Brown mould on walls and ceilings
Common on wood and in damp basements, and easily confused with staining from old water damage.
4 likely answers.
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.
The 4 likely answers
1.Aspergillus
InvestigatePowdery colonies with a granular surface, in an unusually wide colour range — green, yellow-green, white, brown or black — often with a paler edge and darker centre as the colony matures.
How to know it is this one
Powdery, granular surface that puffs when disturbed.
What it means: Persistent damp. If anyone in the household is immunocompromised, has asthma or has a lung condition, this is the case where professional remediation is proportionate rather than excessive.
More on aspergillus →2.Dry Rot
InvestigateWhite 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.
How to know it is this one
Wood cracks into cube-shaped blocks across and along the grain.
What it means: 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.
More on dry rot →3.Alternaria
RoutineDark grey-brown to black with a distinctly velvety or downy texture and slightly raised, woolly growth. Common in showers, under sinks and around window frames.
How to know it is this one
Velvety, slightly woolly texture with a raised profile.
What it means: Standing water or a slow leak. Find the drip before cleaning the growth.
More on alternaria →4.Cladosporium
RoutineOlive-green, brown or near black with a suede-like velvety texture, growing in discrete rounded patches that merge. Dry to the touch and powdery when disturbed.
How to know it is this one
Velvety, suede-like texture that is dry rather than slimy.
What it means: Relative humidity persistently above roughly 60%, or surfaces cold enough to condense. Ventilation and a dehumidifier address it.
More on cladosporium →Still not sure?
The key adds texture and surface, which separate these far better than colour does.
Identify it step by step