The black in the tile lines is alive: why bleach only hides grout mould, and the kill that holds

Grout mould is two separate problems wearing one face: the black you can scrub, and the damp that grows it back in six weeks. Households attack the first forever and win nothing; the guide below is weighted accordingly - half kill, half the boring moisture discipline that makes the kill stick.

Why bleach disappoints, precisely

Bleach on non-porous tile surface looks heroic - it bleaches the pigment of the mould, not always killing the roots inside porous grout, and on a wet, poorly ventilated wall the survivors recolonise within weeks. The chemistry that does better in a bathroom: hydrogen peroxide or a borax paste left to dwell - the two things almost nobody does is leave it long (thirty minutes under a cover, not thirty seconds) and dry the room properly after. For the routine case, make a paste (baking soda plus a little peroxide, or borax with water), brush into lines, dwell, rinse; for stubborn patches a proper mould remover per its own label, ventilated, and never mixed with anything else, ever.

The moisture half, which is the real half

• The squeegee: one pass down walls and floor after the last bath, ten seconds a day, worth more than every product in this page - mould needs standing film water, and you just took it away.
• Ventilation with a habit: extractor fans left running ten minutes past the shower, or the door-open-and-window strategy in houses without fans; a bathroom that stays visibly humid for an hour is a bathroom that grows black lines whatever you scrub.
• Textiles are reservoirs: the plastic curtain and the mats get washed or wiped on a schedule (monthly minimum; mould grows on soap scum on the curtain folds first), because a clean wall behind a mildewed curtain recycles the problem.

When the grout is telling you something worse

Press a fingertip into the grout line: it should be hard as the tile. Grout that crumbles, softens near the shower base, or a tile that sounds hollow when tapped with a knuckle means water behind the finish - a failed seal at the tray corner, a cracked silicone join, weeping behind the board. Re-grouting over an active leak is decorating a damp. Fix order: find the water route (shower-tray perimeter and the vertical inside corner are the usual), re-seal with proper sanitary silicone on a bone-dry joint, let it cure the full day, then renew the grout lines last. And if black staining keeps returning to the same patch beside the ceiling regardless of scrubbing: that is the tell of a slab leak above - and the water-stain chapter is where this desk sends you next.