Dry rot is a variety of fungi that can attack and destroy wooden components in a building. This is why these fungi are popularly known as wood sponge. A total of around 65 house rot fungi are known. The most dangerous among them is the real dry rot (Serpula lacrymans). Since it can even decompose walls, it is popularly known as wall sponge. A fungal infestation usually occurs behind paneling or floor coverings, so it goes undetected for a long time. And that’s what makes him so insidious.

damage picture

The real dry rot is one of the surface fungi, but usually grows in secret for a long time. Therefore, it is not recognizable in its early growth phase. Only when the conspicuous fruiting body suddenly appears does a homeowner become aware of it. Therefore, an infestation is usually only recognized so late that a costly remediation is necessary.

  • large amounts of rusty brown fungal spores (on furniture and floor)
  • lumpy decomposition on wooden parts
  • Mushroom mycelium: hemp-like fibers
  • large, flat fruit body (brownish with a white edge)
  • up to 2 cm in height and 1 m in length

In the case of large-scale growth, however, Serpula lacrymans excretes small liquid droplets, so-called glutation droplets. The dry rot owes its Latin name (lacrima = tear) to these “tears”. In order to achieve absolute certainty, it is advisable to consult a specialist. He can use various test methods for identification and, in case of doubt, finally arrange for a laboratory examination of the affected material.

Tip: If there is still no fibrous mycelium or a brown fruiting body, but the wood can be pressed in with a finger, then an infestation with dry rot is very likely.


The harmful fungus can be found in old half-timbered houses with ground-level foundations as well as in poorly heated or leaky buildings with damp basements, ground floors and attics. But even new buildings with construction defects are not spared from Serpula lacrymans if, for example, cold bridges lead to the formation of condensation. House rot fungi like dry rot require cellulosic materials with high moisture levels to grow. The cellulose is not only contained in wood, but also in many other building materials such as

  • Paper
  • Textiles (cotton)
  • Plant fibers such as straw and reed (in thatched roofs)
  • chipboard

In addition, dry rot can also grow through inorganic materials such as walls and spread without direct contact with wood.

growing conditions

In addition to suitable food, the fungi need above all moisture and warmth in order to be able to grow. Increased moisture levels in the air and thus also in the wood or masonry mainly occur in the basement, on the ground floor or in the attic if a damp environment occurs there due to inadequate sealing or structural damage.

  • high humidity
  • little air movement
  • Growth from 25% wood moisture
  • best conditions: 35 to 60% wood moisture content
  • ideal temperature: 18 to 22 degrees
  • Fruit body formation in light and fresh air
  • sensitive to drafts
  • Stopping growth: above 26 degrees or below 20% wood moisture content

Degradation by the wood-destroying fungus varies greatly depending on the type of wood. It is particularly high for woods such as alder, pine, larch and beech, while it is comparatively low for heartwood such as oak and elm.


There are many reasons why house rot develops. Older buildings are mainly affected, but newer houses with construction defects or damage to the outer shell can also be affected.

  • leaks in the roof
  • Defective roof drainage (downpipes or gutters)
  • water damage
  • rising moisture from the ground

obligation to report

According to the Federal Building Act, an infestation with dry rot is one of the serious construction defects. The infestation and extent must therefore be determined by an expert. An obligation to report an infestation with Serpula lacrymans is regulated in the building regulations of the individual federal states. Incidentally, in most of our federal states there is no obligation to register or it has been removed over time. Only in two of the 16 German federal states is there an obligation to report an infestation with dry rot:

  • Saxony
  • Thuringia

But regardless of whether there is an obligation to report or not, when this wood destroyer occurs, professional remediation is mandatory nationwide.

infestation assessment

How big the actual infestation has progressed should also be determined by a specialist company, an appraiser or an expert. The assessment of the damage is mainly based on visual aspects. The following measurement methods are also used:

  • Swing and hammer test
  • Bohrwiderstand
  • endoscopic examination
  • Measurement of material moisture

The infested wood or walls often have to be completely exposed.


A sponge remediation should always begin with the identification of the harmful fungus. If this is not possible, the worst case – an infestation with Serpula lacrymans – is dealt with. Elimination of the cause of moisture is necessary for permanent control. Dry rot must be removed by a specialist company because there are legal regulations and requirements (regulated in DIN 68800) that prescribe the sanitation measures. When renovating a building infested with wood fungus, the effort is usually considerable.

  • Drainage of walls and wooden structures
  • root cause analysis
  • Revealing hidden constructions
  • Replacement of infested wood and walls
  • Safety distance when exchanging: 1 m
  • proper disposal of the contaminated material
  • New installation of the removed parts of the building (according to DIN)
  • additional measures may be necessary
  • Biocide treatment of the damaged areas

Sometimes the removal of infested materials is not possible. This can be the case, for example, with buildings that are listed as historical monuments. In this case, a thermal process comes into question: the so-called hot air process. At temperatures above 50 degrees, the fungi that destroy the wood or masonry are killed. This control method must also be carried out by a specialist and is also associated with a great deal of effort.

preventive measures

The best precaution against an infestation with dry rot is to make the living conditions for the harmful fungus as unattractive as possible. That is why it is important to regularly maintain a building and make repairs when necessary. In the case of new buildings, care must be taken to ensure that they have dried out sufficiently before they are closed.

  • Correct damage and defects promptly
  • Clean gutters and downspouts regularly
  • remove any vegetation
  • Eliminate thermal bridges or leaks
  • Waterproofing and vapor barriers against moisture
  • create drainages
  • Check water connections and pipes regularly
  • fix leaks
  • if moisture penetrates: arrange for drying (construction fan)
  • Adequately ventilate subfloor spaces
  • ventilate generously on a regular basis

Also, never install damp wood or store firewood in a damp basement. Mushroom residues of the real wood sponge can survive for decades in dry wood and then become active again under optimal conditions.

Tip: Special tannic acids and other substances in oak wood act as natural fungicides. Therefore, oak wood makes a bad base for Serpula lacrymans.

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