A bathroom leaking into the flat below is almost always one of two things: a leaking pipe, or a failed waterproofing membrane under the tiles. The fix for each is completely different, so the first job is always to find out which one it is — before anyone lifts a tile.
Pipe leak or membrane leak? The quick test
Ask the neighbour below one question: does it leak all the time, or mainly after showers?
- All the time, even overnight: most likely a pressurised supply pipe in the wall or floor. Your water meter may move with every tap off.
- Mainly after showers or baths: most likely the membrane under the shower or floor, or the seal around the floor drain.
A professional confirms it with a pipe pressure test (isolating hot and cold lines to see if they hold pressure) and moisture mapping across the bathroom floor and walls. Our leak detection service does both before any quote.
Why regrouting and silicone rarely fix it
Tiles and grout are not the waterproof layer — they are the finish. In a properly built bathroom, a continuous membrane sits under the tiles, turned up the walls and sealed into the floor drain. Water that gets through the grout runs on top of that membrane to the drain.
If the membrane is missing, cracked at a corner, or not sealed into the drain flange, water goes straight into the slab. New grout on top changes nothing underneath.
The most common failure points
- The floor drain. The membrane must lap into a clamped drain flange. Without it, shower water runs under the tiles.
- Wall-to-floor corners. Joints that move crack first; they need reinforcing tape or fabric.
- Shower niches and screens. Every fixing drilled through the tile can pierce the membrane.
- No membrane at all. More common than you would think in older buildings.
Who is responsible?
As a general rule, the owner of the unit where the leak starts is responsible for the repair, and in a rental, tenancy contracts usually place major maintenance with the landlord. Shared risers and common pipes are generally the building's responsibility. Every contract and building is different, so check yours — and use a written leak-detection report as evidence when you raise it with your landlord, neighbour or building management.
The proper fix
- Confirm the source — pipe, drain seal or membrane.
- Lift the tiles in the wet area if the membrane has failed.
- Reinforce corners and the drain and apply a two-coat membrane wall-to-floor.
- Flood-test for 24–48 hours before tiling, with the ceiling below checked.
- Retile with a suitable grout, and keep the photos of each layer.
That is exactly how our bathroom waterproofing works. If the source turns out to be a pipe, you only pay for the pipe repair — not a new bathroom.