Why grout turns black
Standard cement grout is porous — it is full of tiny holes. Every time you mop, dirty water is drawn into those holes. Over months and years, dirt, grease and cleaning residue build up inside the grout, not just on top. That's why the lines turn grey, then dark brown or black.
The grout in a typical home also sits slightly lower than the tile surface, so mops and brooms push dirt into the groove.
Why scrubbing doesn't last
- Scrubbing only cleans the very top; the stain inside the grout soon wicks back up.
- Bleach can lighten grout temporarily but weakens it and can discolour it unevenly.
- Grout pens and paint wear off with mopping.
The permanent fix: marble gum grouting
Marble gum grouting removes the dirty top layer of grout and fills the joints with a resin-based marble gum that is coloured to match your tiles. Unlike cement grout, marble gum is non-porous, so dirty water can't soak in. It is then ground perfectly flush with the tiles, so there's no groove for dirt to collect in.
The whole floor is polished at the same time, so you end up with clean, near-invisible joints and glossy tiles. Marble gum grouting with polishing is $4 per sq ft.
Before and after
