Bleach turns black grout white, and that is exactly the problem. You have taken the colour away, not the mould. Two weeks later it is back along the same line, and you assume you scrubbed badly.
Short answer
Grout is porous cement. Scrubbing reaches the top layer, heat gets into the pore.
That is why steam does more here than anything in a bottle, and why the black comes back if all you did was bleach it. The rest of this page is the order to work in, which home remedies are worth the effort, and the point where cleaning stops being the answer.
Why grout goes grey and then black
Cleaning grout only works once you know which three things are sitting in the line.
Grout is cement, and cement is porous. What you see is a smooth line between two tiles. Under magnification it is a landscape full of holes, and everything that passes over it collects in them.
- Limescale, from water drying on the surface. That is the white-grey haze on the grout around the shower and the tap.
- Soap residue and skin oils, which cling to the limescale and build a tacky film that holds on to dirt.
- Mould, which grows in that film as soon as it stays damp. That is the black.
Each of the three wants something different from you. Limescale dissolves in acid. Grease dissolves in soap. Mould dies of heat. There is no product that does all three at once, and that is why one bottle never turns out to be enough.
It also explains why the same wall can be three different jobs. The line beside the tap is mostly limescale, the line at shoulder height is mostly soap film, and the line in the corner where the water sits is the one that goes black. Treat the whole wall as one problem and you will do two of those three badly.
How to clean grout in four steps
Cleaning grout between tiles in the shower: the order is not arbitrary.
- Clean the tiles first, not the grout. Soap residue left on the tile rinses straight back down into the line the moment you start. Wipe the whole wall over with a damp cloth before you go near the grout itself.
- Lift the grease with a little washing-up liquid on a damp brush. There is no prize for scrubbing until your arm gives out. All you are doing at this stage is loosening the tacky film that the dirt is stuck in.
- Then go into the pore with heat. Steam is the strongest thing here, because heat and moisture together reach further than bristles do. If you have no steam cleaner, use a cloth with water as hot as you can get it and let it sit on the line for a moment before you work it.
- Dry the grout off. This is the step everybody skips, and it is the one that decides how long the result lasts. Grout that is dry does not grow mould.
Four steps, and only one of them is scrubbing. If you are spending most of your time on step two, you are treating a film as if it were stone.
How to get black mould out of grout
This is where colour gone and mould gone part company.
Black grout in the bathroom is almost always mould. Chlorine bleach turns that black patch white, and it does it fast. What has actually happened is that the pigment has been bleached, while the mould itself stays exactly where it was, down in the pore.
So it looks solved for a week, and then the black comes through on precisely the same stretch. Not because you did too little, but because you went at the wrong thing.
What does work is heat. Mould is an organism and it dies at a high temperature. Steam carries that heat into the pore where no brush can reach, and it leaves nothing behind that feeds the next lot of growth.
The practical approach, then: steam first to deal with the mould, and only afterwards bleach, if you still want to shift the discolouration that is left over. In that order, not the other way round.
After that, keep the bathroom dry, because mould needs damp to get going at all. The same rule runs through mould in textiles: killing it and removing it are two separate jobs. And if the black is on the ceiling and in the corners as well as in the grout, the line is not your problem, the room is. That case is covered in removing mould from the bathroom.
Whitening grout: what is possible and what is not
White is not the same as clean, and you can find out which one you are dealing with.
Grout that is grey with dirt goes white again once you get the dirt out of it. Grout that has yellowed over the years is discoloured cement, and that colour does not come back.
How do you know which of the two you have? Take a patch about ten centimetres square, clean it completely, and let it dry. If that patch is clearly lighter than everything around it, there is still dirt in the rest and carrying on is worth your time. If it dries the same yellow as the rest, you are looking at the colour of the material itself.
In that second case there is one option left that genuinely works: a grout pen, or regrouting. A grout pen is paint, and it covers for a few months. Regrouting is a day's work and it lasts for years.
What does not work is scrubbing harder with a scouring powder. You take the top layer off the cement, the grout becomes more porous than it was, and from then on it goes dirty faster. The line looks better for a fortnight and worse for the rest of its life.
The home remedies measured up
Six things people search for, in order of how much they are actually worth.
White vinegar and cleaning vinegar
It deals with limescale, and limescale is genuinely part of what you are looking at. That makes vinegar worth reaching for on the white-grey haze around the shower and the tap. One caveat: vinegar is an acid and cement is alkaline, so with repeated use the vinegar slowly eats the grout away. Once a year is fine. Every week is not.
Bicarbonate of soda and soda crystals
Two different products, and here they do the same thing: they work as an abrasive, not as a cleaner. Bicarbonate of soda, also called baking soda, is the milder powder of the two. Soda crystals are the stronger one. Either way the fine powder scours the film off the grout, which helps visibly, and which also wears the grout down a little. Use it on one stubborn stretch, not across a whole bathroom.
Vinegar and soda crystals together
The best-known tip on the internet and the least useful one. They cancel each other out: what you are left with is water with a bit of salt in it, fizzing. The fizzing looks convincing and does nothing at all. Use one or the other, never both at once.
Washing-up liquid
Exactly right for the greasy film, and that film is a real layer that is really there. Use a small amount and rinse it off properly, because soap that stays behind pulls fresh dirt straight back into the line.
Bleach and chlorine bleach
Takes the colour out of the mould, not the mould itself. See the section above for why that matters. And it does not get on with acid: never mix bleach with vinegar or with a limescale remover, because that releases chlorine gas.
Steam
The strongest thing on this particular subject, and that is not a sales line, it follows from how a grout line is built. Heat plus moisture get into the pore, and nothing is left behind that becomes food again a fortnight later.
Cleaning grout without damaging it: which brush, and how hard
More force is almost always the wrong answer on a grout line.
A narrow grout brush or an old toothbrush is enough. What you want is to get into the line itself, not to work over the tile on either side of it. A toothbrush has the advantage that the head is already the right width, which is most of the argument for it.
No wire brush and no scouring pad on grout. Those take the smooth top layer off the cement. That layer is precisely what keeps the grout more or less closed, and once it has gone the grout draws water and dirt in like a sponge. You will have made the next clean harder than this one.
Work along the length of the line, not across it. Going across, most of what you move ends up pushed into the edge of the tile rather than out of the grout.
A motor does not change any of that. A brush head on a drill or a powered scrubbing brush is the same force, applied faster and with less feel for what you are doing. The question to ask is the one you would ask of a hand brush: are these bristles soft enough that the smooth layer survives them? If the answer is no by hand, it is still no at a thousand revolutions a minute. A long handle is a different matter, because it changes nothing about what the bristles do to the cement and it keeps you off your knees on a floor.
The other two ways people wear their grout out are in the section above: acid used week in, week out, and an abrasive powder taken across the whole room instead of the one patch that needed it.
Grout in the kitchen and on floor tiles
Different dirt from the bathroom, so a different order of work.
In the kitchen the problem is grease, not limescale. Start there with washing-up liquid or with soda crystals, and leave the vinegar in the cupboard, because there is little for it to do. Above the hob there is usually a layer that lets go far more easily with heat than with force, which is the same lesson as in the bathroom arriving by a different route.
On a tiled floor the dirt has been walked in, so it is dark and it is even, rather than sitting in patches. Mopping does not take it away, because mop water runs into the grout and dries there. That is the reason a floor can look clean while its grout gets steadily darker year after year.
On floor grout, steam is the quickest route, because a narrow nozzle lets you follow the whole line without kneeling down to scrub. Go over it afterwards with a dry cloth, otherwise the dirt you loosened settles back into the same pores as the water dries.
After new tiles have been grouted there is a thin cement haze left on the face of the tile. An ordinary cleaner will not take that off. There is a dedicated cement haze remover for it, and you want to use it within a few days, because after that the haze bonds to the tile.
How to keep it clean
The three minutes a week that prevent the whole job.
Mould needs damp. A bathroom that is dry within half an hour of a shower does not get black grout. That is the whole of prevention, and there is nothing clever hiding behind it.
So: window open or the extractor on during and after your shower, and a quick wipe over the wall and the shower screen with a dry cloth. That costs you half a minute, and it saves you this job.
If you want to go further, seal the grout once you have given it a thorough clean. An impregnating sealer sinks into the pore and makes it water-repellent, so limescale and soap get much less of a hold on it. That lasts a year or two before it needs doing again.
Our cloths are at cleaning cloths. The shower screen follows the same rule as a window, and that one is in washing windows without streaks.
When cleaning is no longer the answer
The limit belongs in the piece, and it is what makes the rest of it credible.
If the grout is loose, if it crumbles when you press it, or if you can see that the black runs right through the cement instead of sitting on the surface of it, cleaning is finished. The old grout has to come out and new grout has to go in.
The same goes for sealant. Black mould in silicone sealant has gone into the rubber. You can kill the mould and the sealant stays black, which is the clearest illustration there is of colour and organism being two different things. Replacing sealant costs you an evening and it is the only fix that works.
That is not an admission of defeat. Grout that has stood damp for years is material that has been attacked, and no product on any shelf undoes that.
Frequently asked questions
How do you get dirty grout clean again?
Wipe the tiles down first, then loosen the greasy film with a little washing-up liquid on a narrow brush, then go into the pore with heat, and finally dry the line off. Steam works best at the third step because heat reaches further into porous cement than bristles can. Drying at the end is what decides how long it stays clean.
How do you get black mould out of grout?
With heat, not with bleach. Bleach takes the colour out of the mould while the organism stays down in the pore, so the black is back within a few weeks on the same stretch. Steam first, and only then bleach if you want to shift whatever discolouration is still showing. After that, keep the room dry, because damp is what the mould needs.
How do you get grout white again?
If it is grey with dirt, clean it the way described above and it comes back. If it has yellowed over the years, the cement itself has discoloured and cleaning will not touch it. Test it by cleaning one patch of about ten centimetres square completely and letting it dry: clearly lighter means there is dirt left to get, the same yellow means you are seeing the material.
Does vinegar help against dirty grout?
Against limescale yes, against grease and mould no. Vinegar is also an acid while cement is alkaline, so with repeated use it eats the grout away. Once a year is fine, weekly is not. On the white-grey haze around the shower it earns its place; on a black line it is the wrong tool.
Does bicarbonate of soda work on grout?
As an abrasive, not as a cleaner. The fine powder scours the film off the surface of the grout, which helps visibly and which also wears the line down a little every time. The same is true of soda crystals, which are the stronger of the two powders. Use either on one stubborn patch rather than across the whole bathroom.
Can I mix vinegar and soda crystals?
Better not. They cancel each other out and you are left with water containing a little salt, fizzing away. The fizzing looks convincing and achieves nothing. Pick one of the two: vinegar for limescale, the powder as a mild abrasive on a stubborn patch.
Can I use bleach on grout?
You can, but be clear about what it does. It takes the colour out of the mould and not the mould itself, so on a porous line it is cosmetic. Never mix it with vinegar or with a limescale remover, because that releases chlorine gas. If you use it at all, use it after the heat, not instead of it.
Does hydrogen peroxide clean grout?
Hydrogen peroxide is a bleach, so everything in the bleach section applies to it. It works on the pigment, which means it lightens the black rather than getting the mould out of the pore. The order stays the same: heat first for the organism, a bleach afterwards only for the discolouration that is still visible. And as with any bleach, keep it away from vinegar and limescale removers.
Does a steam cleaner work on grout?
Yes, and on this particular job it is the strongest thing there is. Grout is porous cement, and heat plus moisture get into the pore where a brush cannot go at all. It also leaves no residue behind that turns into food for the next growth. On floor grout a narrow nozzle lets you follow the line without kneeling to scrub.
Which brush should I use on grout?
A narrow grout brush or an old toothbrush. No wire brush and no scouring pad: those take the smooth top layer off the cement, and after that the grout soaks up water and dirt like a sponge. Work along the length of the line rather than across it, because across it you push the dirt into the edge of the tile.
Is a drill brush or an electric brush safe on grout?
It depends entirely on the bristles, not on the motor. A powered brush applies the same force faster and with less feel, so the test is the one you would use by hand: if those bristles would take the smooth layer off the cement when you pressed with your arm, they will do it sooner with a drill behind them. A long handle is different, since it changes nothing about the bristles and only saves your knees.
How do I clean grout on a tiled floor?
Mopping will not do it, because the mop water runs into the grout and dries there, which is how the line gets darker while the tiles look clean. Follow the lines with steam, or with hot water and a narrow brush, and go over the result with a dry cloth afterwards. Without that last step the loosened dirt settles back into the pores as the water dries.
How do I stop grout going black in the bathroom?
By getting the room dry quickly. Ventilation on during and after your shower, and a short wipe over the wall and the screen with a dry cloth. Mould needs damp, so a bathroom that is dry within half an hour does not get black grout. Sealing the grout after a thorough clean buys you another year or two on top of that.
When does grout need replacing?
When it is loose or crumbling, or when the black runs through the whole thickness of the cement instead of sitting on the surface. Silicone sealant is the same story: the mould goes into the rubber, so the sealant stays black even after the mould is dead. At that point replacing it is the only thing that actually works.
More around the house
Everything sorted by job is under cleaning.