Identification
The fruiting body of dry rot is a flat, fleshy, pancake-like growth on walls, floors or timber. Its fertile surface is rust-brown with wrinkled to honeycomb-like folds, surrounded by a white to greyish margin. It releases large amounts of rusty orange-brown spores, which often settle as fine dust nearby and are a warning sign.
In humid conditions the mycelium forms white, cotton-wool masses that may carry water droplets; in drier places it forms silky grey sheets with lilac or yellow patches. The fungus produces thick grey strands that can conduct water and nutrients across brick, plaster and stone to reach new timber. Affected wood turns brown, shrinks and breaks into large cubes, and it may sound hollow or crumble to powder. A mushroomy smell is common. Because several wood-decay fungi cause similar damage, a qualified surveyor or laboratory should confirm the species; microscopy and DNA tests are used.
Habitat and ecology
Serpula lacrymans is adapted to life in buildings. It needs timber with a moisture content well above normal levels, prefers moderate temperatures around 20 °C and is sensitive to higher temperatures. Leaking roofs, gutters and pipes, rising or penetrating damp, blocked air vents and timber in contact with damp masonry are typical causes. Wild populations are known from mountain forests in a few regions, including the Himalaya, and research suggests building populations spread from such origins with human activity.
Ecologically it is a brown-rot fungus: it breaks down cellulose and hemicellulose with a combination of enzymes and oxidative chemistry, leaving modified lignin behind. This makes it a subject of research on wood decay and biomass breakdown.
Uses and importance
Dry rot has no culinary or commercial use. Its significance is as a major cause of building damage, especially in older buildings in Europe, where it can lead to costly repairs and, in serious cases, to structural failure of floors, roofs and joists. Its genome has been sequenced, and studies of its decay mechanism help explain how brown-rot fungi evolved and how wood can be protected.
Safety and edibility
Serpula lacrymans is not eaten and is not considered an infectious risk to people. Large amounts of spores and dust from decayed materials can irritate the airways, so wear suitable respiratory protection during inspection and removal work, and seek medical advice if you develop breathing problems. The main danger is structural: weakened timber may fail.
Prevention means keeping buildings dry: repair leaks quickly, keep gutters and drains working, maintain ventilation under floors, and separate timber from damp masonry. If you suspect dry rot, have it assessed by a qualified specialist; remediation focuses on removing the source of moisture, removing affected material with a safety margin and drying the structure.
If you suspect mushroom poisoning, call your local emergency number or poison centre straight away, even before any symptoms appear, and keep the remains of the mushrooms.
Look-alikes
Other building fungi cause similar damage. The cellar fungus Coniophora puteana (wet rot) needs wetter wood, forms thin dark brown to blackish strands and does not spread far over masonry. Fibroporia vaillantii forms white, flexible strands and white pore surfaces. Serpula himantioides is a close relative found mainly outdoors. White moulds on damp walls are surface growths without strands or deep decay. Professional diagnosis is recommended.
Around the world
Dry rot is a well-known problem in older buildings in temperate climates, especially in northern and central Europe, the British Isles and other regions with cool, damp conditions. Building codes and standards for timber protection and remediation differ by country.