In short: Tanking is defined as a continuous waterproof membrane applied to walls and floors before tiling, so that water passing through grout and tile joints is stopped by the membrane instead of reaching the structure behind. Tiles and grout are not waterproof. Tanking is what makes a shower waterproof, it is buried permanently once tiling starts, and it is the single most commonly omitted item on a cheap bathroom quote.
What's in this guide 6 sections
01 What is tanking?
Tanking is a continuous waterproof layer — a liquid membrane, a sheet membrane, or waterproof backer boards with sealed joints — applied to the walls and floor of a wet area before any tiling goes on. Its job is to catch the water that gets past the tiles.
That last point surprises people, so it is worth stating plainly: tiles are not a waterproof layer, and grout is not a waterproof layer. Ceramic and porcelain tiles are impervious, but the joints between them are not, and cement-based grout absorbs and passes water steadily. In a shower used twice a day, water is behind the tiles within months. Whether that matters depends entirely on what is behind them.
Tanking is therefore not a finish or an upgrade. It is the part of a bathroom that does the waterproofing, and everything you can see is the decorative layer over the top of it.
02 Where does a bathroom need tanking?
Not every surface in a bathroom needs a membrane. These do.
| Area | Tanking required? | Why |
|---|---|---|
| Inside a shower enclosure | Always | Direct, sustained water contact every day |
| Wet room floor, whole room | Always | No tray — the floor is the waterproof layer |
| Wall behind a bath with a shower over | Always | Same exposure as a shower enclosure |
| Floor immediately outside a shower | Yes, minimum 300mm | Splash and foot-carried water |
| Wall behind a basin | Splash zone only | Intermittent splashing, not sustained |
| Wall behind a WC or a towel rail | No | No meaningful water exposure |
| Timber floor anywhere in a wet room | Always, plus decoupling | Timber moves and rots; tile does not move |
03 The three tanking systems, and when each is used
Most real bathrooms use two of the three. A typical wet room gets waterproof boards on the studwork, a sheet membrane across the floor and up 150mm onto the walls, and liquid membrane over the board joints and around the drain — three products, one continuous system.
The important word in that sentence is continuous. A membrane that stops at the floor-to-wall junction is not a waterproof layer, it is two waterproof layers with a gap in the wettest part of the room.
| System | What it is | Best for |
|---|---|---|
| Liquid membrane | Paint-on or roll-on elastomeric coating, two coats, with reinforcing tape at every joint and corner | Shower enclosures and splash zones over plasterboard or backer board |
| Sheet membrane | Bonded polyethylene sheet with taped seams and pre-formed corners | Wet room floors, and anywhere movement is expected |
| Waterproof boards | Extruded polystyrene boards with a cement facing, joints sealed with tape and sealant | Stud walls, and building out to correct a badly out-of-plumb wall |
04 How high should tanking go?
In a shower enclosure, tanking should run the full height of the tiling and at least 1,800mm from the floor, whichever is higher. In a wet room, the floor membrane should turn up the walls by a minimum of 150mm on every side and lap under the wall membrane, so water running down a wall is carried onto the floor membrane rather than behind it.
Around a bath with a shower over, tank the full tiled height on the bath wall and at least 300mm along the return walls at each end. The overlooked one is the floor: tanking should extend 300mm out from the shower opening, because the water that damages a floor is usually carried out on feet rather than sprayed.
05 Signs a bathroom was never tanked properly
You cannot inspect a membrane once it is tiled over. You can read the symptoms.
- Grout that stays dark in one patch long after the rest has dried — water is sitting behind it.
- A loose or hollow-sounding tile low in the shower. Tap across a wall; a change in note means the adhesive has failed, usually because the substrate is wet.
- Cracked grout along the internal corners, where a flexible silicone joint should have been used instead of rigid grout.
- A damp patch, a musty smell or blown plaster on the other side of a shower wall — that is water that has already crossed the structure.
- Staining on a ceiling below an upstairs shower. By this point the substrate has usually failed, not just the seal.
- Springy floor tiles or a cracked grout line across a timber floor, meaning no decoupling layer was used.
None of these are repairable by regrouting. Regrouting a failed shower is putting a new lid on a leaking box — it buys a few months and hides the evidence while the substrate keeps getting wetter.
06 How to check a quote actually includes tanking
Tanking is invisible in a finished bathroom, so it is the easiest thing in the world to leave out of a job and the hardest for a customer to catch. It is also a genuine cost — materials plus the better part of a day — which is why the gap between a cheap quote and a proper one so often lives here.
- Ask each quote to name the tanking system it includes. "We will tank it" is not an answer; "Sheet membrane to the floor and 150mm up, liquid membrane to the shower walls to 1,800mm" is.
- Ask how high it goes, and how far out from the shower across the floor.
- Ask what happens at the internal corners and around the drain. Pre-formed corners and a drain sealed into the membrane are the marks of someone who has done it properly.
- Ask whether internal corners will be siliconed or grouted. The correct answer is silicone, because those joints move.
- Ask to see it before it is tiled. Any fitter who tanks properly is glad to be asked — it is the part of the job they are proudest of and nobody ever gets to see it.
