A tiled bathroom looks completely sealed, which is exactly why the waterproofing underneath so often gets skipped — until the substrate underneath has been silently absorbing water for years. Tiles themselves don't stop water; grout lines are porous by nature, and even the best silicone sealant around a bath or shower eventually shrinks, cracks, or lifts. Tiles and grout alone are not a waterproofing system, they're a wearing surface applied over one.

The standard approach — often called 'tanking' — is to apply a liquid-applied membrane directly to the substrate before any tile adhesive goes down. It needs to cover the whole wet-area floor and extend up the adjoining walls, typically at least 150mm above floor level, with particular attention to internal corners and the wall-floor junction, which is where reinforcement tape is usually embedded into the membrane to prevent cracking at that flex point.

Substrate preparation matters as much as the membrane itself. The surface has to be clean, dry, sound, and free of dust or loose material before application, and any low spots or unevenness in the floor should be corrected with a levelling compound first — a membrane applied over a poorly prepared surface won't bond properly regardless of product quality. After application, the membrane needs its full cure time (commonly anywhere from a couple of hours to two days, depending on the product) before tiling starts, and a flood test — filling the shower or wet area with water and checking for leaks over 24 hours — is the only reliable way to confirm the membrane is intact before it disappears under tiles for good.

The cost of doing this properly is small compared with the alternative: water reaching a timber or steel substrate, or saturating plaster and screed, typically isn't discovered until tiles start delaminating or a ceiling below starts staining — at which point the repair means ripping out finished work that was only a few years old.