Biot: Clay Soil, Subsidence and Cracks – Official Data

What the official clay mapping says

The GéoRisques platform (BRGM) classifies Biot at high exposure to clay shrink-swell (RGA), with some sectors at medium exposure. This is the benchmark level used by insurers and notaires to assess the risk on a plot. Note also: radon potential of class 2 in the commune (IRSN/GéoRisques data) – a building-health point of vigilance in the affected zone.

This ranking complements the decree history above: the hazard tells you what can happen, the decrees tell you what has already happened. A house in a “high exposure” area that has been through one or more dry summers carries a risk of damage that is all the better documented.

What the official clay mapping says

The GéoRisques platform (BRGM) classifies Biot at high exposure to clay shrink-swell (RGA), with some sectors at medium exposure. This is the benchmark level used by insurers and notaires to assess the risk on a plot. Note also: radon potential of class 2 in the commune (IRSN/GéoRisques data) – a building-health point of vigilance in the affected zone.

This ranking complements the decree history above: the hazard tells you what can happen, the decrees tell you what has already happened. A house in a “high exposure” area that has been through one or more dry summers carries a risk of damage that is all the better documented.

What characterises Biot from a building surveying perspective

In Biot, I am never surprised to find cracks as soon as I open a survey file. The commune steps down from the old perched village on its hill to the plain near the coast, by way of hillsides planted with pines and oaks whose roots, in periods of drought, pump the moisture from the clay soils and compound the shrinkage phenomena. The housing stock is highly heterogeneous: 1970s-80s villas built on lightly reinforced shallow foundations, Provençal mas renovated over the decades, and more recent schemes from the estates that have multiplied on the heights since the 2000s. This diversity of constructions, set on sloping ground where the clay alternates with pockets of marl, favours differential settlement as soon as the ground starts moving – swelling in winter, shrinking in summer. The same damage is systematically observed on individual houses built without a prior soil survey, particularly those erected before the generalisation of seismic standards and deep foundations. The tree-filled gardens, very much part of this sought-after residential area, compound the phenomenon further when the planting sits too close to the load-bearing walls. For me, Biot combines all the ingredients of an area to watch: pronounced relief, sensitive clay and an older housing stock little prepared for ground movement.

Drought in the rear-view mirror: 35 years of measured rainfall

The weather archives for the area cover 35 complete years (1991-2025). The driest ever recorded: 2017, with 475 mm over the year, against a local average of 908 mm. 2022 ranks as the 3rd driest year out of 35 (600 mm against an average of 908 mm); 2023 follows in 4th place (635 mm).

This ranking gives the true measure of the recent sequence: when the years 2020-2023 crowd into the top of the drought table, the clay has not had time to rehydrate between episodes – the classic scenario for delayed cracks. The reverse is just as instructive: a very wet year following an exceptional drought causes abrupt swelling of the ground.

Biot: 23 Cat Nat decrees since 1982, in chronological order

Each line = an event recognised by an interministerial decree published in the Official Gazette (GASPAR database):

• 🔴 09/1995 → 12/1995 – Drought (clay shrink-swell, RGA) (decree of 17/12/2002)
• 🔴 07/2003 → 09/2003 – Drought (clay shrink-swell, RGA) (decree of 11/01/2005)
• 🔴 01/2007 → 03/2007 – Drought (clay shrink-swell, RGA) (decree of 07/08/2008)
• 🔴 07/2007 → 09/2007 – Drought (clay shrink-swell, RGA) (decree of 07/08/2008)
• 🔴 04/2016 → 09/2016 – Drought (clay shrink-swell, RGA) (decree of 24/10/2017)
• 🔴 04/2022 → 09/2022 – Drought (clay shrink-swell, RGA) (decree of 03/04/2023)
• 🔴 03/2023 → 06/2023 – Drought (clay shrink-swell, RGA) (decree of 23/09/2024)

Expert’s reading: 7 droughts and 16 floods. Clusters (several decrees close together) mark the crisis periods – the summer of 2022-2023 is the best recent example. An owner whose damage dates from a covered period has a solid Cat Nat file; outside a covered period, the route is the action against the seller.

The complete chronology of the 23 natural disasters recognised in Biot

Each line = an event recognised by an interministerial decree published in the Official Gazette (GASPAR database):

• 🔵 11/1982 → 11/1982 – Cyclones / Storms (decree of 15/12/1982)
• 🔵 10/1987 → 10/1987 – Floods (decree of 02/12/1987)
• 🔵 10/1993 → 10/1993 – Floods (decree of 19/10/1993)
• 🔴 09/1995 → 12/1995 – Drought (clay shrink-swell, RGA) (decree of 17/12/2002)
• 🔵 01/1996 → 01/1996 – Floods (decree of 02/02/1996)
• 🔵 12/1996 → 12/1996 – Floods (decree of 24/03/1997)
• 🔵 10/1999 → 10/1999 – Floods (decree of 03/03/2000)
• 🔵 10/2000 – Floods (decree of 29/05/2001)
• 🔵 10/2000 → 10/2000 – Floods (decree of 19/12/2000)
• 🔵 12/2000 – Floods (decree of 03/04/2001)
• 🔴 07/2003 → 09/2003 – Drought (clay shrink-swell, RGA) (decree of 11/01/2005)
• 🔵 09/2005 → 09/2005 – Floods (decree of 16/12/2005)
• 🔴 01/2007 → 03/2007 – Drought (clay shrink-swell, RGA) (decree of 07/08/2008)
• 🔴 07/2007 → 09/2007 – Drought (clay shrink-swell, RGA) (decree of 07/08/2008)
• 🔵 11/2011 → 11/2011 – Floods (decree of 18/11/2011)
• 🔵 11/2014 → 11/2014 – Floods (decree of 17/02/2015)
• 🔵 11/2014 → 11/2014 – Floods (decree of 03/03/2015)
• 🔵 10/2015 – Floods (decree of 07/10/2015)
• 🔴 04/2016 → 09/2016 – Drought (clay shrink-swell, RGA) (decree of 24/10/2017)
• 🔵 11/2019 → 11/2019 – Floods (decree of 28/11/2019)
• 🔵 12/2019 → 12/2019 – Floods (decree of 13/01/2020)
• 🔴 04/2022 → 09/2022 – Drought (clay shrink-swell, RGA) (decree of 03/04/2023)
• 🔴 03/2023 → 06/2023 – Drought (clay shrink-swell, RGA) (decree of 23/09/2024)

Expert’s reading: 7 droughts and 16 floods. Clusters (several decrees close together) mark the crisis periods – the summer of 2022-2023 is the best recent example. An owner whose damage dates from a covered period has a solid Cat Nat file; outside a covered period, the route is the action against the seller.

Damp, water ingress and rising damp in Biot

Damp in a wall can have four causes – rising damp, water ingress, condensation or a leak – and the treatments are completely different depending on the origin.

A stained wall, a swelling skirting board, a musty smell in Biot: before signing a resin injection quotation or having a ventilation system installed, the cause has to be precisely identified. NOT so fast: a wrong diagnosis pays for a useless treatment while the real problem goes on damaging the building.

The four causes and how to recognise them

• Rising damp: groundwater rises into the masonry by capillary action. Stains climbing from the foot of the wall, often as a tide mark, whitish salts, render blistering near the ground.
• Water ingress: water penetrates from outside (roof, cracked façade, poorly sealed joinery, poorly waterproofed terrace). Localised stains, often high up or near an opening, worsening after rain.
• Condensation: humid indoor air condenses on a cold wall, through lack of ventilation or a thermal bridge. Surface mould, often in corners, bathrooms or poorly aired bedrooms, with no link to the ground.
• Leak: leaking pipe or buried run. Localised stain near a water supply, drain or beneath a bathroom, sometimes with an unexplained excess water consumption.

The trap of the diagnosis sold by the treatment contractor

In Biot, many owners call a damp-treatment company directly. The problem: that company makes its living from selling injections and waterproofing products, so it has no interest whatsoever in looking elsewhere. Frequent outcome: a wall injected against rising damp that never existed, when the real problem was a blocked gutter or a missing ventilation system. An independent diagnosis, with no works to sell at the end, identifies the true origin before any expenditure is committed.

Before any treatment in Biot, request an independent diagnosis. To go further, consult the damp guide for the 06, 83 and 13.

Biot: drought, clay and cracks – what you should check in your home

If your house in Biot is showing cracks, here is the quantified context before calling anyone: the GASPAR database lists 7 drought decrees (full list below) and 16 flood decrees, i.e. 23 natural disaster declarations (Cat Nat) for this commune. The local weather records confirm the scenario: four deficit years in a row (2020, 2021, 2022, 2023), then an abrupt rebound in 2024 to 1,239 mm (+50%) – exactly the kind of alternation that puts foundations to work between Valbonne and the coast, in the heart of the glassmakers’ village.

Cassini Expertise, an independent building practice, works in Biot and throughout the 06 diagnosing drought-related cracks. This page sets out the commune’s official data, with no commercial angle, and tells you what to do with it in practical terms.

Is my house in Biot covered by a drought decree?

According to GASPAR, the national database listing decrees published in the Official Gazette since 1982, Biot has recorded a total of 23 natural disaster decrees:

• 7 drought decrees (differential ground movement linked to clay shrink-swell)
  • event of 01/09/1995 (decree of 17/12/2002)
  • event of 01/07/2003 (decree of 11/01/2005)
  • event of 01/01/2007 (decree of 07/08/2008)
  • event of 01/07/2007 (decree of 07/08/2008)
  • event of 01/04/2016 (decree of 24/10/2017)
  • event of 01/04/2022 (decree of 03/04/2023)
  • event of 31/03/2023 (decree of 23/09/2024)
• 16 decrees flood and storm

What to take from it for your file: if your cracks appeared during one of these windows, Cat Nat compensation is conceivable – provided you declared the claim to your insurer within 30 days of the decree. If the date matches no decree, there remains the recourse against the seller, for latent defect or fraudulent misrepresentation, and it is the precise dating of the damage that will make the difference.

Is my plot in a clay zone in Biot?

The GéoRisques platform (BRGM) classifies Biot at high exposure to clay shrink-swell (RGA), with some sectors at medium exposure. This is the level your insurer and your notaire already consult to assess the risk on your plot – you can check it yourself on georisques.gouv.fr by typing in your address. Note also: radon potential of class 2 in the commune (IRSN/GéoRisques data) – a point to monitor if you spend time in basement or poorly ventilated rooms.

This ranking complements the decree history above: the hazard tells you what can happen, the decrees tell you what has already happened. If your house is in a high-exposure area and has come through one or more dry summers, the risk of damage is no longer a hypothesis, it is documented.

Why do cracks appear several months after the drought?

Annual rainfall measured on the Biot weather grid (Open-Meteo data):

• 2019: 1,167 mm · 2020: 679 mm · 2021: 639 mm · 2022: 600 mm · 2023: 635 mm · 2024: 1,239 mm
• Average over the period: 826 mm/year, low point in 2022 with 600 mm
• Four consecutive deficit years (2020-2023): 679 mm, then 639 mm, then 600 mm, then 635 mm
• Then a surge in 2024 to 1,239 mm, i.e. +50% against the period average

Clay shrink-swell plays out in two acts: the drought dries the clay at depth, which shrinks and triggers the first cracks; then the return of the rain rehydrates the ground in one go, which swells and accentuates the differential movements. If you live in a 1970-2000 villa built on shallow foundations, watch your walls with a lag of 12 to 24 months after each marked climate episode – that is when cracks linked to a dry summer come to light.

Which cracks should worry me in Biot?

• Stair-step crack, through-wall, open by more than 2 mm
• A crack that is evolving over time – fit a tell-tale (a plaster strip or a commercially bought tell-tale) and check it over several months
• Associated signs to check as well: doors and windows sticking, floor tiles splitting diagonally, render bulging
• Aggravating factors on your plot: clay ground, trees close to the house, failing drainage

In Biot, with its record of 7 drought decrees, a combination of several of these signals should push you to have the house surveyed before any repair – and before any buying or selling decision.

How can I tell whether the ground under my house has really moved?

The piezometric station for the sector (ADES/Hubeau network, regular measurements) tells the story of the drought better than any speech. Mean summer water table level, in metres NGF:

• summer 2021: 12.48 m
• summer 2022: 12.33 m
• summer 2023: 12.4 m
• summer 2024: 12.69 m
• summer 2025: 12.54 m

The water table remains relatively stable over the period (last known summer: 12.54 m). This back-and-forth sums up shrink-swell: the clay ground shrinks when the water table drops, then swells again when it rises. Your foundations do not follow these movements at the same pace – hence cracks appearing 12 to 24 months after the dry summers. If you are buying a property in Biot, systematically ask whether any repair works have already taken place after 2022 or 2023.

Is my cracked house in Biot compensable if I am buying or selling?

Between the recent drought decrees and the volume of transactions under way, quite a few properties change hands with damage passed over in silence. If you are the buyer, you have two levers at your disposal:

• Latent defect (Article 1641 of the Civil Code): serious cracks, not visible at the viewing, predating the sale → you can claim a price reduction or cancellation of the sale, within 2 years of discovery
• Fraudulent misrepresentation (dol) (Article 1137): the seller knew and said nothing → you can claim cancellation and damages, within 5 years; the commune’s GASPAR decree then strengthens your file

To engage either of these procedures, call on a property lawyer – the technical survey comes first, to establish the facts.

What actually happens during a drought/clay survey in Biot?

• On-site visit: mapping and classification of your cracks, measurement of openings, associated functional damage
• Context: clay nature of the soil, the commune’s Cat Nat record (7 drought decrees), local drought sequences
• Repair cost estimates: underpinning, injections, drainage – with several scenarios to suit your budget
• Written report, which you can pass directly to your insurer or your lawyer

Independent expert, with no ties whatsoever to a works company. Request a visit in Biot – also worth reading, our complete guide to RGA and Cat Nat in the 06, 83 and 13.

Soil surveys on clay ground: what the regulations say

The requirement for a prior geotechnical study has applied since 1 October 2020 (ELAN Act, art. 68); the new exposure map of 1 July 2026 widens its scope. This obligation, stemming from ELAN Act no. 2018-1021, article 68 (Decree no. 2019-495 of 22 May 2019), applies particularly to Biot (high exposure in the BRGM ranking).

• What it changes for building: the soil survey is no longer an option – the ELAN Act mandates it in exposed zones for any sale of building land and any construction contract. Adapted foundations, ground treatment, drainage arrangements: the specifications flow directly from the survey.
• What it changes when buying a recent property: ask whether the survey was carried out and which specifications were applied. A house built without heeding the geotechnical recommendations on clay ground is a medium-term cracking risk.
• What it changes when selling: the absence of a survey on a property built before the obligation is not a defect in itself – but if damage appears, a seller who knew about it and concealed it exposes themselves to an action for fraudulent misrepresentation.
• The link with the data above: Biot combines high exposure and a documented history of drought decrees. Building or buying here without checking the nature of the ground means building on a known and measured risk.

Cassini Expertise handles these files: compliance checks between geotechnical specifications and execution, surveys of damage on clay ground, second-opinion expert assessments. Request a consultation.

Soil surveys on clay ground: what the regulations say

The requirement for a prior geotechnical study has applied since 1 October 2020 (ELAN Act, art. 68); the new exposure map of 1 July 2026 widens its scope. This obligation, stemming from ELAN Act no. 2018-1021, article 68 (Decree no. 2019-495 of 22 May 2019), applies particularly to Biot (high exposure in the BRGM ranking).

• What it changes for building: the soil survey is no longer an option – the ELAN Act mandates it in exposed zones for any sale of building land and any construction contract. Adapted foundations, ground treatment, drainage arrangements: the specifications flow directly from the survey.
• What it changes when buying a recent property: ask whether the survey was carried out and which specifications were applied. A house built without heeding the geotechnical recommendations on clay ground is a medium-term cracking risk.
• What it changes when selling: the absence of a survey on a property built before the obligation is not a defect in itself – but if damage appears, a seller who knew about it and concealed it exposes themselves to an action for fraudulent misrepresentation.
• The link with the data above: Biot combines high exposure and a documented history of drought decrees. Building or buying here without checking the nature of the ground means building on a known and measured risk.

Frequently asked questions

Is Biot in a clay risk zone?

Yes. The Alpes-Maritimes is very largely covered by the clay shrink-swell hazard (BRGM mapping, consultable on georisques.gouv.fr). For Biot, the 7 drought decrees recorded in GASPAR confirm that the risk is not theoretical: it has already materialised several times.

Can a cracked house in Biot be covered under natural disaster provisions?

Only if your damage can be linked to a period covered by a decree (the last recognised periods: 2002, 2005, 2008, 2017, 2023, 2024) and if you declared the claim within 30 days. Failing that, the recourse against the seller remains open to you – and it is the dating of the damage by an expert that then becomes decisive for your file.

Can the data on this page be checked independently?

Yes: the decrees come from the GASPAR database (Official Gazette, open data, 2025 update), the rainfall from Open-Meteo archives for the Biot grid. Cassini Expertise merely compiles them, without adjusting them – you can cross-check every figure yourself before making a decision.

What is the rainfall situation? (as at 05/10/2026)

In the commune: no notable rain for 26 days. Rainfall over the last 30 days: 2 mm, against roughly 59 mm expected at this time of year – a 97% deficit.

A sustained deficit over this window, especially in the growing season, often precedes by a few weeks the drops in the water table shown above: it is the first signal owners can track themselves, before the cracks.

What is the rainfall situation today? (as at 12/10/2026)

In the commune: recent rain (5 days ago). Rainfall over the last 30 days: 23 mm, against roughly 69 mm expected at this time of year – a 67% deficit.

A sustained deficit over this window, especially in the growing season, often precedes by a few weeks the drops in the water table shown above: it is the first signal owners can track themselves, before the cracks.

What is the rainfall situation today? (as at 19/10/2026)

In the commune: no notable rain for 12 days. Rainfall over the last 30 days: 23 mm, against roughly 80 mm expected at this time of year – a 71% deficit.

A sustained deficit over this window, especially in the growing season, often precedes by a few weeks the drops in the water table shown above: it is the first signal owners can track themselves, before the cracks.

Water table memory: 30 years of measurements, a well-documented context

The reference piezometric station for the sector (002FFSZ) has been monitoring the water table since 1997 – 30 years of records. As at 26/09/2026, the mean level over the last thirty days is 70.81 m NGF. Across the whole record, the water table has spent only 24.0% of its time below this level: a low level, in the lower quartile of known values.

• Last comparable episode: 30/10/2023 – 2.9 years ago
• Frequency: 19 similar episodes recorded since 1997, i.e. a return every 2 years on average – in hydrologists’ language, an intermediate episode
• Historical minimum: 65.58 m NGF, reached on 24/10/2022

This long memory changes how the risk is read: a low level is not merely a weather episode, it is an event whose frequency is known. For a house on clay, it is that frequency that informs the probability of further settlement – and hence the urgency of precautions.

Survey carried out by Frédéric Faucouneau, independent building expert – Antibes, Alpes-Maritimes (06), Var (83), Bouches-du-Rhône (13). Find out more

📅 Last updated 24/10/2026 - Cassini Expertise, independent building expert – departments 06, 83 and 13.