries, marl pits, cellars, military works, natural caves. The inventory is not exhaustive; it depends on what has been reported or studied. Anyone who knows of an underground cavity whose collapse may cause damage must inform the mayor (art. L563-6 of the Environment Code).
Ground movements cover landslides, rockfalls, collapses and mudflows. They are recorded event by event, with a point on the map. A nearby point is not proof of danger to the house, but a reason to ask the mairie, which sometimes knows of local studies or a ground-movement risk prevention plan.
The servitudes that burden a plot
Reading the Géorisques page for an address
- Check that the point placed is really the house: in the countryside, the address may be placed at the roadside.
- Separate what applies to the commune (radon, seismicity) from what applies to the point (cavities, movements, clay, flooding).
- Note the natural disaster orders and their type: drought, flood, ground movement.
- Compare with the seller’s state-of-risks report: the two should match.
This information does not say whether the house is sound or dangerous. It tells you which questions to ask: the seller, about past claims and works; the mairie, about local studies and prevention plans; a building professional, about basement ventilation, the state of the foundations or the nature of the ground. Write down the answers before the compromis: once it is signed, only the ten-day cooling-off period is left to change your mind.
ScanAdresse takes these registers for one address (radon, seismicity, cavities, ground movements, clay, flooding, natural disaster orders) and shows found, not found or source unreachable for each; the preview is free.
Frequently asked questions
Should you measure radon before buying?
It is not required for a sale. In category 3, a measurement after the purchase, in winter, tells you whether ventilation or floor sealing deserve work.
Does the seismic zone lower the price?
Nothing allows a general answer. Compare with actual sales in the area, which already reflect local conditions.
Is a recorded cavity near the house serious?
It depends on its type, depth and distance. The mairie and, if needed, a soil engineering firm can advise you before the compromis.
Sources
- Géorisques: radon
- Géorisques: earthquakes
- Géorisques: underground cavities
- Géorisques: ground movements
- Environment Code, art. L563-6 and R125-24
- Service-public.fr: state-of-risks report
ScanAdresse is a private service by SRI Group BV (VAT BE1037052239), not connected to any public authority: it brings together public data, with the source and date of every item.
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