Quick answer
Yes — you must waterproof the wet zones of a bathroom before tiling. Tiles and grout are water-resistant, not waterproof; moisture passes through grout joints over time. A waterproof membrane (tanking) behind and beneath the tiles is what actually keeps water out of your walls and floor. Skip it and you’re storing up a leak.
The thing most people get wrong about tiles
There’s a widespread belief that tiles are waterproof. They’re not — and neither is grout. Glazed tiles shed water from their face, but the grout joints between them are mildly porous, and over years of daily showering water works its way through. If there’s nothing behind the tiles to stop it, that moisture soaks into the plaster, the boards and eventually the timber, where it rots, breeds mould and stains the ceiling below. By the time you see a damp patch, the damage has usually been going on quietly for a long while.
Waterproofing — what the trade calls tanking — is the layer that catches that moisture. It’s a continuous membrane (liquid-applied or matting) bonded to the wall and floor of the wet zone before any tile goes up, with extra attention at the corners, the floor-to-wall junction and around the drain. The tiles then become the hard-wearing, good-looking surface on top of a wall that’s already waterproof underneath.
Where waterproofing is essential
Not every wall in a bathroom needs full tanking, but the wet zones absolutely do. We tank:
- The whole inside of a shower enclosure — walls and tray junction
- The entire floor and lower walls of a wet room
- Around and behind a bath that doubles as a shower
- Any area on timber or stud walls in the splash zone
In a wet room it’s the single most important stage of the whole job — the floor and lower walls become one continuous waterproof tray before tiling. It’s why a wet room’s timeline is driven by waterproofing rather than fittings. Get it right and a wet room lasts for decades; cut it short and it leaks.
Tanking is the invisible layer that does the real work — the tiles are just the surface that sits over it.
Why it’s worth the extra day
Tanking adds time and a modest cost to a job — the membrane has to be applied properly and given time to cure before tiling can start, which is part of why we never rush a wet-zone fit. But set against the alternative, it’s some of the cheapest insurance you’ll ever buy. A failed shower wall can mean stripping out new tiles, drying out and replacing rotten timber, and repairing the ceiling in the room below — far more than the tanking would ever have cost. We treat it as non-negotiable on every wet zone, and it’s a stage we’ll happily show you as it’s done.
It also ties into the building standards a bathroom should meet. Proper waterproofing is part of fitting a bathroom that won’t cause damp problems down the line — see our notes on building regulations for a bathroom and how to stop a wet room leaking.
Common questions
Aren’t tiles and grout waterproof?
No — they’re water-resistant. Glazed tile faces shed water, but grout joints are porous and let moisture through over time. Without a membrane behind them, that water reaches the wall structure.
Does the whole bathroom need tanking?
No, only the wet zones — inside the shower, around a shower-bath, and the floor and lower walls of a wet room. Dry walls away from splashes don’t need full tanking, though we always use the correct moisture-tolerant boards.
Can you tank over old tiles?
It’s far better to strip back to a sound surface and tank that, so the membrane bonds to a known-good background. See can you tile over tiles.
Waterproofing is part of every tiling and wet-room job we do — see our tiling & flooring service or the FAQs.
Done right, behind the tiles
Tiling that won’t let you down
We tank every wet zone properly before a single tile goes up — so your bathroom stays dry where it counts. Get a fixed quote that includes it as standard.