Shower cutaway showing tiles, adhesive, a blue tanking membrane and substrate, with arrows marking the fall towards a linear drain.

How to waterproof a wet room before tiling (UK guide)

November 11, 2026

The single most important thing to understand about a wet room: tiles and grout are not waterproof. Water passes through grout lines and sits behind the tiles — so the layer that actually protects the building is the tanking membrane applied before tiling. Skip it or do it badly and water gets into the floor and walls, rotting timber and soaking into rooms below. Get it right and the room is watertight for the life of the tiles.

The golden rule: waterproof first, tile second. The tanking is the real barrier; the tiles are the finish. Every joint, corner and penetration in that barrier must be sealed.

Step 1: Get the substrate and falls right

Start with a sound, rigid, suitable substrate — tile backer boards are commonly used because they're stable and water-resistant. In the shower area the floor needs a fall to the drain so water runs away rather than pooling; this is formed before waterproofing, using a former or a screed to the gully. A floor that doesn't drain will always give trouble, however good the tanking.

Step 2: Treat the joints, corners and penetrations

Before the main waterproofing goes on, the vulnerable lines are reinforced: internal corners, wall-to-floor junctions, board joints, and around the drain and any pipe penetrations. These are sealed with waterproofing tape or jointing bands bedded into the membrane, so the barrier stays continuous exactly where movement and water concentrate.

Step 3: Apply the tanking membrane

A liquid tanking membrane is then applied over the whole wet area — the full floor and the walls in and around the shower zone — in coats, building a continuous, seamless waterproof layer that ties into the reinforced joints. A liquid-applied barrier such as Waterproof Blue Liquid DPM forms exactly this kind of jointless membrane across floor and walls. Carry it up the walls to the required height (full height in the shower), and make sure there are no pinholes or missed areas.

Step 4: Let it cure, then tile

Allow the membrane to cure fully before tiling over it. Tile with a suitable waterproof adhesive and, once set, finish the joints with the correct grout and sealant — but remember the tanking beneath is doing the waterproofing work, not the grout.

Common mistakes

Relying on tiles and grout to keep water out. They don't — the tanking does.
Skipping the reinforcing tape at corners and the drain. These are the first places to leak.
No fall to the drain. Ponding water finds every weakness.
Tiling before the membrane has cured. Follow the stated cure time.

Frequently asked questions

Do I need to tank the whole bathroom or just the shower?

At minimum the shower/wet zone and the floor must be tanked. In a true wet room, the floor and the wetted walls are all waterproofed; many installers tank the full floor and extend up all walls for safety.