Regrouting rarely stops a bathroom leak because grout is not the waterproof layer. Water passes through cement grout slowly, and in a well-built bathroom it is stopped by a membrane under the tiles. If that membrane is missing, broken or never turned up the shower walls, water keeps reaching the slab and the flat below, however new the grout looks.
What actually keeps water in a bathroom
A properly built wet area has these layers, from the slab up:
- The concrete slab, with a screed laid to fall to the drain.
- A waterproof membrane, usually cementitious or PU, across the whole floor and up the walls in the shower.
- Reinforcing tape at the corners, the floor-wall joint and around the drain.
- Tile adhesive, tiles and grout on top.
Grout and tiles are the finish. They shed most water, but the membrane is what stops the water that gets past them. When the neighbour below reports a stain, the question is always whether that membrane is doing its job.
Why the leak comes back
- The membrane was never there. In many older bathrooms, tiles were laid straight onto the screed, and the grout was the only barrier.
- The membrane is broken at the drain. The joint between the membrane and the floor drain is the most common failure point. Regrouting the surface does nothing for it.
- The shower walls were not tanked. Water runs down the wall tiles, behind them, and down into the floor-wall joint.
- It was never a waterproofing leak. A cracked supply or drain pipe in the wall or floor leaks whatever you do to the tiles.
How to tell what is leaking
- Leaks all the time, even when no one showers: likely a pipe. A pressure test confirms it.
- Leaks mainly after showers: likely the membrane, the drain joint or the shower walls. A flood test on the floor confirms it.
- Stain spreads from one corner or under the shower: often the floor-wall joint or the drain.
Non-destructive leak detection answers this before any tiles come up, so you only open what needs fixing.
The repair that lasts
When the membrane has failed, the lasting fix is to rebuild the wet area:
- Remove the tiles and adhesive down to the screed.
- Repair the screed and correct the fall to the drain.
- Fit reinforcing tape at corners, the floor-wall joint and the drain.
- Apply a two-coat membrane across the floor and up the shower walls.
- Flood-test the floor before any tile goes back.
- Retile with a suitable grout.
It costs more than a regrout once, but a regrout repeated every few months, plus the neighbour's ceiling repairs, costs more over time. See bathroom waterproofing for what is included and typical prices.
If you have already regrouted
Do not regrout again. Book a test first. If the pipes pass and the flood test fails, you know it is the membrane, and the quote covers exactly that. Send photos of the bathroom and the stain below on WhatsApp, and we will tell you which test comes first.