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Dry rot fungus, the “building cancer”

Foundation of a house whose beams are extensively colonized by dry rot mycelium. This house, located in the Rimouski area, had to be decontaminated and then demolished in 2016.
Credit: Maxime Boivin
Foundation of a house whose beams are extensively colonized by dry rot mycelium. This house, located in the Rimouski area, had to be decontaminated and then demolished in 2016.
Dry rot fungus, the “building cancer”

The true dry rot fungus, whose scientific name, Serpula lacrymans, combines the words for “creeping” and “making tears,” is a fungus that causes lumber rot, and is often described as the “cancer of a building.” It is part of a group of fungi that break down cellulose and cause brown rot on a number of species, essentially conifers. It is not the only one to cause wood rot in buildings, but it is the most frequent one in temperate regions. The term “making tears” comes from the tiny water droplets exuded at the end of the hyphae and on the edges of the fructifications when the air humidity is high.

An insidious problem

To settle in, the fungus needs water. The rot will be at its peak when the humidity of the wood exceeds 45 percent and the temperature hovers around 20 C. In other words, the fungus is associated with poorly ventilated areas, like a crawl space, or flooring after water damage. If the wood dries out, the rot ceases to advance. However, it can resume expanding with the return of a high level of humidity.

When conditions are favorable in the wood, the fungus forms network of filaments similar to cotton wool, making the wood fragile and breakable. It also produces strings of filaments, called rhizomorphs, capable of transporting water and nutrients a distance of several meters from the initial infection, even through masonry, enabling it to colonize dry wood.

Pancake-shaped fructifications may also appear on exposed surfaces and are sometimes the first visible clue to infestation. Since the rot develops in confined spaces, it may go unnoticed for a long time. In cases of advanced infestation, the building may be beyond repair.

Known for a long time in Europe

In Europe, where it is been known for a very long time, it is estimated that the dry rot fungus is responsible for several million dollars of damage annually. The damage it causes has been identified in a dozen or so countries, especially in the north. A number of historical structures, chateaus and churches are infested, including Strasbourg Cathedral, which was founded over 1,000 years ago.

In France, the Construction and Housing Code contains specific measures for fighting this fungus. But dry rot can be found in much older texts: even a passage in the Bible refers to it (Leviticus 14:33–48).

The situation is shifting in Québec

In Canada the species has been reported since the 1940s, but dry rot became the focus of new attention a few years ago when houses had to be demolished. At the moment, southern Québec has about 50 such cases. In Montréal they have happened most frequently in Plateau Mont-Royal and Verdun.

An association is working in favor of the implementation of disaster assistance measures, and an interdepartmental committee on dry rot submitted its report in June 2017 to the Québec Ministry of Municipal Affairs and Land Occupancy. Legislation is expected that will help contain what is often a catastrophic problem for the owners of affected buildings.

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