What Is Tanking — And Why It Matters for Wet Rooms

Tanking is one of those terms that gets used loosely in the trade and often means different things to different people. A client will sometimes ask whether their bathroom “has been tanked” and it turns out the previous tiler applied one coat of a brush-on membrane to the floor only and called it done. That’s not tanking. Here’s what it actually means and why it matters.

What tanking is

Tanking is the process of applying a continuous, impermeable waterproof barrier to a surface before tiling. The purpose is to prevent water from reaching the substrate — the floor, the walls, the structural elements behind the tiles — even if grout or silicone eventually fails.

In a wet room, this is essential. A wet room has no tray, no containment. Water lands on the tiled floor and runs to the drain. If the waterproofing fails, water gets into the floor build-up, into the joists or concrete, and eventually into the room below or the room around. By the time you see the damage — ceiling staining, damp on a wall — the water has usually been finding its way through for months.

What tanking involves

There are two main approaches:

Liquid applied membranes: A brush or roller-applied waterproof slurry, typically a two-part system (a liquid and a reinforcing fleece tape). Applied in two or more coats. The fleece tape goes into the corners — floor-to-wall and wall-to-wall junctions — before the second coat. These junctions are the highest-risk points for water ingress. The tape bridges the joint and prevents cracking at the corners as the building moves.

Sheet membranes: A polyethylene or similar sheet product bonded to the substrate. More commonly used under floor tiles in wet rooms and shower areas. Products like Schluter Kerdi or similar systems work by bonding a thin membrane to the substrate and then tiling directly onto it. The membrane is waterproof; the tile and adhesive above it are not.

Both methods work when applied correctly. The difference is in the detail — coverage, corner treatment, product selection for the specific application.

What doesn’t count as tanking

I see bathrooms where someone has brushed one coat of a waterproof product onto the wall behind the bath and called it waterproofed. One coat is not enough — most products need minimum two coats to achieve the waterproofing rating on the data sheet. One coat of a membrane that requires two coats is not a waterproofed wall.

I also see bathrooms where the floor has been tanked but the walls haven’t, or the walls have been tanked but not to sufficient height. In a shower area, the walls need tanking to the full height of the shower spray — minimum 1800mm, typically 2000mm or to the ceiling. Stopping at 1200mm because “it’s only the lower wall that gets wet” misunderstands how shower spray works.

The corners — where floor meets wall, where wall meets wall — are where failures concentrate. Any tanking system that doesn’t specifically address these junctions with a flexible detail (tape, membrane strip, or flexible compound) is incomplete.

The substrate matters

Tanking doesn’t fix a bad substrate. If the walls are damp before you start — rising damp, penetrating damp through an external wall — tanking the inside of the wall is trapping moisture. This causes problems. Tanking is a waterproofing layer, not a damp-proofing system.

On timber floors, tanking needs to be compatible with the movement of the substrate. A rigid tank on a flexing timber floor will crack at the stress points. This is why wet rooms on timber floors need a decoupling element as part of the build-up — the tanking system needs to accommodate movement, not resist it.

When tanking is required

Any wet room: Non-negotiable. The entire shower floor area, the entire shower wall area to full height, and the floor-to-wall corners.

Shower areas with a tray: The walls behind and around the tray need tanking. The tray itself is waterproof; the walls it sits against are not unless treated. Water finds the gap between tray and tiles over time — that’s the silicone joint — and when it does, you want a waterproof wall behind it, not a bare plasterboard.

Bathrooms with a known damp history or vulnerable substrate: If there’s been a leak before, if the room is at risk of condensation issues, or if the wall construction is timber-framed rather than masonry, additional waterproofing protection is worth the extra time and cost.

The cost

Good tanking materials aren’t expensive relative to the overall job cost. A proper two-coat liquid membrane system with fleece tape for a standard shower area runs to roughly £80–150 in materials. The labour is a day or part of a day depending on the size of the area.

What costs more is doing the job twice. A bathroom that’s fully stripped, re-waterproofed, and re-tiled because the first tanking failed costs three to four times what a properly tanked installation would have cost. I’ve been called to these re-dos. It’s not a pleasant situation for anyone.

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If you’re planning a wet room and you’re getting quotes, ask each tiler specifically what tanking system they use, how many coats they apply, and how they treat the corners. The answers will tell you a lot about the quality of work you’re likely to get.

Related reading: Wet room vs shower tray — which is right for your bathroom? · Wet room installation service