Water finds the weakest joint in any shower floor, and it finds it fast. Get the wet room tanking wrong and the leak won’t show in the middle of a wall. It shows at a corner, an upstand, or where a pipe pushes through the membrane. This guide walks through the systems and the detailing that keep a tiled wet area watertight long after the tiler has left site.
What the UK standards require
Since the 2018 revision, BS 5385-1 (clause 6.1.1.3) says substrates in wet areas should be protected with a suitable proprietary tanking membrane system before tiling, and that now covers domestic work, not only commercial jobs. BS 5385 and The Tile Association also rule out plasterboard, including moisture-resistant board, for wet areas unless it has been tanked over first.
A few documents sit alongside it:
- BS 5385-4: This is the code of practice for tiling in wet conditions and covers waterproof membranes and underlays used in wet rooms.
- BS EN 14891: This standard sets the performance test criteria for liquid-applied waterproofing used beneath ceramic tiling, including watertightness and crack-bridging.
- Approved Document C: This part of the Building Regulations requires resistance to moisture and protection of the building fabric.
- NHBC guidance: Under Clause 9.2.5, tanking is treated as mandatory for wet rooms and for power showers running at 12 litres per minute or more, using an EAD-certified system.
Who the NHBC rule actually catches
The NHBC requirement is a warranty and insurance condition for registered new-build homes, so the waterproofing has to be certified to EAD 030352-00-0503. For a standard shower under 12 litres per minute, tanking is still required when the board manufacturer calls for it or the substrate has not passed NHBC’s assessment for wet-area backing surfaces. In practice, that means most wet areas end up tanked anyway.
For anyone proving their work met reasonable skill and care, BS 8000 (workmanship) and BS 5385 are the benchmarks a loss adjuster or court reaches for. That alone is reason to specify a certified system and keep the datasheet on file.
Membrane types and where each one earns its place
Three families of product dominate wet room tanking in the UK. Each one suits a different substrate and a different kind of job.
Liquid-applied membranes
Liquid-applied membranes go on by brush or short-pile roller and cure to a continuous film with no joints to seal. They follow awkward shapes, dress neatly into upstands, and wrap around pipe penetrations without fuss, which makes them the common pick for retrofits and bespoke layouts. The catch is thickness. The tanking membrane only works once it reaches the dry thickness the maker specifies, often around 1mm built up in two coats laid at right angles, so the result rides on workmanship.
Sheet and board membranes
Sheet membranes such as butyl or polyethylene systems arrive at a fixed factory thickness, so you know what you get across the whole floor. They cope well with timber subfloors where some movement is likely, and many double as a decoupling layer. The trade-off moves to the joints, because every overlap, corner and penetration has to be detailed and sealed with the system’s own tapes and collars.
Cementitious slurry
Cementitious slurry bonds hard to concrete and blockwork and gives a tough barrier, but it is rigid. On a substrate that might flex or shrink, a flexible grade is the safer choice, since a brittle slurry can crack and let water past.
Mixing systems is normal practice. Plenty of installers run a sheet or liquid membrane across the flat field and use liquid detailing to seal the fiddly corners and pipe exits.
Check the floor before anything goes down
A tiled wet room floor only performs if the structure under it holds firm. On timber, check for deflection and strengthen the joists or deck where a tiled wet area would otherwise move, because a floor that bounces will crack tiles and stress the waterproofing. On concrete or screed, build the fall into the base, not the tiling.
Fall figures to work to:
- Minimum fall: BS 5385-4 sets the range between 1:80 and 1:35, so 1:80 (about 12.5mm per metre) is the shallowest allowed.
- Practical target: A 1:60 fall drains more reliably and sits comfortably inside the standard's range.
- Upper limit: Gradients steeper than 1:35 can be awkward and even unsafe to stand on.
Priming: the step that decides whether the membrane bonds
Priming is not filler before the real work. Absorbent substrates like cement backer boards, screed and render pull moisture out of a liquid membrane too fast, which starves the cure and weakens the bond. A compatible primer evens out that suction so the membrane forms a proper film and sticks.
A few rules keep priming reliable:
- Match the primer: Pair it to both the membrane and the substrate, since a primer meant for a porous board can behave differently on a dense, low-absorption surface.
- Follow the datasheet: Stick to the stated dilution and coverage, because over-thinning on a thirsty board defeats the point.
- Let it cure: Wait for the stated tack or dry state before the membrane goes on, or you trap solvent or water under the waterproofing.
Clean, dry, dust-free and sound comes first. Fill gaps and cracks, and let repairs cure before priming.
Sealing corners and wall-to-floor junctions
Internal corners and the wall-to-floor junction take more movement and more standing water than any flat run, so they get reinforced, not just coated. The method stays consistent across liquid systems:
- Bed the tape: Press a reinforcing tape, scrim or tanking band into the first wet coat at every internal corner and along the floor-to-wall junction.
- Work out the air: Smooth out trapped air and membrane so the tape sits flat with no bridging.
- Overcoat it: Lay the second coat once the first has set, covering the tape fully.
Preformed internal and external corner pieces do the same job at three-way junctions, where tape alone is hard to dress without wrinkles. On sheet systems, the maker’s corner units and band are part of the kit for a reason, because a hand-folded offcut rarely seals as well as the moulded part.
Upstands: carrying the waterproofing up the wall
A tanked floor that stops at skirting height is not tanked. The membrane has to turn up the wall to form an upstand, so splashing and run-off meet waterproofing rather than a bare junction. Around 150mm up the wall at the perimeter is a common baseline for the general floor edge, with the shower zone carried much higher.
For the wet zone, BS 5385-4 sets out the areas that need protecting, and shower walls are routinely tanked well above the rose. Check the standard and the system datasheet for the zone heights on your job rather than guessing, since the right figure shifts with the layout and the shower position. Whatever the height, the upstand has to stay continuous with the floor membrane, and the junction gets reinforced the same way as any other internal corner.
Pipe penetrations and the drain
Every pipe that passes through the membrane is a deliberate hole in your waterproofing, and it is one of the first places a leak starts. Silicone smeared round a pipe is not a seal. The membrane itself has to close down onto the pipe.
- Use a collar: Fit the system's pipe collar or a preformed gasket, bedded into the membrane and dressed tight to the pipe.
- Reinforce liquid systems: Set tape or the maker's collar into the first coat around the exit, then overcoat.
- Check the pipe material: Confirm the pipe type against what the tanking manufacturer approves, since some sealants and collars behave differently on plastic than on metal.
The drain or gully flange deserves the same care, if not more. Guidance on wet room failures points to the drain collar, not the open wall, as the spot that lets go most often. The membrane has to bond to the flange and clamp into it where the design allows, so water running to the fall meets a sealed connection and not a lip it can creep under.
Slip resistance and the finish
Safety does not stop at the waterproofing. BS 5385-4 calls for tiles with a surface profile or treatment suited to wet use, so pick a floor tile with a sensible slip rating for a barefoot, soaking-wet room. A glossy tile that looks sharp dry can turn into a hazard once the shower runs.
Good wet room work depends on getting the details right before the tiles go down. TradeFox helps you practise plumbing and tiling skills through clear, guided simulations you can repeat as needed.
Test before you tile
Where the programme allows, a flood or soak test after the membrane has cured, following the maker’s stated cure time, is the honest way to prove the tanking before tiles hide it. A floor that fails a test is a bad morning. A floor that fails under finished tiles is a strip-out. Build the fall first, then tank, then check, then tile.
Wet room tanking rewards patience at exactly the points the eye skips over. The corners, the upstands and the penetrations carry the risk, and that is where a careful second coat pays for itself.
