In some infested areas, it only takes one water infiltration for the merule to invade your home. - IStock / City Presse

If termites scare you, this is nothing compared to the fear that should inspire you to discover a thick, cottony and white mycelium on your woodwork and frames. These traces can indeed indicate the presence of merule under your roof, or more exactly in your roof.

Infested territories

Among the forty wood-eating fungi - those that feed on wood - that can attack houses, the merule is by far the most dangerous and the most widespread. The Serpula lacrymans is in fact present in around fifty departments in France, especially those in the North-West. In Finistère, professionals estimate for example that 75 to 100% of municipalities are affected. But do not panic, all the homes are not in danger.

As the National Housing Agency (Anah) specifies, this fungus only infests "wet wood in a warm and confined environment". If the house is healthy, well designed and maintained, then you have nothing to worry about. On the other hand, in invaded territories, it is enough for certain incidents for the merule to invite itself to your home. It can be behind degraded masonry, a deformed or broken gutter, an insufficiently ventilated cellar or even under cracked tiles. Infiltration into the roof, a leaking pipe or windows in poor condition are also a breeding ground for its development.

Catastrophic effects

This destructive fungus spreads out of sight, under coverings, in walls and behind baseboards. When its harms become visible, the invasion is already very important. The wood changes color, takes on a charred appearance and eventually breaks and crumbles. Likewise, the fibers lose their strength, which can cause the beams, the frame and the floors to collapse. Fir and pine give way very quickly to the merule, while the oak and chestnut resist only for a short time. Only a few so-called “tender” tropical species, such as doussié or douka, are really armed against this scourge.

To top it off, this fungus deteriorates cement joints, bricks and paint when it crosses masonry and can lead to risks of overvoltage and fire by winding its cottony filaments soaked with water around the electric wires. It is not for nothing that it is called "house leprosy". Only in-depth repairs and a complete drying of the premises can come to an end, at the end of an often prohibitive site.

Mandatory information

In order to protect buyers, the 2014 Alur law provided for information on the presence of this fungus in the event of a property sale. But beware, not everyone is affected. While the sales file must systematically contain a series of compulsory diagnoses, and in particular that relating to termites, mention of the merule is only required if the accommodation is located in a risk zone, the perimeter of which is delimited by a prefectural order. Even then, simple written information about the potential danger is sufficient.

The seller is not obliged to call a professional to make a diagnosis on the state of the frame. This intervention is however recommended in order to avoid any subsequent dispute, and in particular that the seller does not take action for hidden defects.

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The responsible real estate agent

Information on the presence, even old, of merule should not be taken lightly, as recently reminded by the Court of Cassation. A couple who signed a promise to purchase a house in Rennes wanted to retract after learning that work related to the presence of merule had been done in the accommodation in the past. The buyers had not only sued the seller to have the contract canceled, but also the real estate agent in liability, in order to be compensated for their damage.

While the latter indicated that he was not himself aware of an old attack on merule, a judgment of November 14, 2019 of the Court of Cassation confirmed that he had committed a fault. Instead of being based on a recent diagnosis, he should have checked the previous bill of sale.

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