Cracked, crumbling or blackened grout makes even a good bathroom look tired, and it is not just cosmetic. Grout is the seal that keeps water out of the wall behind your tiles. Once it fails, water gets in, tiles loosen, and a small weekend job turns into a damp patch, a mould problem or a full re-tile. Regrouting is one of the cheapest repairs in a bathroom, and it is well within reach of a careful DIYer, but only if you do the preparation properly. Most failed regrouting jobs fail at the same point: the old grout was never raked out deeply enough, or the new grout was laid over a joint that was still dirty and damp.

This guide covers the whole job in order: how to tell whether your grout needs replacing at all, how deep to rake the old grout out, how to mix and apply new grout, how long to let it cure, when to seal it, and why it went mouldy in the first place so it does not happen again.

Does It Need Regrouting or Just Cleaning?

Before you buy anything, work out whether the grout is actually failing or just dirty. The two look similar from across the room, and plenty of bathrooms get regrouted that only needed a good clean.

Start with a clean. Scrub the grout lines with a stiff brush and a bathroom cleaner, or a paste of bicarbonate of soda and water for stubborn marks. If the grout comes back to a reasonably even colour and feels solid under the brush, it was dirt, not damage. A mould remover will shift most black staining on the surface; what it cannot fix is grout that has gone soft, powdery or hollow underneath.

Regrouting is needed when the grout is physically failing. Look for these signs:

• Cracks running along the grout lines, especially around the bath, shower tray and taps, where movement and water are greatest.
• Grout that crumbles or falls out when you run a fingernail or an old toothbrush along the joint.
• Grout that feels soft or spongy, or that you can dent with a screwdriver tip. Sound grout is hard; failed grout gives.
• Tiles that sound hollow when you tap them, or that move slightly when you press them. Water has got behind them through the failed grout.
• Persistent black mould that returns within weeks of cleaning, which usually means the grout is porous and holding moisture deep inside, not just stained on the surface.
• Gaps where grout has washed out entirely, leaving a channel between tiles.

One more honest point, and it is the one most retailer guides skip: you do not need to regrout every joint. If only the shower area and the row above the bath have failed, regrout those joints and leave the sound ones alone. Regrouting a whole bathroom when only a third of it has failed is wasted work, and every joint you disturb is a joint you have to get right again. Work joint by joint, and only rake out what is actually damaged.

There is a limit, though. If large areas of grout are failing, if several tiles are loose, or if the wall behind the tiles is soft or swollen, regrouting will not save the job. That is a re-tiling job, and it needs the wall dried out and made sound first. Our guide to how to tile a bathroom wall covers what a proper tiling job involves, including tanking wet areas so water cannot get behind the tiles again.

Rake Out the Old Grout

This is the step that decides whether the job lasts. New grout needs something to grip, and it needs depth. Laid thinly over the top of old grout, it will crack and fall out within months.

The rule the best small guides agree on is to remove at least two-thirds of the depth of the old grout, and ideally down to three-quarters. For a standard wall tile joint, that usually means raking out to a depth of about 3 to 5mm, leaving a clean channel with the tile edges exposed on both sides. Do not just scratch the surface. If you can still see the old grout at the bottom of the joint, you have not gone deep enough. Equally, do not gouge so deep that you damage the tile edges or the adhesive bed behind them.

You have three tool options, and the right one depends on how much grout you are removing:

• A manual grout rake, which costs about £4 to £8 from Screwfix and other DIY stores, is the cheapest and safest choice for a small area. It is a handle with a carbide blade that you drag along the joint. It is slow, but it gives you control and it will not chip a tile edge if you slip.
• An oscillating multi-tool with a grout removal blade is the fast option for a whole bathroom. A decent one costs around £90, with Bosch’s GOP 30-28 often named as an example. It cuts through grout quickly, but it is also the easiest way to chip a tile or cut into the adhesive, so keep the blade flat in the joint and let the tool do the work rather than forcing it.
• A grout removal attachment on a rotary tool sits between the two on speed and cost, and suits medium areas.

Whichever tool you use, work along the joint, not across it, and keep the blade centred so it does not ride up onto the tile face. Wear safety glasses and a dust mask: old grout dust is fine, sharp and unpleasant to breathe. Vacuum the joints as you go, and when you have finished, brush every joint out with a stiff brush and wipe it down with a damp cloth so no dust or loose crumbs are left. Dust left in the joint stops the new grout bonding, and it is the most common reason a regrout fails early.

One place needs a different treatment, and it matters. Where tiles meet the bath, the shower tray or a worktop, the joint should be flexible sanitary silicone, never grout. Grout is rigid; the bath moves slightly every time it is filled and emptied, and grout in that corner cracks every time. All three of the detailed guides checked for this article agree on this. Rake the old grout out of those corners and replace it with silicone sealant, not grout. The same applies to internal vertical corners where two tiled walls meet.

Before you mix anything, check the wall is dry. If the joints are damp, or the wall behind feels cold and wet, let it dry for at least 24 hours. New grout will not cure properly on a damp joint, and sealing moisture in is how mould starts.

Mix and Apply the New Grout

Choose the right grout before you start, because the choice is not cosmetic. Wall tiles take unsanded grout, which is smooth and suits narrow joints. Floor tiles take sanded grout, which is stronger across wider joints. Ready-mixed grout in a tub is convenient for a small area, but it comes in a limited range of colours; powdered grout, mixed with water yourself, gives a much wider colour choice and is usually the better option for a full bathroom. Two widely stocked powdered grouts in the UK are BAL Micromax2, about £12 for a 2.5kg bag, and Mapei Keracolor. Match the colour to the existing grout if you are only doing part of the room, and remember that grout dries lighter than it looks when wet, so do not panic when the fresh grout looks darker than the old.

Mix only as much as you can apply in one go. Once mixed, grout has a working time of about 20 to 30 minutes, or 30 to 45 minutes according to some guides, before it starts to stiffen. Never add water to grout that has started to set and try to remix it; it will be weak and will crack. Mix a small batch, use it, then mix the next. Follow the manufacturer’s water ratio exactly, and mix to a smooth, thick paste, about the consistency of toothpaste. Let it stand for a few minutes after mixing, then give it a brief final stir before you start.

Apply with a rubber grout float, which costs about £6. Hold the float at roughly 45 degrees to the wall and push the grout firmly into the joints, working diagonally across the tiles so the float does not drag grout back out of the joints you have just filled. Work in sections of about a square metre at a time. Press hard enough that the joints are fully packed with no air pockets; a joint that looks full on the surface but is hollow underneath will crack.

After about 15 to 20 minutes, when the grout has started to firm up but is not yet hard, wipe the tile faces with a damp sponge, rinsing it often. This is the step that separates a tidy job from a smeary one. Use a barely damp sponge and light pressure; a soaking sponge will wash grout out of the joints. Work diagonally, and change the water as soon as it turns cloudy. You will probably need two or three passes, letting the haze dry between them, before the tiles are clean. Leave the finished joints alone after the final wipe and do not be tempted to smooth them with a wet finger, which leaves a weak, shiny skin that cracks.

If you are regrouting a shower, keep the room warm and dry while you work. Cold, damp air slows curing and can leave the grout soft. Open a window slightly for ventilation, but do not point a heater directly at fresh grout; drying it too fast causes shrinkage cracks.

Cure Times

Grout does not dry, it cures, and the difference matters. Drying is water leaving the surface; curing is the chemical reaction that makes grout hard and water-resistant. Rushing it is the second most common reason a regrout fails.

The consensus across the UK guides checked for this article is:

• 24 hours before light use or any water exposure. Do not splash the shower or run the bath against freshly grouted joints within the first day. Victorian Plumbing gives 24 hours for grout to set, which matches the general guidance.
• 48 to 72 hours before regular wet use, and before sealing. A shower used daily needs the full 72 hours before it is back in normal service. If you can, use another bathroom in the meantime; if you cannot, keep the shower off the new joints and wipe any splashes away immediately.

Temperature and humidity change these times. In a cold bathroom in winter, curing takes longer, so add a day rather than cutting one off. In a warm, well-ventilated room, the times above hold. Do not be tempted to speed things up with a hairdryer or a fan heater aimed at the wall; rapid drying causes the surface to shrink and crack while the inside is still soft.

The test is simple: press a fingernail into an inconspicuous joint. Properly cured grout is hard and does not dent. If it still gives, give it another day.

Sealing the Grout

Sealing is optional on a tiled wall that stays mostly dry, and it is close to essential in a shower. Grout is porous by nature, and an unsealed joint in a wet area absorbs water, holds it, and grows mould. A penetrating grout sealer soaks into the grout and closes the pores without changing how it looks.

Wait until the grout is fully cured before sealing, which means 48 to 72 hours after application, not before. Sealing over grout that is still damp traps moisture inside, which is worse than not sealing at all. When the grout is dry, apply the sealer with a small brush or an applicator bottle, working it into the joints and wiping any excess off the tile faces before it dries. Most sealers need two coats, with the first coat dry before the second goes on. Check the product instructions for the exact recoat time.

Sealing is not a one-off job. In a shower, expect to reseal every one to two years, depending on how much the shower is used and how hard your water is. If water stops beading on the grout and starts soaking in, it is time to seal again.

A note on what sealing cannot do: it will not fix grout that was laid badly, and it will not stop mould growing on the surface if the bathroom stays damp. If mould keeps coming back, the cause is ventilation, not the sealer. That is the next section.

Sealant for the silicone joints is a separate product. For the corners where tiles meet the bath or tray, use a mould-resistant sanitary silicone. Everbuild Forever White costs about £8 and UniBond Anti-Mould about £7, and both are widely stocked. Apply it in a smooth bead, tool it with a wet finger or a profiling tool, and leave it to cure for 24 hours before the shower or bath is used.

Why Grout Goes Mouldy

Mouldy grout is usually a symptom, not the disease. Grout goes mouldy because it stays damp, and it stays damp because the bathroom does not dry out between uses. Regrouting without fixing the dampness means you will be regrouting again in a year.

The usual causes, in order of likelihood:

• Poor ventilation. A bathroom with no working extractor fan, or a fan that is never switched on, stays humid for hours after every shower. Warm, damp air condenses on the coolest surfaces, which are the tiles and the grout lines. If your fan is noisy, weak or never runs, that is the first thing to fix; a bathroom should clear its steam within 15 to 20 minutes of a shower.
• No heat. A cold bathroom dries slowly. A heated towel rail or a radiator that actually warms the room makes a real difference to how fast grout dries.
• Constant water contact. Grout lines at the bottom of a shower wall, or around the bath where water pools, stay wet longest and go mouldy first. This is also why the silicone corners matter so much.
• Limescale and soap build-up. Hard water leaves deposits on grout that hold moisture and give mould something to grip. A regular wipe-down after showering helps more than any cleaning product.
• Grout that was never sealed, or sealer that wore off years ago, in a shower that is used daily.

If your bathroom has a broader damp problem, such as condensation on windows or damp patches on walls away from the shower, the cause may be wider than the grout. Our guide to the types of damp in a house explains how to tell condensation, rising damp and penetrating damp apart, because each needs a different fix. And if water is leaking from a tap or fitting and keeping the grout wet, fix the leak first; our guide to how to fix a dripping tap covers the common tap types.

The practical routine that keeps regrouted joints healthy is simple: run the extractor fan during and for 20 minutes after every shower, wipe the shower walls down with a squeegee or towel after use, keep the room heated, and reseal the grout when water stops beading. It takes two minutes a day and it is the difference between grout that lasts a decade and grout that goes black in a year.

When Regrouting Is Not Enough

Regrouting is a repair, not a rebuild, and it has limits. Call in a professional tiler if any of these apply:

• Tiles are loose across a wide area, or the wall behind them is soft, swollen or crumbling. The adhesive bed has failed and the tiles need to come off.
• The wall behind the tiles is wet through, with damp spreading beyond the bathroom or staining the ceiling below. That is a leak or a waterproofing failure, and regrouting will only hide it.
• You are regrouting the same joints every year. Something else is wrong, usually ventilation or a failed tanking membrane behind the shower, and a tiler or damp specialist needs to look at it.

For scale, DIY materials for regrouting a full bathroom cost about £40, according to FixMyHome. A full professional re-tile of the same bathroom typically costs £1,200 to £2,500. That gap is why it is worth doing the regrout properly the first time: a careful job with proper rake-out depth, correct cure times and sealing can last years, while a rushed one can fail within months and push you towards the expensive option anyway.

Frequently Asked Questions

How deep should I rake out old grout before regrouting?

Remove at least two-thirds of the grout depth, ideally three-quarters, which is usually about 3 to 5mm on a standard wall tile joint. If you can still see old grout at the bottom of the joint, you have not gone deep enough, and the new grout will not bond properly.

Can I regrout over old grout without removing it?

No, not if you want it to last. New grout laid thinly over old grout has nothing to grip and will crack and fall out within months. The rake-out is the most important part of the job, and skipping it is the most common reason regrouting fails.

How long should I wait before using the shower after regrouting?

Wait 24 hours before any water touches the new grout, and 48 to 72 hours before regular wet use. In a cold bathroom, allow longer. Do not seal the grout until it is fully cured.

Should I use grout or silicone where tiles meet the bath?

Silicone, always. Grout is rigid and cracks where the bath moves; flexible sanitary silicone absorbs the movement. The same applies to internal corners where two tiled walls meet.

Can I regrout just one section of a bathroom?

Yes. Only regrout the joints that have actually failed, and leave sound grout alone. Match the colour to the existing grout, and remember new grout dries lighter than it looks when wet.

Why does my grout keep going mouldy after I clean it?

Because the grout is staying damp between uses, usually due to poor ventilation. Clean grout in a damp bathroom will go mouldy again within weeks. Fix the ventilation, keep the room warm, wipe the shower down after use, and seal the grout once it is cured.