The door frame gives way when you press with a screwdriver. There is a musty smell in the crawl space. The floorboards under the window are dark and soft. Rot and fungal decay in timber is one of the most serious problems a house can face — because it attacks the structure itself. And the longer you wait, the more expensive it becomes.
Here is what you need to know.
What is rot and fungal decay?
Rot is the degradation of timber caused by fungi. The fungi need three things: moisture, warmth and wood. Remove one of the three, and the fungus dies. But by then, the damage may be enormous.
True dry rot (Serpula lacrymans): The most dangerous. Can spread several metres through masonry and concrete via its rhizomorphs (threads). Degrades timber rapidly and aggressively. Requires professional remediation — often involving removal of masonry and replacement of all affected timber elements.
Cork cap fungus and other pore fungi: Less aggressive, but still serious. Require direct contact with damp timber. Do not spread through masonry.
Brown rot: Degrades cellulose in timber, which becomes brown, brittle and cracks in a cube pattern. Serious structural weakening.
White rot: Degrades lignin — the timber becomes light, fibrous and soft. Less common in buildings.
The signs — what to look for
- Musty, earthy smell. Often the first sign. Especially in basements, crawl spaces and enclosed loft spaces.
- Soft timber. Press with a screwdriver — if it sinks in easily, the timber is attacked.
- Discolouration. Dark, brownish timber (brown rot) or light, fibrous timber (white rot).
- Visible mycelium. White, cotton-like threads or grey-brown rhizomorphs (thicker threads). The rhizomorphs of dry rot can resemble shoelaces.
- Fruiting bodies. Flat, pancake-like formations in brown, orange or white. Release spores as a fine, rust-brown powder.
- Floors that give underfoot. Load-bearing beams, joists or floorboards are attacked.
- Doors and windows that will not close. Settlement in the structure due to weakened timber elements.
The typical causes
Moisture is always the cause. Fungi cannot grow in dry timber (below 20% moisture content). The typical moisture sources:
- Leaking roof. Leaks in the roof structure cause prolonged moisture in rafters, battens and floor decks.
- Condensation in loft. Missing vapour barrier or insufficient ventilation.
- Moisture in crawl space. Insufficient ventilation and ground vapour barrier.
- Leaking bathroom. Water behind tiles and under the floor causes moisture in floor decks and adjacent walls.
- Pipe damage. Slow leakage from hidden pipes.
- Rising damp. Moisture from the foundation is drawn up into sole plates and timber floors.
- Defective vapour barrier. Condensation in insulation and structure.
What it costs
Localised rot repair:
- Window or door frame: 5,000–15,000 DKK
- Sole plate or sill: 10,000–30,000 DKK
- Replacement of individual floorboards: 5,000–20,000 DKK
Fungal remediation in roof space:
- Removal of affected timber, treatment, new rafters/battens: 50,000–200,000 DKK
- Including new underlay and insulation: 100,000–350,000 DKK
True dry rot — comprehensive remediation:
- Removal of all affected timber + 1 m of fresh timber in all directions
- Removal of masonry with rhizomorphs
- Heat treatment or chemical treatment
- Total cost: 100,000–500,000+ DKK
Insurance rarely covers fungal damage caused by lack of maintenance.
What you can do — step by step
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Identify the extent. Test with a screwdriver on suspect timber. Look in loft, crawl space and behind claddings. Commission a professional fungal survey where there is suspicion (3,000–8,000 DKK).
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Find and remove the moisture source. This is the most important step. Repair roof leaks, drainage, pipes or ventilation. Without dry structure, the fungus returns.
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Remove all affected material. All attacked timber must be removed. For dry rot: remove 1 m of fresh timber extra in all directions as a safety margin.
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Treat remaining structure. Fungicidal products are applied to intact timber and masonry near the attack.
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Rebuild with dry, pressure-treated timber. New timber in contact with masonry and concrete should be pressure-treated or separated with a damp-proof course.
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Establish a dry environment. Ventilation in loft and crawl space. Vapour barrier on the warm side of insulation. Correct drainage at the foundation.
Which houses are most at risk?
Houses with crawl spaces. Moisture from ground + stagnant air + timber structure = ideal conditions for fungus.
Houses with leaking roofs. Prolonged leakage in the loft is the most common cause of fungal attack in rafters.
Older houses with timber in contact with masonry. Beam ends embedded in exterior walls (floor decks above basement, floor structures). Moisture from the masonry is transferred to the timber.
Houses with extensions from the 1960s–70s. Often poorly connected, with leaks at the junction with the original house.
Insurance and liability
Fungal damage typically does not fall under home insurance, as it is considered a maintenance failure — not a sudden event. Exceptions can occur if the fungal attack was caused by a covered water damage (e.g. pipe burst).
When buying a house, fungal decay is normally identified in the condition report. Hidden fungal damage not mentioned may give rise to a claim against the seller through the purchase insurance — but this requires the seller to have been aware of the problem.
Conclusion: Prevent and detect early. An annual inspection of crawl space, loft and vulnerable timber structures costs 2,000–5,000 DKK from a building surveyor — and can save hundreds of thousands.
How to move forward
Rot and fungal decay do not wait. If you have suspicions, a professional inspection is the most important step. It costs 3,000–8,000 DKK — and can reveal problems that would cost many times more if left to develop.
Rot and fungal decay almost always starts with moisture. Read also about damp in walls and leaking roof — two of the most common causes of fungal attack in Danish houses.
Sources: BYG-ERFA — fungal attack, Bolius — rot and fungal decay, Teknologisk Institut