How to tell if mould is dangerous or just cosmetic
Surface mould or a structural problem?
The question most people are really asking is whether the mould is just on the surface, or whether it is a sign of water inside the building. Cosmetic mould grows on top of paint, silicone, grout or glass, usually in a room that gets damp in the normal course of things, and it cleans off without damage underneath. Structural mould is a symptom of moisture that has got into plaster, timber, masonry or flooring and is staying there. The first is a cleaning and airing job. The second needs the water found and the damaged material repaired.
This page is about the building. If your question is about health, see what is real and what is myth about black mould, and if you have any symptoms, speak to your GP. In an emergency, call 112.
Signs it is probably cosmetic
- It is small and sits somewhere that gets wet daily, such as shower silicone, tile grout or a window seal.
- It wipes away and leaves a clean, firm, dry surface underneath.
- The paint around it is smooth, with no bubbling, flaking or staining.
- It stays away once you start airing the room or drying the window each morning.
- There is no musty smell once the room has been aired.
Signs the problem is in the structure
The warning signs are about the material and the pattern, not the mould itself. Plaster that feels soft, crumbles or sounds hollow when tapped; paint lifting in bubbles; brown or yellow staining that spreads beyond the growth; a tide mark along the bottom of a wall; skirting or door frames that have swollen; or a patch that is wetter after rain or after the neighbours above shower. Any of these says water is getting into the fabric of the building, through a leak, the ground or persistent condensation, rather than simply condensing on the face of the wall.
Simple tests you can do
Wipe a small area with a damp cloth and a mould cleaner, wearing gloves and a mask with the window open. If the surface underneath is clean and sound, that is a good sign. If a stain remains, or the paint comes away with the cloth, the growth goes deeper. Press the wall gently with a thumb: it should feel hard. Check the other side of the wall and the room above for anything that could be feeding water in. Then watch the spot. Note when it appears and whether it returns, because the pattern often tells you more than the first look. Remember that a stain doesn't need a burst pipe behind it, as mould can grow without a leak when condensation keeps a cold wall wet.
Why a patch that keeps coming back matters most
Recurrence is the clearest dividing line. Cosmetic mould stays gone once the surface is kept dry. Mould that returns to the same spot after careful cleaning is telling you the moisture is still there, and each return pushes further into the material. Painting over it, or cleaning it more often, won't change that; removing mould from walls properly starts with dealing with what feeds it. In Cyprus, recurring patches are common on cold outside walls of older ground-floor flats and in holiday homes left closed over the winter.
Getting a clear answer
If you can't tell which side of the line you are on, a moisture check behind the stain settles it: moisture readings in every affected room, a room-by-room moisture map sent to your phone, and a written report with the quote. Getting to the cause comes before cleaning up the growth, because that's what makes the result last. Where the problem is structural, removal that repairs the cause as well as the wall means the growth is removed, the cause repaired, and the surface finished in an anti-mould system. If you go ahead with the work, the inspection fee is taken off the price.
book an inspection from €75 and know for certain, or call +357 99 759144 and describe what you're seeing.