How a wet room is waterproofed

The common misunderstanding about a wet room is that the tiles keep the water in. They do not. Tiles are a wearing surface, grout is porous, and water passes through both steadily. The waterproofing is a separate layer underneath, and if it is wrong the room will leak regardless of how good the tiling looks.

Tanking is the actual waterproof layer

Tanking means applying a continuous waterproof membrane across the floor and up the walls before any tile goes on. It comes either as a liquid painted on in two coats, or as sheets bonded down, and both work when installed properly.

The word that matters is continuous. A membrane with a gap is not a membrane. The usual failures are at the edges: where floor meets wall, at internal corners, and around a pipe or drain penetration. Those junctions need reinforcing tape or preformed corner pieces bedded into the membrane, not just extra liquid brushed into the angle.

Get the falls right before waterproofing

A wet room floor must slope towards the drain across its whole showering area, typically around one in fifty. That slope has to be built into the floor structure or a prefabricated former, before tanking.

Water that pools instead of draining will find the weakest point in time, and standing water on a tiled floor is also the thing most likely to be noticed as a defect later. This is not a step that can be corrected with grout or sealant afterwards.

Take the membrane higher than you expect

Water does not only fall downwards in a shower. It runs down walls, splashes off a body and travels along the floor further than people assume.

Tank the whole showering wall to full height, not just to shoulder height, and carry the floor membrane up every wall by at least 150mm as an upstand so the floor is effectively a shallow tray. In a small room, tanking the entire floor is a sensible precaution rather than an extravagance.

Think about what is below

On a timber floor, movement is the enemy of any rigid waterproof layer. The deck needs to be stiff enough not to flex underfoot, which usually means additional noggins or an overboard, before tanking starts.

On an upper floor the consequence of failure is a ceiling below, so it is worth being more careful than the room itself seems to justify.

Where this stops being a home job

Forming the falls, cutting into a floor for a drain, and guaranteeing a continuous membrane across a timber upper floor are the three parts that most often go wrong, and all three are hidden once tiled. A leak then means taking the room apart.