Tanking is the quiet hero of a properly-built wet room. It's the layer you never see, the one that doesn't appear in photos, the one nobody talks about — and the one that decides whether your wet room is still working in 10 years or quietly leaking through the ceiling below. If a wet room install costs more than a walk-in shower, this is largely why.
Why Waterproofing Matters
A standard bathroom contains splash, the occasional puddle, and condensation. A wet room contains a primary shower zone with no tray — water lands directly on the floor, runs along the floor, and drains through a single channel. That changes the engineering problem completely.
If even a small percentage of that water finds its way under the floor finish — through a missed silicone joint, a poorly-grouted tile, a pipe penetration that wasn't sealed — it goes somewhere. In a ground-floor bungalow, it sits on the slab and rots the floor structure. In a first-floor flat or upstairs bathroom, it ends up in the ceiling below. Tanking exists to prevent the "somewhere" from happening.
Wait — Isn't Tile Waterproof?
This is one of the most common misconceptions. Glazed ceramic tile and porcelain are waterproof on their own face. The grout between them isn't — it's a porous cement-based mortar. Add silicone in corners and around pipe penetrations, and that's also not a long-term seal — silicone fails after 3–7 years and shrinks back from the joint.
Wet wall panels are slightly better — they're impermeable across the panel face — but the joints between panels still rely on silicone and trim. In a wet room where water hits the floor surface every day, joint reliance alone is not enough.
Tanking is the layer underneath that handles every joint, corner and penetration as a single continuous waterproof skin.
How Tanking Actually Works
The standard process for a wet room:
- Prepare the substrate. Remove all old tile, plaster, plywood. Make good — the tanking layer needs a sound, dry, dust-free base.
- Install the shower-former panel and floor grade. A pre-formed cement-fibre panel built into the floor structure, grading the wet zone toward the linear drain at 1:50 to 1:80.
- Prime. Apply a primer compatible with the membrane system to the substrate and former panel.
- Apply the membrane. Either a brush-and-roller liquid system applied in two coats with a reinforcement scrim in corners and joints, or a sheet membrane bonded with adhesive. Both lap onto the floor, up the walls (minimum 1.5 m on shower walls, 100 mm elsewhere), and around every pipe penetration with bonded collars.
- Cure and test. Membrane needs to cure properly before any finish is applied. We flood-test the floor (block the drain, fill the wet zone with water for 24 hours) before tile or wet wall goes down.
- Apply the visible finish. Tile or wet wall goes on top of the cured, tested membrane.
Liquid vs Sheet — Which We Use and Why
Two main systems on the market:
- Liquid membrane systems (e.g. Schlüter Kerdi-Coll, Mapei Mapelastic, Bostik Tanking Kit) — applied with a brush and roller in two coats. Conforms easily to corners, pipe penetrations and complex shapes. The mainstream choice for residential wet rooms.
- Sheet membrane systems (e.g. Schlüter Kerdi sheets) — bonded sheets that overlap with sealed joints. Faster on simple flat areas, more expensive on complex layouts. Often used in commercial wet rooms.
For most Cumbernauld wet rooms — typically a 2.0 × 2.0 m or smaller wet zone in a domestic bathroom — we use a liquid system. The corners, pipe penetrations and floor-to-wall transitions need a continuous bond, which a liquid membrane handles cleanly. Sheet systems work brilliantly on big flat areas but the joints add risk in a small irregular space.
Shower-Former Panels and Grading
The other half of the engineering problem is grading the floor. A wet-room floor needs to fall toward the drain at a consistent gradient — typically 1:50 to 1:80 — across the wet zone. On a suspended timber floor, this almost always means installing a pre-formed shower-former panel: a cement-fibre board with the grade pre-cut into it, screwed down to the joists.
Trying to grade a timber substrate by hand on-site is technically possible but rarely gets a clean result. The pre-formed panels (Wedi, Schlüter, Bushboard) save time and produce a guaranteed grade.
On a concrete-slab ground floor, we sometimes pour a sloped screed instead — common in Carbrain and Greenfaulds bungalows. Either approach works.
What Happens When It's Done Badly
The classic failure modes — most of which we see when we strip out a previous installation that's started leaking:
- No tanking at all, with the installer relying on tile and grout. Lasts 12–24 months. Then a slow brown stain appears on the ceiling below.
- Tanking that stops short of the wall. The floor membrane should lap up the wall by at least 100 mm; in shower walls, by 1.5 m. We've seen jobs where the membrane stops at floor level — the joint between floor and wall becomes the leak path.
- Pipe penetrations sealed with silicone alone. The waste pipe through the floor needs a bonded collar (a moulded membrane piece that wraps the pipe and is sealed into the surrounding membrane). Silicone alone fails.
- No flood test. If the membrane wasn't tested with standing water before tile went down, you don't know whether it works until the bathroom is in use. By then it's too late to fix without strip-out.
A Walk-In Shower vs a Wet Room — What's Tanked, What Isn't
One of the reasons a walk-in shower is cheaper than a wet room is that the tanking requirement is much smaller. The shower tray itself is the main waterproof layer; we only need to tank the wall area around the tray (typically 1.5 m up the back and side walls of the enclosure) and dress the joints between tray, wall and screen with the right sealants.
A wet room tanks the full floor and the entire shower zone walls. Different scope, different cost. We cover the choice in detail in our wet room vs walk-in shower guide.
Related reading: wet room vs walk-in shower, Scottish Building Regulations, wet wall vs tiles, and our wet room and walk-in shower service.
Call us on 01236 801802 to book a free home visit. We'll talk you through the tanking spec for your specific room — and you'll see exactly what's going under the visible finish before you commit.