What is tanking, and do I need it for a wet room?

Tanking is applying a continuous waterproof layer to a wall or floor before the tiles go on — and it is what separates a wet room that works from one that quietly destroys the structure around it.

The misconception it corrects. People assume tiles and grout are waterproof. They are not. Tiles are largely impermeable but grout is porous, and so is the adhesive bed beneath. Water passes through the joints steadily. In a conventional shower enclosure, the amount is small and the substrate dries between uses. In a wet room — where water hits the walls and floor across a much larger area, repeatedly — it does not.

What tanking involves:

A liquid membrane painted or rollered on in two or more coats, or a sheet membrane bonded in place.

Reinforcing tape or matting at every internal corner, junction and change of plane — corners are where failures occur, because that is where movement concentrates.

Sealing collars around pipe penetrations and the drain outlet.

Continuity onto the floor, lapping wall membrane over floor membrane so water always runs onto rather than behind.

The floor also needs a fall toward the drain, formed either with a pre-formed sloped tray or by building the screed to a gradient. Water must not pond, and it must not run out of the room.

Where tanking is genuinely required: a full wet room, without question; the inside of a shower enclosure, strongly recommended and standard practice; and any tiled area exposed to regular direct water. Steam rooms have their own requirements.

Substrate matters as much as the membrane. Standard plasterboard is a poor base for a wet area. Tile backer board — cement- or foam-cored — is stable, does not degrade if wetted, and is what most specifications call for.

Where wet rooms go wrong: no tanking at all; tanking that stops short of the full wet zone; a failed junction at the drain; insufficient fall; and movement in a timber floor, which cracks grout and lets water through faster.