Cassis: Clay Soil, Subsidence and Cracks – Official Data

The geological and climate context of Cassis

In Cassis, yes, it is a serious hypothesis worth checking: the GASPAR database lists 3 drought decrees (full list below) and 14 flood decrees, i.e. 17 Cat Nat decrees for the commune. The local rainfall matches the classic shrink-swell scenario: two dry years (2020, 2022), then a rebound to 730 mm in 2024 – exactly the kind of alternation that cracks foundations between the calanques and the Cap Canaille.

Cassini Expertise, an independent building expert, handles drought-related cracks and damage in Cassis and across the whole department. This page brings together the commune’s official data to help you work out whether your case fits the pattern, with no sales talk.

Cassis: drought, clay and cracks – what the official data says

The GASPAR database (decrees published in the Official Gazette since 1982) lists 17 natural disaster decrees for the commune:

• 3 drought decrees (differential ground movement linked to clay shrink-swell)
  • event of 01/01/2007 (decree of 07/08/2008)
  • event of 01/07/2007 (decree of 07/08/2008)
  • event of 01/04/2022 (decree of 03/04/2023)
• 14 flood and storm decrees

Why these dates matter to you: each decree corresponds to a precise window of time. If your damage dates from a recognised period (2007 or 2022), the Cat Nat compensation route is open – provided you declared the claim to your insurer within 30 days of the decree being published. If that deadline has passed, or your cracks date from another period, the claim against the seller for hidden defect or fraudulent concealment remains (see below).

What the official clay mapping says

The GéoRisques platform (BRGM) classifies Cassis at high exposure to clay shrink-swell, with some sectors at medium exposure. This is the level your insurer and notaire already consult to assess the risk on a given plot – you can run the same check yourself on georisques.gouv.fr with the exact address of the property. Note also: the commune is classed as radon potential class 2 (IRSN/GéoRisques data), a point to watch if you are buying or spend time in a basement or poorly ventilated ground floor.

This ranking complements the decree history: the hazard tells you what can happen, the decrees tell you what has already happened. A house in a high-exposure area that has already come through one or more dry summers carries a risk of damage that is all the better documented – an argument worth using when negotiating a purchase.

Cassis: ground that demands a local reading

In Cassis, we are regularly called to villas clinging to the limestone slopes above the bay, but also to buildings set in the valley and on the first rises between the historic centre and the heights towards the Col de la Gineste. The stock is a typical mix: older Provençal constructions with shallow foundations, 1970s-80s detached houses from the estates that accompanied the commune’s residential growth, and a few more recent builds on hillsides. The soil is heterogeneous, with clay pockets alternating with rocky outcrops, which complicates the uniformity of the bearing surface under a single house. On sloping plots, we frequently find shallow foundations that were not dimensioned to absorb ground movements, and the surrounding Mediterranean vegetation – pines and evergreen oaks especially, but also olive trees planted near buildings when gardens were laid out – deepens the local drying of the clay through root uptake, particularly during the summer droughts characteristic of the Cassidain climate. This shrink-swell phenomenon produces differential settlement that translates into façade cracks, often diagonal at corners or openings. The houses built on the steepest plots in the commune, where earthworks have sometimes weakened the natural stability of the ground, are the ones where this type of structural disorder is most often observed.

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: 2007, with 400 mm over the year, against a local average of 666 mm. 2022 ranks as the 5th driest year out of 35 (488 mm against an average of 666 mm); 2023 follows in 9th place (524 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.

Cassis: 17 Cat Nat decrees since 1982, in chronological order

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

• 🔵 11/1982 → 11/1982 – Floods (decree of 18/11/1982)
• 🔵 11/1982 → 11/1982 – Cyclones / Storms (decree of 18/11/1982)
• 🔵 08/1983 → 08/1983 – Floods (decree of 15/02/1984)
• 🔵 08/1986 → 08/1986 – Floods (decree of 11/12/1986)
• 🔵 10/1988 – Floods (decree of 20/04/1989)
• 🔵 01/1994 → 01/1994 – Floods (decree of 06/06/1994)
• 🔵 11/1994 → 11/1994 – Floods (decree of 21/11/1994)
• 🔵 08/1996 – Floods (decree of 01/10/1996)
• 🔵 09/1998 – Floods (decree of 29/12/1998)
• 🔵 09/1999 – Floods (decree of 28/01/2000)
• 🔵 12/2003 → 12/2003 – Floods (decree of 12/12/2003)
• 🔴 01/2007 → 03/2007 – Drought (RGA – clay shrink-swell) (decree of 07/08/2008)
• 🔴 07/2007 → 09/2007 – Drought (RGA – clay shrink-swell) (decree of 07/08/2008)
• 🔵 05/2012 → 05/2012 – Floods (decree of 11/07/2012)
• 🔵 10/2019 → 10/2019 – Floods (decree of 30/10/2019)
• 🔵 10/2021 → 10/2021 – Floods (decree of 15/10/2021)
• 🔴 04/2022 → 09/2022 – Drought (RGA – clay shrink-swell) (decree of 03/04/2023)

How an expert reads this: 3 droughts and 14 floods. Clusters (several decrees close together) mark crisis periods – the summer sequence 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 a claim against the seller.

Damp, water ingress and rising damp in Cassis

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.

You have a damp problem in Cassis, and you may already have read online that resin injections are needed, or a ventilation system. My message: not so fast. Without a diagnosis, you risk paying for an ineffective treatment aimed at the wrong cause.

The four causes and how to recognise them

• Rising damp: marks low on the walls, rarely climbing above 1.5 m, often with saltpetre and blistering render. Typical of older houses.
• Water ingress: a localised stain, often linked to a rain-exposed façade, a roof terrace or a failing window seal. Worsens after rain.
• Condensation: diffuse damp, mould in cold corners and poorly ventilated bathrooms, with no link to the ground.
• Leak: a stain that appears with no relation to the rain, sometimes with a smell, often close to a pipe run or an upstairs bathroom.

The trap of the diagnosis sold by the treatment contractor

A company that sells resin injections and bills the diagnosis has an obvious interest in concluding “rising damp”, even when the true cause is water ingress or a ventilation defect. It is a structural conflict of interest, not a question of individual dishonesty. A reliable diagnosis must be carried out by someone who sells no treatment afterwards.

Before committing to works in Cassis, have an independent diagnosis drawn up: ask for an independent survey. To understand the four causes and the treatments suited to each, see the damp guide for the 06, 83 and 13.

Why do cracks appear several months after the drought?

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

• 2019: 686 mm · 2020: 496 mm · 2021: 695 mm · 2022: 488 mm · 2023: 524 mm · 2024: 730 mm
• Average over the period: 603 mm/year, driest year: 2022 (488 mm)
• Two markedly dry years: 496 mm in 2020, 488 mm in 2022
• Then a rebound to 730 mm in 2024

Clay shrink-swell plays out in two stages: the drought dries the clay at depth (the soil contracts, the first cracks appear), then the return of the rains rehydrates the ground in one go (swelling, new differential movements). If your house sits on shallow foundations – the case for most villas built between 1970 and 2000 – it absorbs the first damage, often with a 12 to 24 month lag after the dry episode. In practical terms: a crack opening in 2025-2026 may well be the delayed consequence of the 2022 drought, amplified by the wet year that followed.

Cracks in Cassis: the signs that should push you to call an expert

• A stair-step crack, passing through the wall, open by more than 2 mm – measure it with a ruler, not by eye
• A crack that is evolving: fit a plaster or paper tell-tale and read it again a few months later to see whether it moves
• Other signals to watch together: doors and windows that stick, floor tiles splitting diagonally, render that bulges
• Aggravating factors on your land: a house built on clay, trees close to the façade, failing drainage around the foundations

With 3 drought decrees already issued for the commune, as soon as two or three of these signs stack up in your home, do not simply fill the crack in: have the disorder recorded and dated by an expert before any repair – and before any buying or selling decision, because a hastily filled crack becomes invisible again at a viewing.

Is my cracked house in Cassis compensable? What the law says when 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 and discover cracks after the sale, you have two levers:

• Hidden defect (vice caché, Article 1641 of the French Civil Code): serious cracks, invisible at a viewing, predating the sale → you can claim a price reduction or cancellation of the sale
• Fraudulent concealment (dol, Article 1137): the seller knew and hid the damage → you can claim cancellation of the sale plus damages, within 5 years

To choose between the two and build your file, instruct a French property lawyer.

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

• On-site technical visit: mapping and classification of each crack, width measurements, identification of functional disorders (doors, floor tiles, render)
• Context analysis: clay zoning, the commune’s Cat Nat history (3 drought decrees), local drought episodes cross-checked against the date the damage appeared
• Costing of repair works: underpinning, injections, drains – with several scenarios depending on severity
• A written report you can pass directly to your insurer or your lawyer to support a declaration or proceedings

Independent expert, with no ties to any works company – the report you receive leads to no works commission. Request a visit in Cassis – see also 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 survey has applied since 1 October 2020 (ELAN law, art. 68); the new exposure map of 1 July 2026 widens its scope. This requirement, introduced by the ELAN law no. 2018-1021, article 68 (decree no. 2019-495 of 22 May 2019), applies to Cassis (high exposure under the BRGM classification).

• What it changes for building: a soil survey is no longer optional – the ELAN law makes it compulsory in exposed zones for any sale of unbuilt 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 following the geotechnical recommendations on clay ground is a medium-term cracking risk.
• What it changes for sellers: the absence of a survey on a property built before the requirement is not a defect in itself – but if damage appears, a seller who knew about it and concealed it exposes themselves to a claim for fraudulent concealment (dol).
• The link with the data above: Cassis combines high exposure with a documented record 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 what was actually built, damage surveys on clay ground, second-opinion expert assessments. Request a consultation.

Frequently asked questions

Is Cassis in a clay risk zone?

Yes. The Bouches-du-Rhône is very largely covered by the clay shrink-swell hazard (BRGM mapping, consultable on georisques.gouv.fr). In Cassis, the 3 drought decrees recorded in GASPAR confirm the risk is nothing theoretical: it has already materialised twice, in 2007 and in 2022.

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

Only if your damage can be tied to a period covered by a decree (last recognised drought periods in the commune: 2008, 2023) and if you declared the claim to your insurer within 30 days of the decree being published. Otherwise, the action against the seller remains – and precisely dating your damage through a survey then becomes the central issue of the file.

Can the data cited on this page be verified?

Yes, you can cross-check it yourself: the decrees come from the GASPAR database (Official Gazette, open data sets, 2025 update), the rainfall from the Open-Meteo weather archives for the Cassis grid. Cassini Expertise does not adjust these figures.

Weekly drought update (as at 04/10/2026)

In the commune: recent rain (3 days ago). Rainfall over the last 30 days: 47 mm, against roughly 53 mm expected at this time of year – an 11% deficit.

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

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

In the commune: recent rain (4 days ago). Rainfall over the last 30 days: 47 mm, against roughly 54 mm expected at this time of year – a 13% deficit.

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

Weekly drought update (as at 12/10/2026)

In the commune: recent rain (5 days ago). Rainfall over the last 30 days: 76 mm, against roughly 61 mm expected at this time of year – a surplus of 25%.

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

Drought right now (as at 19/10/2026)

In the commune: recent rain (1 day ago). Rainfall over the last 30 days: 79 mm, against roughly 67 mm expected at this time of year – a surplus of 18%.

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

The water table against its historical record: what 30 years of measurements show

The reference piezometric station for the sector (002KZYQ) has been monitoring the water table since 1984 – 30 years of records. As at 01/10/2026, the mean level over the last thirty days is 103.09 m NGF. Across the whole record, the water table has spent only 11.3% of its time below this level: a very low level, rarely reached in the station’s record.

• Last comparable episode: 04/09/2024 – 2.1 years ago
• Frequency: 28 similar episodes recorded since 1984, i.e. a return every year on average – in hydrologists’ language, a rare event
• Historical minimum: 102.04 m NGF, reached on 30/10/2022

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.

Find out more: Article 1641 of the Civil Code and the hidden defects warranty (garantie des vices cachés), explained.

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

📅 Last updated 06/11/2026 - Cassini Expertise, independent building expert – departments 06, 83 and 13.