Two Types of Bathroom Joints
Grout lines are narrow gaps between tiles on flat walls or floors. They are filled with tile grout, which can be cement-based or epoxy-based.
Corner joints are where tiles change planes or meet another material: vertical wall corners, wall-to-floor junctions, and edges around tubs or shower trays. These are filled with flexible silicone sealant rather than grout.
This distinction is essential. Walls and floors shift slightly due to changes in temperature and humidity. Rigid grout in a corner cannot withstand this movement, cracks, and lets water in. Flexible silicone expands and contracts with the structure, keeping the corner sealed. That is why tiling industry standards require sealing corners and transitions with silicone sealant.
Throughout the rest of this article, "grout" refers to the material between tiles, and "silicone" refers to the sealant in corners.
Why Mold Appears on Joints
Glazed tiles barely absorb water, but cement grout is porous. It absorbs moisture after every shower and releases it slowly. In a warm bathroom where humidity stays high, mold easily grows inside the grout pores. That is why tiles can stay clean while the joints between them turn dark.
Silicone does not absorb water, but over time mold grows into its body and under its edges, especially if the sealant is old, pulling away from the tile, or was poorly applied in the first place.
Identify Where the Mold Is
Your next step depends entirely on where the mold sits.
On grout:
- Dark spots scrub off with a brush, leaving clean grout underneath. This requires cleaning.
- Grout stays dark after several cleanings, crumbles, or flakes out. This requires grout replacement.
On silicone:
- Mold sits on the surface and washes off easily. This requires cleaning.
- Black spots are visible inside the sealant and will not scrub off. This requires replacing the silicone.
Cracks in corner grout. This means the corner was filled with grout instead of silicone. The crack lets water through, so this corner needs to be redone with silicone sealant.
Mold returns quickly and spreads from one area. Moisture might be coming from under the tile rather than from above. More on this below.
How to Clean Grout
Wear gloves and safety glasses. For heavy mold infestation, add an N95 or FFP2 respirator. Open a window or turn on the exhaust fan.
- Wash tiles and grout with warm water, standard detergent, and a stiff brush to remove surface buildup mechanically.
- If dark spots remain, apply a specialized anti-mold spray designed for grout and masonry surfaces. Follow the packaging instructions strictly for dilution and dwell time.
- Rinse thoroughly with water, wipe the joints dry, and let the bathroom dry completely. Avoid using the shower for several hours.
- Once cement grout is completely dry, apply a grout sealer. The sealer closes the pores so water stops soaking into the grout and beads on the surface. Epoxy grout does not need a sealer because it is already virtually non-porous.
Use a stiff brush, but avoid wire or metal brushes. Metal scratches the glaze and digs grout out of the joints, creating grooves where dirt and mold stick even faster.
Can You Use Bleach?
Chlorine bleach works well on smooth tiles. On porous cement grout, it lightens the stains, but a lighter joint does not mean the mold is gone from deep inside. Bleach is not a substitute for mechanical scrubbing and applying a grout sealer.
If you decide to use bleach, follow CDC recommendations:
- Dilute it to no more than one cup of household bleach per gallon of water (roughly a 1:16 ratio).
- Apply it with a sponge or cloth instead of spraying it.
- Work with an open window, wearing gloves, goggles, and a respirator.
- Test colored grout on a hidden spot first, as bleach can cause discoloration.
The most important safety rule: never mix bleach with ammonia or acidic cleaners, such as toilet bowl or limescale removers. Mixing them releases toxic gases. If you cleaned the joints with another product beforehand, rinse it away completely with water before applying bleach.
Silicone in Corners: Clean or Replace?
If mold sits on the surface of the silicone, wash it off with a mild detergent and a soft brush. If black spots are visible deep inside the sealant, no cleaner can reach them because the mold has grown into the material itself.
In this case, replace the silicone. Cut out the old sealant completely, clean and degrease the surface, let the corner dry thoroughly, and apply fresh sanitary silicone containing fungicides. If you do not dry the corner completely before replacing it, mold will return beneath the new sealant.
When Cleaning Will Not Help
Certain signs indicate that the issue goes deeper than grout or silicone, meaning water is trapped behind the tiles:
- Mold returns within a few weeks after thorough cleaning and drying.
- Tiles sound hollow when tapped or feel loose.
- The bathroom smells musty even when dry.
- Dark patches spread outward from the shower area or drain.
- Dampness or spots appear on the opposite side of the wall or on the downstairs neighbor's ceiling.
In these cases, water is leaking through the joints and beneath the tile. Cleaning the surface is useless while the backing remains wet. You need to locate where the wet area barrier failed: waterproofing layers, corner joints, or slopes toward the drain. We explain how a reliable wet area is built in our article on bathroom waterproofing in Batumi.
Replacing Grout
Replace grout if it crumbles, flakes out, or stays dark after several cleanings, provided the tiles are firmly attached and show no signs of moisture underneath. Scrape the old grout out of the joints to a sufficient depth, clear away dust, dry the area completely, and apply new grout.
In showers and other high-water zones, consider using epoxy grout. It absorbs practically no water, making it much harder for mold to take root. Epoxy grout is trickier to work with because it sets quickly, so homeowners often hire a professional for application. Keep in mind that corners and transitions still require silicone sealant, as epoxy grout is rigid and will crack in corners just like cement grout.
You can replace only the darkened sections, but fresh grout almost always differs in shade from the old one. In visible areas, replacing it across the entire wall or shower zone looks much cleaner.
One important note: if moisture is trapped behind the tiles, new grout will not stop it. It will only hide the problem temporarily. Fix the root cause before replacing the grout.
How to Keep Mold from Returning
Grout and silicone stay clean only if the bathroom dries quickly. An effective exhaust fan, airing out the room after showers, wiping down the shower area, and leaving a gap under the door for airflow do more to prevent mold than any chemical cleaner. Inspect corner silicone once a year: if it cracks or peels away from the tile, replace it promptly. Read more about moisture, ventilation and protecting wet areas in Batumi's humid climate in our article on bathroom waterproofing.
Frequently Asked Questions
Can I cover up dark grout with a grout pen?
Only after thorough cleaning and complete drying. Applying a marker over active mold does not eliminate it, and spots will bleed back through.
Why did mold return a month after cleaning?
Either spores remained deep in the grout pores, the bathroom lacks proper drying, or moisture is seeping from behind the tiles. If you did not seal cement grout after cleaning, it quickly absorbs water again.
Do I need to replace all the grout or just the dark patches?
Technically, you can replace specific sections, but new grout usually varies in color from the old material. In visible zones, replacing the grout across the entire wall or shower area gives a far better result.
Can I fill corner joints with epoxy grout instead of silicone?
No. Corners and transitions shift over time. Epoxy grout is rigid and will crack, so corners must be sealed with silicone sealant.
Planning a bathroom renovation in Batumi?
Clean, light grout lines for years start during the renovation: a dry, level substrate, waterproofing in the wet area, a slope to the drain, epoxy grout where there is a lot of water, and silicone in the corners. Tile-Maker in Batumi handles tiling from start to finish, from substrate preparation and waterproofing to tile setting, grouting and sealing the corners. We document the hidden stages with photo and video updates on WhatsApp and guarantee our work. Send photos of the space and we'll discuss the scope and a cost estimate.