Tiles are the easiest surface in the house to clean. The grout between them is one of the hardest — and the reason is structural, not cosmetic.
Standard grout is cement-based, which means it is porous. Water, soap, body oils and pigment soak into it rather than sitting on it. That's why a shower wall can be spotless while the joints between the tiles read as grey, and why a surface spray does nothing.
It also explains the second problem: most of the things people reach for to fix discoloured grout are acidic, and acid dissolves cement.
Quick Answer: Scrub with a bicarb soda paste and a stiff narrow brush for general discolouration. Use oxygen bleach (sodium percarbonate) for stubborn staining, and a diluted chlorine bleach solution only for black mould. Avoid vinegar and other acids for routine cleaning — they erode cement grout over time. Once it's clean and fully dry, seal it. Reseal shower grout annually.
Why Grout Gets Dirty (and Why It Stays That Way)
Three separate things discolour grout, and they need different answers.
Soap scum and body oils build a dull film that traps everything else. This is surface grime — mechanical scrubbing removes it.
Absorbed staining happens because unsealed cement grout drinks. Tea, dirt tracked in on shoes, hair dye, iron in the water supply. The pigment is inside the grout, not on it, and no amount of surface scrubbing reaches it.
Mould is a living colony rooted in the pores. Wiping the black off the surface leaves the roots, which is why it returns within a fortnight. CSIRO scientist Anthony Wright makes the point that indoor mould is fundamentally a moisture problem — treat the surface without fixing the moisture and you're on a permanent cycle.
The Cleaners That Quietly Damage Grout
This is the part most guides skip, and it's why a lot of grout looks worse three years in than it did at year one.
Acids. Vinegar, lemon juice and acidic tile cleaners dissolve the calcium compounds that hold cement grout together. An occasional use won't ruin it, but manufacturers of tile and grout systems are consistent that acidic cleaners are not appropriate for routine maintenance of cement-based grout — repeated use discolours it and weakens the joint. Mapei's care and maintenance guidance directs routine cleaning to pH-neutral products for exactly this reason.
Acids on natural stone are worse. If your tiles are marble, limestone, travertine or honed stone rather than ceramic or porcelain, acid etches the tile itself — a dull white mark that cannot be cleaned off, only re-polished.
Steel wool and metal brushes. These score the grout, and a scored surface holds dirt better than the one you started with.
Chlorine bleach as a routine cleaner. It has a place on mould, covered below. Used weekly it degrades grout and, on coloured grout, fades it unevenly.
If you're unsure what your tiles are, test any new product in an inconspicuous corner — behind the toilet, or a floor tile under the vanity — and wait a day.
What You'll Need
Bicarbonate of soda
Oxygen bleach (sodium percarbonate — sold as a laundry soaker or oxygen-based stain remover)
A stiff nylon grout brush — narrow, so the bristles sit in the joint rather than skating over it
An old toothbrush for corners
Rubber gloves and eye protection
Two microfibre cloths
A spray bottle of warm water
Penetrating grout sealer (from a tile shop or hardware store)
⚠️ Safety note: Never mix bleach with vinegar, ammonia or any acidic cleaner. The Washington State Department of Health treats the chlorine gas this produces as a genuine poisoning risk. If you use one product and then want to try another, rinse thoroughly and ventilate the room in between.
How to Clean Grout, Step by Step
Step 1: Clear the Surface First
Clean the tiles normally with a pH-neutral cleaner and rinse. There is no point attacking the joints while soap film is still spreading dirt back into them.
For a shower, deal with any soap scum before you start on the grout.
Step 2: Bicarb Paste and a Stiff Brush
Mix bicarb soda with just enough warm water to make a thick paste. Press it into the joints with your fingers or the brush — you want it sitting in the channel, not smeared across the tile.
Leave it for fifteen minutes, then scrub along the joint, not across it. Short firm strokes following the line of the grout. Work a square metre at a time so the paste doesn't dry out.
Rinse with warm water and wipe with a clean damp cloth. For a lot of household grout, this is the whole job.
Don't finish by spraying vinegar over the bicarb. As Dr Nathan Kilah, Senior Lecturer in Chemistry at the University of Tasmania, explains, acid and base neutralise on contact — the fizz is the two products cancelling each other out, and on grout you're also feeding acid to the cement for no gain.
Step 3: Oxygen Bleach for Stubborn Discolouration
If the joints are still grey, move to oxygen bleach rather than escalating to chlorine.
Sodium percarbonate releases hydrogen peroxide in water. It lifts absorbed organic staining, it's colour-safe on coloured grout, and it's far gentler on cement than either acid or chlorine.
Mix to a thin paste with warm water, following the packet ratio.
Apply along the joints and leave for 15–30 minutes — do not let it dry out completely. Mist it with water if it starts to.
Scrub with the grout brush, then rinse thoroughly.
Repeat once if there's improvement but not enough. Two moderate applications beat one aggressive one.
Step 4: Chlorine Bleach — Mould Only
For black mould in shower grout, a diluted chlorine bleach solution is the appropriate tool. Better Health Victoria recommends diluted bleach for hard, non-porous surfaces, and NSW Health's mould fact sheet sets out the ventilation and protective equipment to use.
Apply to the affected joints, leave the contact time on the product label, scrub, and rinse thoroughly with clean water. Ventilate the room throughout and afterwards.
Bear in mind what bleach does and doesn't do: it kills and it whitens. On deeply rooted mould in porous grout it can bleach the visible colour away while leaving the colony below, which is why the black seems to come back so fast. If it returns within weeks, the fix is moisture control, not a stronger chemical.
Step 5: Dry Completely, Then Seal
This is the step that makes the difference between cleaning grout once a year and cleaning it every month.
Let the grout dry fully — at least 24 hours in a bathroom, longer in winter. Sealer applied to damp grout traps the moisture inside.
Then apply a penetrating (impregnating) sealer with the applicator or a small brush, working joint by joint. Wipe any excess off the tile face before it hazes. Follow the product's cure time before using the shower again.
How to tell if grout is already sealed: drip water onto it. If it beads and sits, it's sealed. If it darkens and soaks in within a minute or two, it isn't.
How often: annually for shower and splashback grout, every two to three years for floors, and never for epoxy grout — it's non-porous and doesn't need it.
Stopping It Coming Back
Mould in bathroom grout is a ventilation problem wearing a cleaning problem's clothes. The Australasian Society of Building Biologists is direct that controlling moisture is the only durable answer.
Four habits, in order of effect:
Run the exhaust fan during the shower and for 20 minutes after. Most people turn it off with the light. That last twenty minutes is when the room actually dries.
Squeegee the walls and screen after the last shower of the morning. Thirty seconds, and it removes most of the water mould needs.
Leave the bathroom door open afterwards so humid air can move out.
Keep the grout sealed. Sealed grout can't absorb the water and soap that feed the colony.
Where the joint has gone soft, crumbly, or is missing in sections, cleaning is the wrong tool — that grout needs raking out and replacing. A grout pen will cosmetically re-whiten a sound joint, but it won't fix a failing one and it will peel over a damp substrate.
Frequently Asked Questions
Q: What is the best thing to clean grout with?
A bicarb soda paste and a stiff narrow grout brush for everyday discolouration, and oxygen bleach for stubborn staining. Both are effective and neither damages cement grout. Save chlorine bleach for visible black mould.
Q: Does vinegar clean grout?
It cuts soap scum, so it looks like it's working — but it's acidic, and repeated use on cement-based grout erodes the joint and can discolour it. On natural stone tiles it will also permanently etch the tile. Use a pH-neutral cleaner or bicarb instead.
Q: How do I whiten grout that's gone grey?
Scrub with bicarb, then apply oxygen bleach and let it dwell 15–30 minutes before scrubbing again. Grey usually means absorbed staining rather than surface dirt, so contact time matters more than pressure. If two rounds don't shift it, the grout is stained through and your options are a grout pen on sound joints, or regrouting.
Q: Why does mould keep coming back in my shower grout?
Because the moisture is still there and the grout is still unsealed. Bleach whitens the surface without necessarily killing the colony rooted in the pores. Run the exhaust fan for twenty minutes after every shower, squeegee the walls, and seal the grout once it's clean and dry.
Q: Should I seal my grout?
Yes, if it's cement-based — and virtually all standard grout is. Sealing is what keeps it clean rather than what makes it clean. Epoxy grout is the exception; it's already non-porous.
Q: Can I use a steam cleaner on grout?
Yes, and it works well — heat and pressure lift grime out of the joints without any chemicals. Two cautions: keep the nozzle moving rather than parking it on one spot, and don't steam grout you've recently sealed, as it can lift the sealer.
Q: How often should I clean grout?
Wipe shower walls down after use and the joints will need a proper clean about twice a year. Sealed grout in a well-ventilated bathroom can stretch to annually; unsealed grout in a bathroom without a working exhaust fan will need attention every month or two.
