In short
- The black on shower grout is mold that has settled into the micro-cracks, fed by humidity and a lack of ventilation.
- A tile grout line (cement, between the tiles) cleans up well with sodium percarbonate.
- A silicone sealant bead (around the shower tray or the bath) is more fragile: hydrogen peroxide or baking soda, but no bleach.
- If the silicone is peeling, cracked, or if the black comes back within a few weeks, it has to be redone, and you usually wait 24 hours before showering again.
- To stop it happening again: wipe the grout after showering and ventilate well.
Those little black spots invading the shower grout: we’ve all tried to scrub them with a toothbrush on a Sunday morning, often without much success. Before you get stuck in, ask yourself the right question: can this grout still be cleaned, or is it time to redo it? Here’s how to decide, then how to deal with each case.
Why does shower grout turn black?
The black isn’t ordinary dirt: it’s mold. It settles into the micro-cracks in the grout and thrives there because the bathroom offers everything it likes: humidity, warmth, insufficient ventilation and surfaces that never dry. Products that are too aggressive, which damage the grout, also open the door to it.
This type of mold poses little health risk, but it signals a humidity problem in the room. Cleaning without fixing that problem guarantees you’ll be back at it in a few weeks.
Tile grout or silicone sealant: which one is black?
In a shower there are two kinds of joints, and they aren’t treated the same way:
- Tile grout: the cement-based mortar that fills the lines between the tiles. It is tough and withstands a fairly hard brush.
- Silicone sealant: the flexible bead that makes the seal around the edges, between the shower tray or bath and the wall, or in corners. It is more fragile and doesn’t like aggressive products or hard brushes.
How do you clean blackened tile grout?
Sodium percarbonate, first
It is the most suitable product: it whitens thanks to the active oxygen it releases in hot water.
- Make a paste with percarbonate and hot water.
- Apply it to the grout with an old toothbrush.
- Leave it for 20 to 30 minutes, then scrub and rinse thoroughly.
Wear gloves and be careful with your eyes. To compare it with the other natural products, see our guide to vinegar, baking soda, soda crystals and percarbonate.
Diluted bleach, occasionally
For very dirty tile grout, diluted bleach (1 part to 4 parts water), applied with a sponge or spray and left for 5 to 10 minutes at most, can get rid of the stains. With gloves, with good ventilation, and not too often: it weakens the grout over time. Avoid it on colored grout.
And above all, never mix it with vinegar or any other acid: the mixture releases chlorine.
How do you clean blackened silicone sealant?
Silicone is more delicate. Forget bleach, which risks weakening it, and hard brushes.
- 10-volume hydrogen peroxide, on its own or as a paste with baking soda: apply it to the joint, leave it for 30 minutes, then rub gently and rinse.
- Baking soda: as a paste, left on the joint, without rubbing hard.
Be careful: if you used white vinegar just before, rinse thoroughly before moving on to hydrogen peroxide. The two products mixed form peracetic acid, which irritates the skin and eyes.
When should you redo the silicone sealant?
No product will save a silicone joint in these cases:
- it has come away from the wall or the shower tray, even over a few centimeters;
- it is cracked or crumbles when touched;
- it stays black after a thorough clean: the mold is in the bead itself;
- the black comes back a few weeks after cleaning.
A peeling joint also means water getting behind the shower tray or the tiles with every shower. It’s better not to wait.
How do you redo a shower silicone joint, step by step?
What you need: a cartridge of bathroom silicone sealant (anti-mold, and definitely not an acrylic sealant), a caulking gun, a joint remover or a utility knife, a sealant remover gel, a smoothing tool, masking tape, acetone, gloves and a cloth.
1. Remove the old joint
With a joint remover tool, score the old silicone, then slide the blade underneath to lift it, and remove the whole bead. If you use a utility knife, wear gloves and keep the blade flat, right against the surface, so you don’t scratch the enamel or the tiles.
2. Get rid of the residue
For stubborn leftovers, apply a generous layer of sealant remover (a dissolving gel), leave it for about 30 minutes, then remove the residue with a sponge or a damp cloth.
Be careful if your walls are made of PVC panels, such as Dumawall wall panels: these solvents must not be applied to plastic wall coverings. Check the instructions, and limit yourself to scraping if necessary.
3. Degrease and dry
The new joint only sticks to a clean, dry surface, with no trace of old silicone. Degrease with a cloth soaked in acetone, then dry thoroughly with a dry cloth. Never apply new silicone over old.
4. Prepare the cartridge
Cut the end of the cartridge neck with a utility knife, without damaging the thread. Screw on the nozzle and cut its tip at an angle, to the width of the joint you want. Put the cartridge in the gun and squeeze the trigger 4 or 5 times to bring the product up. If your old cartridge is blocked by dried silicone, a wood screw pushed into the nozzle often clears it.
5. Mask the edges
This step is optional, but it changes everything for a beginner: stick masking tape on both sides of the future joint, on perfectly dry surfaces. You’ll get a straight line with no smears.
6. Lay the bead
Apply the silicone in one go, without stopping, keeping constant pressure on the trigger. Don’t worry if the bead isn’t perfect: smoothing will correct small imperfections.
7. Smooth it straight away
Smooth within 15 minutes of applying it, before the silicone forms a skin. Use a joint smoothing tool in a slow, even movement, or your index finger dipped in soapy water so the silicone doesn’t stick. Remove any excess straight away with a lint-free cloth, then peel off the masking tape very gently.
8. Let it dry
Follow the drying time given on the cartridge. In general, you need to wait 24 hours before exposing the joint to water splashes. This is the time to shower somewhere else.
How much does it cost?
Prices noted at Castorama (France) in October 2026:
- Sanitary silicone sealant (280 ml cartridge): from €10.90 to €15.50.
- Sealant remover (80 ml tube): about €10.50 to €10.90.
- Joint remover tool: from about €4.30 to €10.
- Caulking gun: from €5 to €10 for a manual model.
- Joint smoothing tool: from about €4.30 to €5.50.
So allow €35 to €52 for all the equipment, and much less if you already have the gun. Prices vary from store to store: check them before you buy.
How can you stop the joints turning black again?
- Wipe the joints with an absorbent cloth after showering, and run the squeegee over the screen, as you would to keep a glass shower door free of streaks.
- Ventilate: a mechanical ventilation system that works well is your best ally. Use and maintain it properly.
- Choose suitable products: an anti-mold silicone for flexible joints, and a water-repellent grout for tiles.
Frequently asked questions
Can I apply new silicone over the old joint?
No. Silicone only sticks to a clean, dry surface, with no residue from the old joint. Applied on top, it will quickly peel off, and the mold will stay trapped underneath.
How long should I wait before taking a shower?
The time given on the cartridge, and generally 24 hours before wetting the joint.
My tile grout stays gray despite cleaning: what can I do?
If the grout is sound but dull, several passes of percarbonate may be enough. If it is crumbling or hollowed out, you’ll need to scrape it out and redo it.
This article is part of our guide to cleaning your bathroom and toilet naturally, with the full daily, weekly and monthly routine.
So, more of a toothbrush person or a “redo everything” person? Tell us in the comments how you got rid of your black grout.