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Dry Rot

When wood processed in buildings has a moisture level of more than 21% for a prolonged period of time, it is likely for fungi to grown on and in this moist food.

The most prevalent wood harming fungus in the Netherlands is the Serpula lacrymans Both conifers and broad-leaved trees can be harmed by these fungi.

The fungus can exist in beams and / or other wooden parts of the ground floor flooring. On or in wood outdoors, the dry rot is hardly ever found.

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.

Dry Rot, 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.

Appearance / Lifestyle

Wood affected by dry rot has a brownish discolouration. As the wood is affected further, it turns soft and loses its strength. An an advanced rotting stage, the wood is brown with deep parallel shrinkage cracks perpendicular to the fibre direction.

The dry-rot fungus Serpula lacrymans is among the most feared wood decaying fungi in the built environment because of its ability to rapidly decompose cellulose and hemicellulose from coniferous wood and timber.

The fungal threads in the wood cannot be seen by the naked eye. These threads penetrate the wood to a very deep level.

The cross-section of such threads is very small, just 0.0016 mm. At the surface of the wood the fungal threads are usually somewhat thicker, and sometimes visible in the form of white flakes.

Fungal threads don’t just exist on wood, but also on stone or concrete floors of basements for instance, and in and on damp walls etc. Sometimes bundles of fungal thread are found with a diameter between 5 to 8 mm. These are called strands, and are white at first, and later turn grey.

In places where the fungal threads grow in the wood, substances are formed that degrade the cell walls.

A portion of the wood is converted into carbon dioxide and water by the fungus. In this process, energy is released that is used for the growth of the fungus. Even dry wood can be affected, because dry rot is able to transport moisture from moist places using the fungal threads.

After a while, fruiting bodies are formed on the wood or wall, consisting of compact tissue. The fruiting bodies are fairly flat, 1 to 3 cm, thick, brown with a white edge and vary in size from several centimetres to 1 meter.

The fruiting bodies form the base for the spores that cause the species to spread. A fruiting body can form billions of spores. Large numbers of spores together look like rust-brown powder.


Dry rot develops the fastest in temperatures of 23 degrees Celsius. In 28 degrees C. or higher, the fungal threads die off.

The spores however can withstand higher temperatures. At freezing temperatures the fungal growth comes to a halt, but the dry rot doesn’t die. After the frost period, the growth simply continues.

Developmental stages of S. lacrymans include fruiting bodies (basidiomata), aerial mycelia and rhizomorphs . Fibre hyphae and generative hyphae as well as ‘vessel’ hyphae are observable in tissues with microscopy.

The colonization of wood by S. lacrymans is characterized by fast growth of vegetative mycelia and the formation of thick (up to 2 cm diameter, mycelial cords that are used to transport nutrients and water to hyphae exploring new wood substrates. This allows S. lacrymans a rapid growth and makes it a successful invader in built environments resulting in particular economic importance of dry rot.


Prior to control measures, it is important to take all possible measures to prevent leakage, condensate, rising damp, resounding walls, water entry or high ground water levels.

After the control measure, the wood should not be allowed to get moist again, because that would make it appealing for dry rot again.

The wood affected by fungus must first be removed. In addition, the adjacent, not visibly affected wood across a length of 1 meter must be removed and replaced, preferably with wood pre-emptively treated with a fungicide.

Affected elements that cannot be removed must be treated with a curative agent, such as azaconazole or tributyltin phosphate. It isn’t simple to impregnate the wood with the necessary amounts of the substance in such a curative treatment. For that reason, the treatment must sometimes be carried out twice.

Walls that may contain fungal threads must also be treated. Loose stucco must be removed and the joints must be scraped out. Afterwards, the walls must be treated with the agent.

In case of serious deterioration due to dry rot, it is recommended to drill diagonal holes downward in the holes. The depth of the holes must be 2/3 of the wall thickness, 30 cm apart and 20 cm crosswise to each other.

Afterwards, the drill holes are injected with the fungicide several times in a row. After drying, stucco can be applied to the wall.

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