Surface Mildew
RoutineVarious surface fungi · also called bathroom mildew, grout mould
Mildew is a loose term for superficial surface growth rather than a species, and it is what the overwhelming majority of bathroom 'black mould' actually is. The distinguishing feature is that it is flat and superficial — if it wipes off and the surface underneath is sound, you have a cleaning and ventilation issue, not a building problem.
Quick facts
- Appearance
- A flat, superficial film of black or grey speckling on grout lines, silicone sealant and painted surfaces. It sits on the surface rather than penetrating, and it wipes away with a cloth.
- Moisture
- Grows on humidity alone — no leak needed
- Smell
- Faint musty.
- Colours
- black, grey, white, pink
- Texture
- powdery, velvety
- Surfaces
- grout, drywall, fabric
How to recognise it
- Flat and superficial. Wipes off, leaving clean material underneath.
- Confined to grout lines, sealant and painted surfaces.
- Returns predictably in the same spots after showers, which points at ventilation.
- No soft or crumbling material beneath it.
What it tells you about the building
Inadequate ventilation. If it comes back within a fortnight of cleaning, the extractor fan is undersized, not vented outside, or not run long enough.
What it gets confused with
One observation settles each of these.
Stachybotrys grows into soaked material and does not simply wipe away. Mildew is superficial.
Pink or orange slime in a shower is Serratia bacteria, not a fungus.
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.