Why grout goes black, and how to stop it coming back

By Darwin Tile & Grout · Updated 2026-10-17 · Darwin NT

What grout is, and why it stains

Most grout is a cement-based mortar. Under a microscope it is full of tiny pores and channels, which is why a drop of water on unsealed grout darkens and disappears. Anything liquid that reaches the grout goes the same way: mop water, spilt coffee, cooking grease carried on bare feet, soap and body oils in the shower. The tile glaze shrugs it off; the grout drinks it.

The lines also sit a little below the tile surface. On a floor that means every mop pushes a thin film of dirty water across the tiles and into the lines, where it sits and dries. On a shower wall it means water runs down the tile and collects in the horizontal grout lines and in the corners, which is why the bottom rows go black first.

Mould or dirt? A one-minute test

The treatment is different, so it is worth knowing which you have.

1. Dry the grout lineWipe a short length of grout with a dry cloth so you are looking at the grout, not a film of water.
2. Dab on hydrogen peroxideUse a cotton bud with 3% hydrogen peroxide from the chemist. Keep it off natural stone.
3. Watch for fizzingFine bubbles within a minute mean organic growth: mould or mildew. No reaction means dirt, grease or staining, which needs a degreaser and scrubbing rather than a mould treatment.

Shower grout: mould

Mould and mildew need moisture, warmth and food, and a Darwin shower supplies all three for months at a stretch. Soap residue and skin oils settle into the grout, the room is warm, and in the build-up and wet season a recess without a strong exhaust fan never fully dries. Mould roots into the pores, so a surface wipe lightens it for a week and then it is back. The fix is a treatment that kills and lifts it, a scrub of every line, and a rinse that takes the dead growth out. Step by step, with what to use and what to avoid, is in our guide to getting mould out of shower grout.

Floor grout: dirt, grease and the mop

Living-area and kitchen grout rarely goes black from mould. It goes brown, then grey-black, from years of mopping. The mop lifts dirt off the tile and deposits it in the grout, the water evaporates, and the dirt stays. Near the cooktop, airborne cooking grease settles on the floor and acts as glue for the next lot of dirt. The darkest grout in a house is nearly always along the kitchen bench line and in the walkway between kitchen and living room.

Mopping harder doesn’t help; it just moves more dirty water into the lines. What works is a cleaner that breaks down grease, time for it to work, scrubbing along each line, and then extracting the dirty solution rather than leaving it to dry back in. That last step is what separates a professional clean from a hired machine and a bucket.

Other things that darken grout

• Sealer that has worn off. Grout that was sealed when the house was built stays light for years, then seems to go dark quickly. The sealer has gone and the grout has started absorbing again. The water-drop test in our sealing guide shows whether that is the case.
• Wrong cleaners. Oil-based “shine” floor cleaners and some polishes leave a sticky film that holds dirt. Acidic cleaners roughen cement grout, which then holds more dirt than before.
• Efflorescence. White, powdery deposits rather than black: salts carried to the surface by water moving through the grout. Different cause, different fix.
• Dyed or dark grout fading unevenly. Bleach lightens coloured grout in patches, which reads as “dirty” even when it is clean.

Will it come back to the original colour?

Usually most of the way, not always all of the way. Surface dirt and mould lift. Staining that has worked through the full depth of the grout over years can leave a grey shadow after even a thorough clean. We tell you which to expect before starting, and where the shadow would bother you, colour sealing gives one even colour and seals the grout in the same visit.

Keeping grout light in Darwin

• Seal it after a clean. Sealed grout can’t absorb what it can’t absorb. It is the single biggest difference.
• Mop with less water, and clean water. Wring the mop out, change the bucket often, and dry the floor with a towel or fan afterwards.
• Use a neutral cleaner. No oil-based shine products, no acid, no undiluted bleach.
• Ventilate bathrooms. Exhaust fan on during and after showers, door open, squeegee the walls.
• Deal with the kitchen bench line. A quick degrease of that strip every few weeks stops the worst build-up.