A wet room is only as good as the waterproofing beneath it. Without proper tanking, water migrates through tile grout and adhesive over months and years, saturating the floor structure, causing timber rot or concrete carbonation, and eventually damaging ceilings below. Getting the tanking system right at installation is far cheaper than opening up a failed wet room later.
What Is Wet Room Tanking?
Tanking is the application of a continuous waterproof membrane to all floor and wall surfaces in a wet room (or shower enclosure without a tray), before any adhesive or tiling is applied. It creates a sealed “tank” that contains all water within the finished surface zone, directing it only to the drain.
Unlike a shower tray — which has a factory-formed waterproof base — a wet room relies entirely on site-applied waterproofing. This demands higher skill and attention to detail, particularly at corners, penetrations (drain flanges, service pipes), and junctions between floor and wall.
Tanking Systems: Types and Costs
| System | Description | Material cost per m² |
|---|---|---|
| Liquid applied membrane (LAM) | Brushed or roller-applied cementitious or polymer slurry | £15–£40 |
| Sheet membrane (polyethylene/CPE) | Bonded sheet with preformed corner pieces | £20–£50 |
| Composite board system | Foam boards with integral membrane (e.g. Wedi, Schluter Kerdi) | £40–£90 |
| Shower tray liner (CPE) | Loose-lay liner used with sand:cement screed | £10–£25 |
Most UK wet rooms use a liquid applied membrane over a sand:cement screed or directly onto blockwork/plasterboard, because it is flexible, easy to detail around penetrations, and widely understood by tilers and bathroom fitters.
Total tanking material cost for a typical 3 m² wet room floor plus 1.8 m wall height on three sides (approx. 11 m² total area) runs £165–£440 in liquid membrane, not including screed, primer, or tape for joints and corners.
The Build-Up: Step by Step
1. Structural Assessment
Before any wet room work begins, confirm the floor can carry the dead load. A sand:cement screed at 65 mm depth weighs approximately 130 kg/m². On timber joists, a structural engineer should confirm adequacy; many older UK properties require joist sistering or a concrete board overlay before a screed is viable. Rigid foam board systems (Wedi, Schluter Kerdi-Board) weigh 4–6 kg/m² and are usually suitable for timber floors without structural changes.
2. Falls and Drainage
The floor must fall to the drain at a minimum gradient of 1:80 (12.5 mm per metre), with 1:60 preferred for reliable drainage. This is achieved in the screed layer. A pre-formed shower drain former (linear or point drain) sets the outlet level; the screed is then floated to drain consistently to it.
Popular drain types and supply costs:
| Drain type | Supply cost | Notes |
|---|---|---|
| Point drain (chrome) | £30–£120 | Simplest; floor tiles cut to diamond pattern |
| Linear drain (stainless) | £80–£350 | Edge or centre; long tiles can run parallel |
| Channel drain with tile insert | £100–£300 | Invisible when tiled |
The drain waste must be trapped (minimum 75 mm water seal) and connect to the existing waste run, typically 40 mm pipework to a soil stack. This is plumbing work under Part H of the Building Regulations.
3. Applying the Tanking Membrane
For a liquid applied system on a concrete or sand:cement substrate:
- Prime the substrate with the membrane manufacturer’s primer and allow to cure (typically 1–4 hours).
- Apply reinforcing tape (fabric scrim) into the first membrane coat at all internal corners and around the drain flange.
- Apply first coat of membrane at 1 mm wet film thickness; allow to cure (usually 2–4 hours at 20 °C).
- Apply second coat perpendicular to the first at 1 mm; total minimum DFT (dry film thickness) 1 mm — check manufacturer’s datasheets.
- Flood-test for 24 hours (fill with water to 25 mm depth) before tiling.
The flood test is critical and should not be skipped. It proves the system before expensive tiles go on top. Document the test with photographs showing water level at start and end.
4. Adhesive, Tiles, and Grout
Use only a flexible, water-resistant tile adhesive (EN 12004 Class C2/S1 or S2) in wet areas. Standard class C1 adhesive is not waterproof and will fail over time.
Floor tiles in a wet room must have a slip resistance rating of R10 or better (EN 16165 pendulum test equivalent); R11 for domestic wet rooms with elderly or less mobile users. Mosaic tiles and small-format tiles tend to have higher effective slip resistance due to more grout joints.
All grout in the wet zone should be epoxy or moisture-resistant cement grout. The junction between floor and wall tiles must be finished in silicone, not grout, to allow for differential movement.
Full Wet Room Cost Breakdown
| Item | Typical cost range |
|---|---|
| Structural preparation (if required) | £300–£1,200 |
| Drainage supply and installation | £200–£600 |
| Sand:cement screed with falls | £20–£40 per m² |
| Tanking system (materials + labour) | £35–£90 per m² |
| Wall tiling (materials + labour) | £60–£180 per m² |
| Floor tiling (materials + labour) | £70–£200 per m² |
| Shower valve, head, and rail | £150–£600 |
| Shower screen (optional) | £150–£500 |
| Total for 3–4 m² wet room | £3,000–£8,000+ |
Building Regulations
Wet rooms in domestic properties in England and Wales fall under:
- Part H (drainage): the waste pipe and trap must meet the requirements.
- Part P (electrics): all electrical work within 600 mm horizontally of any shower outlet must be Part P compliant. Zone 1 (inside the shower area) permits only SELV (Safety Extra Low Voltage) fittings; zone 2 extends 600 mm beyond. A bathroom extractor fan connected to the circuit also requires Part P sign-off.
- Part L (energy): if the wet room is a new extension, the roof and wall U-values must meet the current Part L2A standards. Wet rooms in existing bathrooms are not notifiable unless the floor area exceeds limits or structural work is involved.
Wet room conversions that involve removing a load-bearing wall to create an accessible bathroom may also require Building Regs Part A (structure) sign-off.
When to Call a Professional
Wet room tanking is one of those tasks where DIY errors are almost invisible until significant water damage has already occurred. At a minimum, you need:
- A plasterer or floor screeder with experience in drainage falls.
- A tiler experienced in wet room detailing (not all tilers are).
- A plumber for the waste connection.
- A Part P electrician for any electrical work within the wet zone.
Attempting to save money by doing a single layer of membrane, skipping the flood test, or using standard (non-flexible) adhesive is likely to cost far more in remediation within five years.
