How to Clean Grout: What Works, and What Etches Stone

Knowing how to clean grout properly is mostly a matter of knowing what your grout and your tiles are actually made of. The same oxygen bleach paste that brings a cement-grouted porcelain floor back to life will do nothing much for epoxy grout, and the vinegar trick that gets passed around online will permanently ruin a marble bathroom. Get the surface right and most discoloured joints come back with one afternoon of work. Get it wrong and you pay a stone restorer to grind the damage out.

This is the order we work through on customers’ bathrooms and kitchens across Bromley and south east London: cheapest method first, strongest method last, and a clear line where cleaning stops being the answer.

Why grout goes dark in the first place

Standard grout is a cement product, and cement is porous. Under BS EN 13888-1:2022, the standard that classifies grouts for ceramic tiles, cementitious grouts are split into CG1 (normal) and CG2 (improved), with suffix letters for the extras: A for high abrasion resistance, W for reduced water absorption. Resin grouts sit in a separate class, RG.

Those letters matter more than the colour on the tub. A plain CG1 joint drinks in soapy water, shampoo, cooking grease and the fine grit walked in on shoes. The dirt is not sitting on top of the grout waiting to be wiped off; it has soaked in, a millimetre or two down. That is why wiping does nothing and why the joints go patchy rather than evenly dirty. A CG2 W grout absorbs far less and stays presentable much longer, which is worth knowing before you regrout rather than after.

Epoxy grout is a different animal. It is effectively non-porous, so dirt genuinely does sit on the surface. If your grout is epoxy, you need less chemistry, not more.

Black speckling in the corner where the wall meets the bath or the shower tray is usually not grout at all. It is mould growing inside silicone sealant, and no cleaner reaches it.

Start with the cheapest method that might work

Before any chemistry, try hot water, a squirt of washing-up liquid and a stiff nylon grout brush. An old toothbrush is too soft and too small for a floor; a proper narrow grout brush costs a few pounds and does ten times the work.

Work a square metre at a time, scrub along the joint rather than across it, and rinse with clean water before the dirty water dries back into the grout. That last step is the one most people skip, and it is why joints often look worse an hour after cleaning than they did before.

On a floor that has simply gone dull rather than stained, this alone often finishes the job. If a square metre of test area comes up noticeably lighter, keep going and save yourself the stronger methods.

Oxygen bleach does most of the heavy lifting

For cement grout that has genuinely stained, oxygen bleach is the workhorse. The active ingredient is sodium percarbonate, sold as a laundry booster or a dedicated patio and grout cleaner. Dissolved in warm water it releases oxygen, which penetrates the porous grout and breaks the staining down inside the joint.

The practical difference from household chlorine bleach is worth understanding. Chlorine bleach mostly decolourises what it touches, so the joint looks paler for a while but the dirt is still in there and comes back. Oxygen bleach works more slowly and actually lifts the soiling out. It also does not strip the pigment out of coloured grout, which chlorine bleach happily will.

Method: mix it into a thick paste rather than a runny solution, spread it along the joints, and leave it to dwell for twenty to thirty minutes so it stays wet and keeps working. Scrub, then rinse thoroughly with clean water. Badly stained floors often need two passes. Ventilate the room and wear gloves; sodium percarbonate is alkaline and unkind to skin over a long session.

What never to use, and the one combination that is dangerous

Two rules matter more than everything else in this article.

Panel showing which grout cleaner is safe on cement grout, natural stone, epoxy grout and silicone, and a warning against mixing bleach with acid — Which grout cleaner is safe on which surface. Chart by Bromley Tilers. | Chart by Bromley Tilers

Never use acid on calcite stone. Marble, limestone and travertine react with acid. Vinegar, lemon juice, citrus cleaners and limescale removers all etch them, leaving dull cloudy patches where the polish used to be. Etching is physical damage to the stone surface, not a stain, so it cannot be cleaned off afterwards; it has to be honed and repolished. If there is any calcite stone in the room, including a stone windowsill or shelf that overspray could reach, use a pH-neutral stone cleaner and nothing else.

Never mix bleach with an acidic cleaner. Household bleach is sodium hypochlorite. Add an acid to it, which includes vinegar and most limescale and bathroom descalers, and it releases chlorine gas. In a small unventilated bathroom that is a genuine hazard, not a theoretical one. The Health and Safety Executive’s guidance on cleaning chemicals is aimed at workplaces, but the underlying point applies at home: use one product at a time, follow the label, ventilate, and never decant chemicals into an unlabelled container.

Two more to avoid: wire brushes, which scratch glazes and tear the surface off soft grout, and aggressive grinding tools used freehand, which chew the tile edges long before they finish the joint.

Steam cleaners and grout pens: what each actually achieves

A steam cleaner with a narrow detail nozzle is genuinely effective on cement grout, and it uses no chemicals at all, which makes it the sensible option in a bathroom you cannot ventilate well. Steam softens the soiling so a brush can remove it. Keep the nozzle moving, and be careful around silicone joints and any grout that is already crumbling, because concentrated steam will finish off a joint that was on its way out.

Grout pens are a different proposition. They do not clean anything; they paint over the joint with a pigmented coating. On a sound, dry, properly cleaned wall joint in a low-traffic spot, a grout pen gives a tidy result that lasts a reasonable time. On a shower floor or a kitchen floor it wears off in patches and looks worse than the discolouration did. Use one as cosmetic finishing after cleaning, never as a substitute for it, and never over damp or crumbling grout.

When cleaning is not the answer

There is a point where more scrubbing is wasted effort. Stop cleaning and fix the underlying problem if you see any of the following.

Grout that crumbles or turns to powder under the brush. The joint has failed and needs raking out and regrouting.
Cracked joints, or joints that have come away from the tile edge. Usually movement in the substrate, which regrouting alone will not cure.
Black mould in the corners and around the tray or bath. That is silicone, not grout. Cut it out, dry the joint properly and reseal.
Staining that returns within weeks in the same spot. Something is feeding it, normally water getting behind the tiles.
Damp on the other side of the wall. Cleaning the visible surface is the least useful thing you can do at that point.

Seal it so you are not doing this again next year

Cleaning cement grout opens the pores back up, which is exactly when it is most vulnerable. An impregnating grout sealer soaks in and makes the joint water-repellent rather than sitting on top as a film. Applied to grout that is clean and fully dry, it buys you a great deal of time before the joints darken again.

Two conditions decide whether it works. The grout must be dry, which after a heavy wet clean means leaving it a full day or more, not an hour. And the sealer must be the right type for the tile as well as the grout, because some products will haze a polished porcelain or a stone surface. Test on an offcut or an out-of-sight corner first.

If you are choosing grout for a new job rather than rescuing an old one, specify a CG2 W grout for anything in a shower, a wet room or a kitchen floor. The reduced water absorption is the whole point, and the extra cost is trivial next to the labour of regrouting.

For heavily stained floors, failing joints or anything involving natural stone, it is worth getting a tiler to look before you start. You can find a member firm through The Tile Association, or see the rest of our tiling advice for Bromley homeowners.

Frequently asked questions

Does bicarbonate of soda and vinegar clean grout?

The fizzing is the two neutralising each other, which leaves you with a mildly salty solution and very little cleaning power. The bicarbonate on its own works as a gentle abrasive. The vinegar adds nothing useful and, on marble, limestone or travertine, causes permanent etching. Use oxygen bleach instead.

Will bleach whiten my grout permanently?

No. Chlorine bleach lightens the appearance of the stain rather than removing it, so the colour usually creeps back within weeks. It will also strip the pigment from grey, beige or coloured grout, leaving blotches that are harder to live with than the original discolouration.

How do I know whether my grout is cement or epoxy?

Scratch a hidden joint with a coin. Cement grout gives up a little powder; epoxy stays glossy and unmarked. Epoxy also tends to look slightly plasticky and to have been used with glass mosaic or in commercial settings. If it was grouted by a previous owner and there is no record, assume cement and treat it accordingly.

Can I clean grout on a natural stone floor at all?

Yes, with a pH-neutral cleaner made for stone, a soft brush and patience. Avoid every acid, avoid strong alkalis on polished surfaces, and never let any cleaner dry on the stone. Heavily soiled stone is one of the few cases where paying a specialist to clean and reseal is genuinely cheaper than the alternative.

How often should grout be resealed?

In a family shower, expect to reseal roughly every year or two; on a kitchen floor, every two to three years. The test is simple: drip water onto the joint. If it beads, the sealer is working. If it soaks in and darkens the grout, it is time to reseal.

Is it cheaper to clean grout or regrout?

Cleaning is far cheaper while the joints are sound. Once grout is crumbling, cracked or missing in places, cleaning is money spent on a surface that is about to fail anyway, and regrouting is the better use of the same afternoon.

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