Wet Rooms: Waterproofing, Drainage Falls and What to Get Right

A wet room — an open shower area without a tray or enclosure, where the entire floor drains — is increasingly popular in UK bathrooms. Done well, it looks clean, works for people with mobility issues, and is easier to clean than a shower tray. Done poorly, it leaks into the floor structure, causes rot and mould, and becomes an expensive remediation project. The difference between the two comes down almost entirely to preparation, not aesthetics.

Tanking and Waterproofing

The entire wet room floor and walls (to at least 1.8m high in the shower zone) must be waterproofed before tiling. This is called tanking. The most reliable systems use a liquid applied membrane (such as BAL Tandaseal, Schlüter Kerdi, or Mapei Mapegum) applied in two coats with fabric tape reinforcing all corners, joints and pipe penetrations.

Sheet membrane systems (such as Schlüter Kerdi board) are bonded directly to the substrate and tiled over — they're thinner and faster to install, but joints must be properly sealed and the substrate must be flat and sound. Some fitters mix systems inappropriately — always use a complete system from one manufacturer and follow its technical data sheet exactly.

Timber floor structures require particular care. Timber flexes, and membrane systems that cannot accommodate movement will crack at stress points. A decoupling membrane (Ditra, Mapedecoupling) over the tanking layer is recommended for timber-framed floors.

Drainage Falls

The floor must slope (fall) towards the drain — a minimum of 1:80 (12.5mm per metre) is needed for reliable drainage; 1:60 is better in large wet rooms. Achieving this fall in a new build or extension is straightforward. Retrofitting it into an existing bathroom requires either a raised wet room former (which introduces a level change) or cutting back the screed or structural floor to create the fall — significant work that is often underestimated in cost.

Linear channel drains (set along one wall) are easier to fall towards than central point drains, because only one gradient is needed rather than four. They also collect hair and debris more accessibly.

Tile Selection and Slip Rating

Wet room floors must use tiles with an appropriate slip resistance rating. Look for R10 or R11 rating for domestic wet room floors (R10 is the minimum for barefoot wet areas per EN 16165). Large format tiles are visually appealing but harder to achieve the correct fall with — the fewer grout lines, the harder it is to drain small areas around the edges. Mosaic or small-format tiles are more drainage-friendly.

Costs