Dry Rot
InvestigateSerpula lacrymans · also called brown rot, true dry rot
Dry rot is a wood-decay fungus rather than a surface mould, and it is the one entry here that is a structural rather than an air-quality problem. It digests the wood itself, and uniquely among decay fungi it can transport water through its own strands to attack timber that is otherwise dry — which is how it spreads through a building. The cubical cracking of the timber is diagnostic.
Quick facts
- Appearance
- White or grey cotton-wool sheets over timber, with grey-white strands that can run across masonry, and sometimes a flat rusty-orange fruiting body with a wrinkled surface. Affected wood darkens, shrinks and cracks into cubes.
- Moisture
- Needs surfaces that stay genuinely wet
- Smell
- Strong, distinctive mushroomy damp smell.
- Colours
- white, grey, orange, brown
- Texture
- stringy, fuzzy, crusty
- Surfaces
- wood, concrete
How to recognise it
- Wood cracks into cube-shaped blocks across and along the grain.
- Timber is soft, light and crumbles under a screwdriver.
- Grey-white strands running across brickwork or plaster away from the wood.
- A strong mushroomy smell quite unlike ordinary musty damp.
What it tells you about the building
Structural timber decay. This is a building surveyor and a builder, not a cleaning product. It needs the moisture source fixed and affected timber cut out well beyond the visible damage.
What it gets confused with
One observation settles each of these.
Surface mould leaves the wood sound. Push a screwdriver in — if it sinks easily, this is decay, not mould.
Where it turns up
The species matters less than you think
- Public health guidance is to remove all indoor mould regardless of type. The CDC and EPA do not recommend routine species testing for ordinary households, because the response does not change: find the water, stop it, remove the growth.
- “Toxic black mould” is a marketing phrase, not a category. Stachybotrys is real and does need soaked material to grow, so finding it tells you something useful about your building. But the health effects that are well established — allergy, asthma aggravation, irritation — are not specific to it.
- Mould without water does not persist. Cleaning growth while leaving the moisture source is why it comes back. The leak, the condensation or the ventilation is the actual problem.
Call a professional for areas larger than about 10 square feet, for anything in HVAC ductwork, after sewage or flood water, or if anyone in the household is immunocompromised or has a lung condition.