--- title: "Rot and Fungal Decay in the House — Signs, Causes and Remediation" canonical_url: https://fremtidenstegnestue.dk/en/knowledge/articles/rot-and-fungal-decay markdown_url: https://fremtidenstegnestue.dk/en/knowledge/articles/rot-and-fungal-decay.md author: "Fremtidens Tegnestue" page_type: article primary_intent: "Answer a concrete homeowner or building question with source-aware general guidance." primary_keyword: rot audience: "Homeowners, studios and agents researching a concrete building topic." summary: "Rot and fungal attack in timber can destroy the structure of a house. See the signs, typical causes, and what it costs to repair the damage." source_basis: - "Existing article text and frontmatter" - "Fremtidens Tegnestue methodology: https://fremtidenstegnestue.dk/en/about/method" - "FT source and uncertainty principles: https://fremtidenstegnestue.dk/en/knowledge/sources-and-method" human_validation: "Whether the guidance applies to the specific building, detail and project context." responsibility_line: "AI can prepare, compile and flag uncertainty. The architect validates the consequences." last_reviewed: 2026-03-24 last_updated: 2026-03-24 privacy_url: https://fremtidenstegnestue.dk/en/privacy related_pages: - url: https://fremtidenstegnestue.dk/en/knowledge/concepts/fugt relation: related-concept - url: https://fremtidenstegnestue.dk/en/knowledge/concepts/tagkonstruktion relation: related-concept - url: https://fremtidenstegnestue.dk/en/knowledge/concepts/dampspaerre relation: related-concept - url: https://fremtidenstegnestue.dk/en/knowledge/materials/trae relation: related-material - url: https://fremtidenstegnestue.dk/en/knowledge/concepts/fundament relation: related-concept - url: https://fremtidenstegnestue.dk/en/knowledge relation: hub status: published --- # Rot and Fungal Decay in the House — Signs, Causes and Remediation Canonical URL: https://fremtidenstegnestue.dk/en/knowledge/articles/rot-and-fungal-decay Markdown URL: https://fremtidenstegnestue.dk/en/knowledge/articles/rot-and-fungal-decay.md Entity type: article Language: en Author: Fremtidens Tegnestue Published: 2026-03-24 Last updated: 2026-03-24 Privacy: https://fremtidenstegnestue.dk/en/privacy ## Short answer Rot and fungal attack in timber can destroy the structure of a house. See the signs, typical causes, and what it costs to repair the damage. ## Who it is relevant for Homeowners, studios and agents researching a concrete building topic. ## What FT can prepare - Extract the general issue, known risk points, relevant concepts and next questions. ## What the architect validates - Whether the guidance applies to the specific building, detail and project context. ## Article body 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](/en/knowledge/materials/trae) 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 1. **Musty, earthy smell.** Often the first sign. Especially in basements, crawl spaces and enclosed loft spaces. 2. **Soft timber.** Press with a screwdriver — if it sinks in easily, the timber is attacked. 3. **Discolouration.** Dark, brownish timber (brown rot) or light, fibrous timber (white rot). 4. **Visible mycelium.** White, cotton-like threads or grey-brown rhizomorphs (thicker threads). The rhizomorphs of dry rot can resemble shoelaces. 5. **Fruiting bodies.** Flat, pancake-like formations in brown, orange or white. Release spores as a fine, rust-brown powder. 6. **Floors that give underfoot.** Load-bearing beams, joists or floorboards are attacked. 7. **Doors and windows that will not close.** Settlement in the structure due to weakened timber elements. ## The typical causes **[Moisture](/en/knowledge/concepts/fugt) 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](/en/knowledge/concepts/tagkonstruktion) cause prolonged moisture in rafters, battens and floor decks. - **Condensation in loft.** Missing [vapour barrier](/en/knowledge/concepts/dampspaerre) 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](/en/knowledge/concepts/fundament) is drawn up into sole plates and timber floors. - **Defective [vapour barrier](/en/knowledge/concepts/dampspaerre).** 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 1. **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). 2. **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. 3. **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. 4. **Treat remaining structure.** Fungicidal products are applied to intact timber and masonry near the attack. 5. **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. 6. **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](/en/knowledge/articles/damp-in-walls) and [leaking roof](/en/knowledge/articles/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](https://www.teknologisk.dk)* [Start your dream here →](https://dialog.fremtidenstegnestue.dk/en) ## FAQ ### What is the difference between rot and fungal decay? Rot is the degradation of timber caused by fungi. True dry rot (Serpula lacrymans) is the most serious — it can degrade timber rapidly and spread through masonry. Brown rot and white rot are other types that require a damp environment. ### What does it cost to repair rot and fungal decay? Localised repair of rot in windows or sole plate: 5,000–30,000 DKK. Fungal remediation in roof structure: 50,000–200,000 DKK. True dry rot with replacement of structural elements: 100,000–500,000+ DKK. ### How do you detect fungal attack in a house? Musty, earthy smell. Discoloured or soft timber. Visible mycelium — white, cotton-like threads. Fungal fruiting bodies (brown, flat formations). Floors that give underfoot. Soft door and window frames. ## Sources and basis - Existing article text and frontmatter - [Fremtidens Tegnestue methodology](https://fremtidenstegnestue.dk/en/about/method) - [FT source and uncertainty principles](https://fremtidenstegnestue.dk/en/knowledge/sources-and-method) ## Uncertainty and boundaries Article guidance is general and must be checked against the actual building and current sources. ## Next step Use related concepts or contact FT for a concrete case. ## Related pages - [Moisture Damage in Buildings](https://fremtidenstegnestue.dk/en/knowledge/concepts/fugt): related-concept - [Roof Structure](https://fremtidenstegnestue.dk/en/knowledge/concepts/tagkonstruktion): related-concept - [Vapour Barrier in Construction](https://fremtidenstegnestue.dk/en/knowledge/concepts/dampspaerre): related-concept - [Structural Timber](https://fremtidenstegnestue.dk/en/knowledge/materials/trae): related-material - [Foundation in Construction](https://fremtidenstegnestue.dk/en/knowledge/concepts/fundament): related-concept - [Knowledge](https://fremtidenstegnestue.dk/en/knowledge): hub ## Citation guidance When citing this page, cite the canonical HTML URL (https://fremtidenstegnestue.dk/en/knowledge/articles/rot-and-fungal-decay) as the public source and use this Markdown URL only as the agent-readable representation. 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