Dry rot (Serpula lacrymans, known in France as merule) is a wood-destroying fungus capable of destroying the timber structures of a building within months. In older Parisian buildings, where floors, beams, and joists are made of wood, its presence can compromise the structural integrity of the building and reduce a property’s value by 20% to 40%.
Introduction
Paris has approximately 70% of buildings constructed before 1945, a large proportion of which rely on timber structures. Dry rot thrives in damp, poorly ventilated, and dark environments: conditions found in certain cellars, ground floors, and poorly maintained apartments in the capital.
In 2026, although a dry rot diagnosis is not systematically mandatory in Paris, vigilance is essential. An informed buyer knows how to spot the warning signs and demand the necessary checks. Since 2011, our property hunters have identified cases of dry rot in properties that could have cost unaccompanied buyers dearly.
Table of Contents
- What is dry rot and why is it dangerous
- Risk zones and buildings in Paris
- Legal framework for dry rot diagnosis in 2026
- How to detect dry rot during a viewing
- Diagnosis and treatment costs
- The property hunter’s role in managing dry rot risk
What is dry rot and why is it dangerous
Dry rot (Serpula lacrymans) is the most destructive fungus affecting indoor woodwork. It feeds on the cellulose in wood and can spread several centimetres per week under optimal conditions: a temperature between 20 and 26 degrees Celsius, a wood moisture content above 22%, and a lack of ventilation.
Its danger stems from three characteristics. First, it develops in darkness, behind partition walls, under floors, and within wall linings, making it invisible for months. Second, it can penetrate masonry through its mycelial cords, contaminating several units within the same building. Third, attacked wood irreversibly loses its structural strength: it fragments into characteristic brown cubes.
In a Haussmann building, floor beams, joists, roof rafters, and door frames all constitute potential targets. A floor weakened by dry rot can collapse, posing a direct safety risk to the occupants.
Risk zones and buildings in Paris
Unlike the departments of Finistere or Nord, Paris is not classified as a major risk zone for dry rot. However, cases do exist and are concentrated in specific property types.
Ground floors with adjoining cellars in older buildings in the 3rd and 4th arrondissements present an increased risk due to rising damp. Buildings located near the Seine or on land with a high water table are also affected.
Apartments that have suffered untreated water damage represent the most common risk factor. A leak behind a partition wall, a defective bathroom seal, or a leaking roof can create ideal conditions for dry rot within weeks.
Unventilated attics, apartments vacant for several months, and buildings with defective rainwater downpipes complete the picture of situations to watch for. In the 18th arrondissement, where many 19th-century faubourien buildings are found, our property hunters pay particular attention to these signals.
Legal framework for dry rot diagnosis in 2026
The ALUR Law of 2014 introduced article L.133-8 of the French Construction and Housing Code, which obliges the seller to inform the buyer of any known presence of dry rot in the building. This information obligation covers the property being sold as well as the communal areas.
A dry rot diagnosis becomes mandatory only in zones defined by prefectoral decree as areas of dry rot risk. In 2026, Paris proper is not subject to such a decree, unlike many municipalities in Brittany and Normandy.
However, the absence of an obligation does not exempt the seller from their duty of transparency. If the owner or the co-ownership management company is aware of dry rot in the building, this information must be included in the technical diagnostic file (DDT) appended to the preliminary sales agreement.
If dry rot is discovered after the sale, the buyer may invoke the hidden defects guarantee if the seller was aware of the problem. The time limit for action is two years from the discovery of the defect.
How to detect dry rot during a viewing
During an apartment viewing, several clues should alert a vigilant buyer.
Visual signs include warping or buckling of floors, skirting boards coming away, paint blistering without apparent cause, and cottony white or orange traces in dark corners. A pronounced mushroom smell in damp rooms or cellars is a characteristic olfactory signal.
The screwdriver test is simple and revealing: pressing a screwdriver tip against suspect woodwork, healthy wood resists while wood attacked by dry rot gives way as though it were cardboard.
In cellars and communal areas, the mycelial cords of dry rot, grey to brown filaments a few millimetres in diameter, may be visible on stone walls. These cords allow the fungus to transport the water it needs to spread through even dry materials.
It is advisable to request the minutes of the co-ownership general meetings from the last three years. Any dry rot issue reported in the building will appear in them.
Diagnosis and treatment costs
A dry rot diagnosis carried out by a certified expert costs between 200 and 400 euros in Paris in 2026. This diagnosis includes a visual inspection, a moisture survey, and, if necessary, a sample for laboratory analysis.
Treatment varies considerably depending on the extent of the infestation. For a localised problem, such as a skirting board or door frame, fungicide injection treatment costs between 3,000 and 5,000 euros. For a widespread infestation requiring the replacement of beams or joists, works reach 15,000 to 30,000 euros.
In the most severe cases, involving structural work on floors and roof timbers, the bill can exceed 50,000 euros. These figures explain why dry rot heavily impacts a property’s value and justify absolute vigilance before purchase.
Treatment must be carried out by a CTB-A+ certified company (Centre Technique du Bois). It involves removing contaminated wood within a one-metre perimeter around the infested zone, fungicide treatment of the masonry, and ventilation of the treated areas.
The EPC (Energy Performance Certificate) does not cover dry rot risk, but an energy diagnostic report flagging moisture problems should prompt further investigation.
The property hunter’s role in managing dry rot risk
At Home Select, detecting dry rot risk is part of our viewing protocol. Our property hunters systematically inspect cellars, foundations, and woodwork in older properties, particularly for ground floors and buildings showing signs of moisture.
If any doubt remains, we recommend a dry rot diagnosis before signing the preliminary agreement. This 200 to 400 euro expense can prevent works costing tens of thousands of euros.
Our knowledge of the Parisian property stock, built over more than 1,200 mandates since 2011, allows us to identify at-risk building types before even visiting. A building in the 5th arrondissement with vaulted stone cellars does not call for the same checks as a 1960s concrete building in the 13th.
FAQ
Is dry rot common in Parisian buildings?
Dry rot is relatively rare in Paris compared to the north-western regions of France, but it does exist in older buildings with moisture problems, particularly ground floors, cellars, and poorly ventilated apartments. Arrondissements with dense older building stock (3rd, 4th, 5th, 11th, 18th) see occasional cases.
Is a dry rot diagnosis mandatory in Paris?
In 2026, a dry rot diagnosis is not mandatory in Paris for a property sale. It only becomes required in risk zones defined by prefectoral decree. However, the seller is obliged to inform the buyer if they are aware of the presence of dry rot in the building.
How much does dry rot treatment cost?
The cost of dry rot treatment in Paris ranges from 3,000 to 15,000 euros for an apartment, depending on the extent of the infestation. In severe cases requiring the replacement of beams and joists, the bill can exceed 30,000 euros. The diagnosis alone costs between 200 and 400 euros.
Considering buying an older property in Paris? Our property hunters inspect each property with a rigorous protocol, including detection of dry rot risk. Contact Home Select for secure support.
Frequently asked questions
Is dry rot common in Parisian buildings?
Dry rot is relatively rare in Paris compared to the north-western regions of France, but it does exist in older buildings with moisture problems, particularly ground floors, cellars, and poorly ventilated apartments. Arrondissements with dense older building stock (3rd, 4th, 5th, 11th, 18th) see occasional cases.
Is a dry rot diagnosis mandatory in Paris?
In 2026, a dry rot diagnosis is not mandatory in Paris for a property sale. It only becomes required in risk zones defined by prefectoral decree. However, the seller is obliged to inform the buyer if they are aware of the presence of dry rot in the building.
How much does dry rot treatment cost?
The cost of dry rot treatment in Paris ranges from 3,000 to 15,000 euros for an apartment, depending on the extent of the infestation. In severe cases requiring the replacement of beams and joists, the bill can exceed 30,000 euros. The diagnosis alone costs between 200 and 400 euros.
