Last Updated on October 26, 2026 by Umar Farooq
Can mould come through tiles? Not through a sound glazed tile, but it can reach the face of a tiled wall or floor by three other routes: through the grout, round the edges, or from damp in the wall or slab behind. So when black patches appear on tile, the useful question is where the water behind it is coming from.
That distinction matters because scrubbing the grout fixes the cosmetic problem and leaves the cause alone. I would rather you knew which of the three routes you have before you pick up a brush.
Can mould come through tiles themselves? Rarely
A glazed tile is a very poor path for water. Porcelain, by the trade definition, absorbs 0.5 per cent or less of its weight in water in the standard boil test, so a sound glazed tile gives water very little to travel through. Mould needs moisture and something to feed on, and a clean glazed face offers little of either.
That is why mould on tiles is usually sitting on soap scum, body oils and dust on the surface, or growing in the grout and silicone. There are a few exceptions. Unglazed natural stone, terracotta and some cheap, soft-bodied tiles are far more absorbent, and a stone stored damp before it was fitted can carry the problem with it. But for a normal bathroom or kitchen tile, the surface is rarely where the story starts.
Route one: through the grout
Cement grout is the usual door. One flooring guide says it is porous by nature and that even small pores let capillary action draw water through, and that sealers reduce absorption but do not make an impervious membrane. Sealer wears off, and in a shower the joints are wet twice a day.
So the grout stays damp, and the damp moves in two directions. Outwards, it feeds the black growth you see along the lines. Inwards, it can soak the adhesive and whatever sits behind, which is the second route.
A crude check: drip a few drops of water on a dry joint. If it darkens and soaks in within a minute or two rather than beading, the grout is absorbing water and is likely overdue for sealing or replacement. This is a rule of thumb, not a standard test.
Route two: round and behind the tile
In a wet area the tile is not meant to be the waterproofing at all. The grout and tile stop most splashes, and the layer behind (a tanking membrane, backer board or waterproof render) is what protects the wall. When that layer is missing, cracked or has a gap at a corner, tap or waste pipe, water gets behind the tile, and mould appears on the face because the moisture eventually works its way out.
Signs that point behind the tile rather than at the surface:
- Mould that returns along the same joint within days of cleaning.
- Loose, hollow-sounding tiles. Tap along a row with a coin; a dull rattle suggests the adhesive has let go.
- Grout that crumbles or powders, or tiles that have lifted at a floor-to-wall corner.
- A damp patch on the wall on the far side, in the next room or below the floor.
- Silicone that is black through its full thickness, not just on the surface.
If you suspect the membrane has failed, the fix is not a cleaning job. Water that has been getting behind the tiles for months needs the tiles off, the substrate dried and the waterproofing redone. How to tell if a wall is damp and how to use a damp meter will tell you how wet the wall behind is before you commit.
Route three: damp from the wall or floor behind
Sometimes the moisture is not coming through the joints at all. It is coming through the wall. A tiled external wall that is cold will collect condensation behind and on the surface, and a ground-floor tiled slab can feel cold and sweat in warm, humid weather. Mould on the tiles is then a symptom of a wall or floor that is wet from some other cause, not a tile problem.
A useful test is to look at where the mould sits. If it is on the cold external wall and corners only, and the room is steamy, condensation is the first suspect. If it is on a wall that backs onto a bathroom, a kitchen or a pipe run, look for a leak. If it follows a wet-looking floor in a cellar or garage, why a concrete floor sweats explains the mechanism.
Surface mould or hidden problem? A 10-minute check
Do not mix bleach with other cleaners, and wear gloves and eye protection when you clean mould. Open the window while you work.
- Wipe a patch with warm water and a little washing-up liquid. If it comes off cleanly and stays off for a fortnight, it was surface growth on soap scum.
- Check it a few days later. If the same line is black again, the joint is staying damp.
- Tap and press. Loose tiles, soft grout or a spongy board behind point to water behind.
- Look at the other side. Is there a damp patch, peeling paint or stained plaster on the room next door?
- Count the area. The EPA’s brief guide says most people can clean up an area of less than about 10 square feet themselves; larger areas call for the EPA’s remediation guidance. The same guide says to fix plumbing leaks and other water problems as soon as possible.
For the cleaning itself, how to get rid of mould in a bathroom covers the safe method.
Dry the room, not just the tile
Whichever route you have, the air in the room decides how long the tile stays wet. Keep the extractor fan running for a good while after a shower, wipe the screen, and leave the door ajar once the steam has gone. A dehumidifier in a bathroom that is slow to dry will help, but it works on the air, not on water already trapped behind the tile. Why mould keeps coming back explains why the water source matters more than the cleaner.
Mould on tile is a message about moisture, so follow the water
A sound tile does not let mould through. The mould you see on tiled surfaces has either grown on the surface, come up through porous grout, escaped from a failed membrane behind, or travelled from a wet wall or slab. Work out which with the wipe, tap and next-door-wall tests, and fix the water before the cosmetic clean.
FAQ
Can mould grow behind tiles?
Yes. If water gets behind the tiles through failed grout, a cracked seal or a missing membrane, the adhesive and the substrate stay damp and mould can grow there out of sight. Loose tiles, crumbling grout and damp on the far side of the wall are the clues.
Does mould grow on tiles or in the grout?
Mostly in the grout and silicone, and on the film of soap scum and dust that sits on the tile face. Cement grout is porous and holds moisture, which makes it the usual starting point.
Can mould grow under floor tiles?
It can where moisture comes up from the slab or in from the edges, or where a leak has soaked the layers beneath. A floor that stays cold and damp, or tiles that feel hollow or lift, suggests moisture underneath.
Should I seal my grout?
Sealing reduces how much water cement grout absorbs, but it is not a waterproof barrier and has to be renewed. In a shower, the membrane behind the tile does the waterproofing; sealed grout is a supporting measure.
Will a dehumidifier stop mould on tiles?
It reduces the humidity that keeps tile and grout damp, which helps prevent new surface growth. It cannot dry a wall that is leaking or a membrane that has failed, so fix that first.
