Quick answer
Tanking is the waterproof membrane applied to a wet room’s floor and lower walls before tiling. It’s the layer that actually holds water back — tiles and grout don’t. Without full tanking, water soaks through the joints into the structure and you get a leak. It’s the single most important stage of a wet room, and the one you can never see once it’s finished.
What tanking actually is
“Tanking” simply means turning the wet area into a watertight tank. We apply a waterproof membrane — either a paint-on liquid that dries to a flexible skin, or sheet membranes bonded down — across the whole floor and up the lower walls, with extra reinforcement at corners, the floor-to-wall junction and around the drain. Once it’s cured, that membrane is a continuous, sealed barrier. Whatever water gets through the tiles and grout above it hits the membrane and runs to the drain instead of soaking into the floor.
Think of the tiles as the wearing surface and the tanking as the real waterproofing underneath. Grout is porous and tile joints aren’t sealed, so without tanking, water has a clear path into the boards, screed or joists below. That’s why every genuine wet room is tanked — it’s not optional.
The membrane
A liquid or sheet waterproof layer over floor and lower walls — the actual barrier against water.
The reinforcement
Tape and matting at corners and joints, where movement and water pressure are highest.
The drain bond
The membrane is sealed to the drain flange so water can’t sneak around the edge of it.
Why a wet room can’t do without it
In a normal bathroom, a shower tray and enclosure contain the water — the tray is itself a waterproof vessel. A wet room has no tray. The floor is the shower, so the floor has to be the waterproof vessel instead. That’s the job tanking does. Skip it and these are the consequences we get called out to fix:
- Water soaking into floorboards or screed, leading to rot and damp
- Leaks into the ceiling of the room below an upstairs wet room
- Tiles lifting as the substrate swells and moves
- Persistent musty smells and black mould you can’t clean away
None of these show up on day one. They appear months later, by which point the only real fix is to strip it back and tank it properly — far more expensive than doing it right first time. It’s why we treat tanking as the thing that stops a wet room leaking.
The membrane laps up the walls and bonds around the drain, turning the whole wet zone into a sealed, watertight tank before any tile is laid.
How we tank a wet room — and why it takes time
Tanking sits in the middle of the build, after first fix and the floor falls are formed, and before tiling. We prepare a clean, stable, rigid base — reinforcing any timber floor so it can’t flex and crack the membrane. We prime where needed, then apply the membrane in coats, working tape and matting into every corner and around the drain. Then we wait. The membrane has to cure fully before tiling; tile over it too soon and the seal never sets properly. That curing time is built into the 4–7 working day wet room timeline and it isn’t us being slow — it’s the difference between a wet room that lasts twenty years and one that fails in two.
Because tanking is hidden, it’s also where cheaper installs cut corners you can’t see. We don’t, and our wet rooms carry our workmanship guarantee. You can see the whole process and the costs involved on our wet room installation hub and our wet room cost page. If you’re still choosing between a fully tanked wet room and a tray-based walk-in, our wet room vs walk-in shower guide explains how the waterproofing differs.
Common questions
Is tanking the same as waterproofing?
Yes — tanking is the building trade’s word for waterproofing a wet area. It’s the membrane that seals the floor and lower walls so water can’t reach the structure beneath.
Do all wet rooms need tanking?
Every genuine wet room does, because there’s no tray to contain the water — the floor is the shower. Any installer who skips or skimps on tanking is setting the room up to leak.
Can you tank over old tiles or a timber floor?
Not directly. We prepare a clean, rigid, suitable base first — reinforcing timber floors so they can’t flex — then tank onto that. Tanking onto a moving or unsound surface is a common cause of failure.
Done right, hidden well
Get a wet room tanked the proper way
We never cut corners on the part you can’t see. Tell us about your room and we’ll give you a fixed written quote and a guaranteed, watertight finish.