The short answer
- Plenty of bathroom exhaust fans discharge into the roof cavity instead of outdoor air, so the moisture leaves the room and stays in the building, in a dark space full of timber, insulation and dust.
- Grout is porous. Water sits inside it long after the tile face feels dry, which is why the same grout line keeps darkening on the same cycle.
- Silicone that darkens again within days of a proper clean has growth below the surface film. Replace it.
- The 20 minutes after a shower do more for recurrence than any product: squeegee the glass and tiles, dry the seals and lower corners by hand, leave the fan running with the door ajar.
- Mould does not have to smell or be visible. Most significant problems have no odour, and nose blindness sets in within days, so a bathroom that smells fine tells you nothing.
You clean it properly. It looks perfect. 3 weeks later the same shadow is back in the same corner, and you assume the clean was not good enough or the product was not strong enough. In a bathroom that keeps regrowing mould in the same 3 places, the variable is drying time.
The room looks dry long before it is dry
Tile faces and glass dry fast, which is why they mislead you. Grout, silicone seams, the plasterboard around the fan, the ceiling above the shower and the window reveal all hold water for hours after the visible surfaces have cleared. Given the right conditions, mould can start growing in around 48 hours. A bathroom that stays damp for 2 or 3 hours after every shower, twice a day, never fully resets.
Most of the moisture in a house is generated inside it. Showering, cooking, drying laundry indoors and breathing all add water to the air, and in Brisbane the outside air is often no drier than the inside air. A bathroom is rarely a closed system, and if the rest of the house sits at high humidity it has nowhere to send its moisture. There is more on that in our guide to damp and mould in Brisbane homes.
Start with the fan, and find out where it actually ends
This is the biggest cause of recurring bathroom mould that people never check. An exhaust fan is supposed to discharge to outdoor air, and current building requirements expect that. Plenty of installations, particularly older ones and cheap retrofits, terminate in the roof space: a short stub of duct stopping in mid-air, or no duct at all, just a fan blowing straight up through the ceiling. The bathroom feels better while the water stays in the building, now in a dark cavity full of timber, insulation and dust.
You can check this yourself. Put your head into the roof space with a torch and look directly above the bathroom. You want a duct running to an eave, a wall vent or a roof cowl. If it ends loose in the cavity, has fallen off its collar, or sags with a low point where water can pool, you have found your problem. Other things worth checking while you are up there:
- Whether the fan moves air at all. Hold a square of toilet paper against the grille; if it does not hold on its own, the fan is not doing much.
- Whether the grille is loaded with dust. A furred-up grille cuts airflow badly, and it is a 5 minute clean.
- Whether there is makeup air. A fan can only extract what it can replace, so a tight door with no gap underneath leaves it working hard and moving very little.
- Whether the fan is sized for the room. A small fan in a large bathroom with a big shower will never keep up, no matter how long you run it.
Grout and silicone are where the water stays
Cement-based grout is porous. It draws water in and releases it slowly, so a grout line can still be wet inside when the tile either side is bone dry. That is the whole explanation for grout that keeps darkening: you are cleaning the surface of something damp all the way through.
Silicone gets there by a different route. Soap, skin oils and shampoo residue form a thin film on the seal, and that film is food. New silicone often contains a fungicide, which works for a while and then stops. If the seal has cracked, lifted at an edge or pulled away in a corner, water gets in behind it and sits in the join. Nothing you wipe on the front will reach that.
Colour tells you very little. The same species can appear in different colours on different surfaces in the same room, and some growth is effectively transparent. "Black mould" describes a colour, not a species, and identification is a laboratory job. What matters is that something is growing, and why.
Why it is always the same 3 spots
Recurrence clusters where 3 things overlap: the coldest surface, the stillest air and the slowest drainage. In most bathrooms that gives you the lower corners of the shower, the silicone where the wall meets the floor or the hob, and the ceiling above the shower or the window reveal.
Condensation forms first on the coldest surface. An external wall, a window frame, an uninsulated ceiling above a shower recess: these run several degrees cooler than everything around them, so water lands there first. Airflow then decides how long it stays, and corners, the space behind a screen frame, the underside of a hob and the back of a vanity are all dead air. Add gravity, and the base of the shower receives everything running down the walls. Same room, same water, 3 spots that dry hours after the rest.
Which is why moving the shampoo bottles, the loofah and the shower caddy matters. Clutter blocks airflow, holds water underneath and hides early growth until it is well established.
The 20 minutes after the shower
Change this if you change nothing else. The 20 minutes after the water stops decide whether the room dries in half an hour or 3 hours, and drying time is the lever you control.
- 1Squeegee the glass and the tiled walls. It takes about 45 seconds and removes most of the standing water before it evaporates into the room and lands somewhere colder.
- 2Wipe the silicone seals, the lower corners and the shower shelf with a dry cloth. These are the slowest-drying points in the room, so a dry cloth short-circuits the whole cycle.
- 3Leave the fan running for at least 20 minutes after you get out, and open the door or leave a gap so replacement air can get in. A timer switch removes the need to remember.
- 4Hang the towel where air moves around it. A wet towel folded on a rail in a closed bathroom is a small, steady moisture source for the next several hours.
- 5Open the window only if the outside air is drier than the inside air. On a humid Brisbane morning it will not be, and the fan is the better option.
None of it is glamorous and all of it works. It will not fix a fan that vents into the roof or rescue silicone that has already failed. On an otherwise sound bathroom it is often the difference between cleaning the shower corner monthly and cleaning it twice a year.
When resealing beats recleaning
There is a point where more scrubbing is just wear on the material. The test I use is time-to-return. If a seal or a grout line has been properly cleaned, dried, and discolours again within days, the growth has extended into the material, below where any cloth can reach. Replace it.
Signs it is time to replace: silicone that is lifting, cracked, spongy or peeling at an edge; discolouration sitting under a clear film; grout that is powdery, eroded or missing in sections; tiles that sound hollow when tapped. There is another that changes the conversation entirely: a permanently damp patch on the wall outside the bathroom or on the ceiling below it. That suggests water is getting past the waterproofing membrane, which needs a licensed trade.
Replacing a run of shower silicone is a small job next to what a failed seal eventually turns into. Regrouting is a bigger commitment, but a bathroom where the grout has gone will keep beating you otherwise.
What actually removes bathroom mould
The goal is physical removal. Dormant spores and dead growth still matter, so sterilising a surface without clearing it does not finish the job. On non-porous surfaces (glass, tile faces, plastic, sound painted ceilings) a cloth, a detergent and real physical effort remove it properly. On porous materials the growth extends below the surface, and what happens next depends on the material and how far it has gone. Sometimes the material has to come out.
I do not use bleach. It removes the colour while leaving the growth in place on anything porous, so you get a clean-looking result and the same problem back. It is also mostly water, which is the last thing a damp surface needs. Vinegar has its own limits, which we have gone through in what vinegar can and cannot do. For scale and soap residue, a mild acid and the right abrasive for the surface do the work, without adding fragrance to a room that already has a moisture problem.
A note on scope: I clean homes. I am not a remediator, not a builder and not a health professional, and nothing here is health or medical advice. Everything above is about the building itself: where water sits, how long it stays, and what removes growth from a surface. If anyone in the household has a health concern, that is a conversation for their GP, not for a cleaner.
Where I fit, and where I do not
In a regular clean I remove surface mould where it is safe and appropriate, using fragrance-free products and physical effort. I am not a remediator. Full remediation is specialist work, and a bathroom where water has got behind the waterproofing needs a builder and a licensed trade.
What I can do is look at the whole house rather than the single corner you asked about. A Home Health Review is a walk-through and a conversation, with no testing or sampling: fans that are not doing their job, rooms with no cross-flow, laundry drying indoors, seals on their way out, the cold external wall behind the wardrobe. Most of what comes up is fixable with a habit change or a tradesperson, and knowing which saves a lot of scrubbing.
A bathroom that regrows mould on a predictable cycle is wet for longer than it looks.
Common questions
Why does mould keep coming back in the same spot in my shower?
Because that spot is the coldest, stillest or slowest-draining point in the room, so it stays wet for hours after everything around it has dried. Condensation forms first on the coldest surface, corners and areas behind screen frames have almost no airflow, and water runs down to the base of the shower. Cleaning removes the growth but does not change the conditions, so it returns on the same cycle. Dry the room faster and the cycle breaks.
Can a bathroom exhaust fan vent into the roof cavity?
It should not. An exhaust fan is meant to discharge to outdoor air, but plenty of installations, especially older or retrofitted ones, simply blow into the roof space. That moves the moisture out of the bathroom and leaves it inside the building, where the timber, insulation and dust are. Check by looking into the roof space above the bathroom with a torch. You want a duct running to an eave, wall vent or roof cowl. A duct that ends loose in the cavity is the problem.
Should I use bleach on bathroom mould?
It is a poor choice. Bleach removes the colour from the growth while leaving it in place on porous materials like grout, so the surface looks clean while the problem continues below it. It is also mostly water, which adds moisture to a surface that already has too much. Physical removal with a cloth, a detergent and genuine effort works better on non-porous surfaces, and on porous ones the material itself may need replacing.
How long should I run the bathroom fan after a shower?
At least 20 minutes after the water stops, with the door open or a gap under it so replacement air can get in. A fan can only extract as much air as it can draw in, so a sealed bathroom limits how much it achieves. A timer switch is worth fitting because it removes the need to remember. Squeegeeing the glass and tiles and drying the silicone seals by hand in the same window makes a bigger difference again.
When should I replace shower silicone instead of cleaning it?
When it discolours again within days of being properly cleaned and dried, which means the growth has extended into the material. Other signs are silicone that is lifting, cracked, spongy or peeling at an edge, or discolouration that appears to sit under a clear film. Replacing a run of shower silicone is a small job and it usually ends a recurrence that no amount of cleaning was going to fix.
Does bathroom mould always smell?
No. Most significant mould problems have no smell at all, so the absence of an odour is not reassurance. People also become nose blind to a persistent smell within a few days, which means a household can stop noticing something a visitor picks up immediately. Colour is equally unreliable as a guide, since the same species can appear in different colours on different surfaces and identification requires a laboratory.