Wet room waterproofing explained (tanking, falls and where leaks start)

Why tiles are not the waterproof layer

Porcelain does not let water through, but the joints between porcelain do. Cement grout is porous by design, and every internal corner moves slightly. Over a few years, a shower with no membrane behind it will soak the wall, rot the timber and appear as a stain on the ceiling below — usually blamed on the shower valve.

The tanking system is the actual waterproofing. Everything you can see is a decorative, hard-wearing covering over it.

What a full tanking system includes

A compliant installation is a kit, not a bucket of paint:

• Primer suited to the substrate (different products for board, plaster and screed).
• Two coats of liquid membrane, or a bonded polythene sheet membrane.
• Reinforcing tape or collars at every internal corner and wall/floor junction.
• Pipe and valve collars where anything penetrates the wall.
• A drain flange bonded directly into the membrane, not sealed around afterwards.

Getting the falls right

Water has to arrive at the drain. In a formed wet-room floor we build a fall of roughly 1:60 across the showering area — enough to drain fast, gentle enough that a large tile still sits flat and a shower screen still seals. Bigger tiles need the fall built into the screed and the tile cut into facets around a centre drain, which is why linear drains at one edge have become the default for large-format floors.

A wet room that pools water is almost never a drain problem. It is a falls problem, and it cannot be fixed with sealant.

The five places leaks actually start

Internal corners grouted instead of siliconed. Pipe penetrations sealed after tiling instead of collared before. The junction where the tray or formed floor meets the wall. Behind a recessed niche with no membrane returned into it. And the movement joint that was never installed where a tiled wall meets a different material.

Every one of those is decided before tiling starts. That is why the invisible day of the job is the one that matters.

How long before you can use it

The membrane needs its full cure time before tiling — usually 12–24 hours depending on product and temperature. After tiling, grout the following day and silicone last. The shower can be used 24 hours after the silicone goes in. Rushing any of those cures is how a good specification becomes a callback.