A wet room is a room designed to get wet. That sounds obvious, and it is exactly the point that gets missed. A conventional bathroom relies on a tray and a screen to contain water. A wet room has no containment: water lands on the floor, runs across it, and leaves through a drain. Everything that keeps that water out of the building is hidden underneath the tiles, and it is either done properly or it is a leak with a delay on it.
If you are planning one in a Central London flat, this is the page to read before you choose a tile. Tanking is not a coat of paint applied at the end. It is a system, it has to be designed into the build-up from the start, and in a flat above another flat the consequences of getting it wrong are not yours alone.
What tanking actually is
Tanking — waterproofing — is a continuous, unbroken membrane forming a tray under the finished surfaces. Tiles and grout are not waterproof. Grout is porous. Tiles are bedded in adhesive that will pass water. Water gets through the finish; that is normal and expected. The membrane underneath is what catches it and sends it to the drain.
Two broad families of system. Liquid-applied membranes are painted or rolled on in coats, with reinforcing tape at every junction. Sheet membranes are bonded down as a physical layer. Both work. Both fail in exactly the same place: not in the middle of a floor, but at the edges, the corners, the pipe penetrations and the drain.
The critical principle is continuity. The membrane must be continuous from the drain, across the floor, up the walls to well above any splash zone, around every pipe that comes through, and behind every fixing. One unsealed screw into the floor, one pipe penetration taped badly, one internal corner that was not reinforced, and you have a route through. Water is patient and it only needs one.
The fall — the thing that cannot be fixed later
Water leaves a wet room because the floor is sloped towards the drain. That slope has to be built in, it has to be consistent, and it has to be right across the whole wetted area — not just near the drain.
Get it wrong and you get ponding: standing water sitting on the tiles after every shower. That is not cosmetic. It is water sitting on the finish for hours instead of minutes, driving through the grout, staining, and eventually finding whatever weakness the membrane has. A wet room that ponds is a wet room that will fail early.
The awkward part in period stock is that the fall has to come from somewhere. Either the substrate is cut or formed to fall, or a sloped former is built in, or the floor level rises. Rising floor levels in a Victorian flat means door thresholds, ceiling heights, and the joinery around them — and if the room has original skirting, architraves or a panelled door, that becomes a conservation question rather than a plumbing one.
Timber floors — the Central London problem
Most of the flats in W1, W2, NW8 and W9 have suspended timber floors. Joists, boards, and a plaster ceiling below that belongs to somebody else. This is genuinely the hardest context for a wet room, and it is worth being honest about why.
Timber moves. Joists deflect when you walk on them. Boards expand and contract. A rigid waterproof membrane over a floor that flexes is a membrane that will eventually crack at the point of most movement — which is usually a corner or a junction, which is exactly where you cannot afford it.
The substrate has to be right. That usually means overboarding with a suitable rigid board, properly fixed, on a deck that is stiff enough. Deflection has to be dealt with before waterproofing, not accommodated afterwards. Sometimes that means additional support to the joists — which, in a flat, may engage structural questions and freeholder consent.
The drain has to go somewhere. A wet room drain needs depth for the trap and fall for the waste run. In a timber floor that means notching or working between joists, and joists are structural. There is a limit to what you can cut and where, and pretending otherwise is how floors get weakened. If the joists run the wrong way relative to the stack, the geometry can genuinely defeat the design.
Everything below is somebody else's ceiling. The failure mode is not a wet floor. It is a stain appearing in the flat below, months later, and an escape of water conversation — which typically engages the building's insurance, arranged by the freeholder or managing agent, and which is exactly the kind of dispute nobody wants. How liability lands depends on your lease and the policy; check both.
None of this makes a wet room impossible on a timber floor. It makes it a properly detailed job rather than a tiling exercise. In some flats, the honest answer after looking at the joists and the drain route is that a low-profile tray with a level-access screen achieves nearly the same look with a fraction of the risk. We would rather say that than build something that will fail quietly.
Concrete floors and basements
A concrete or screeded floor solves the movement problem and is a much better host for a wet room. Newer blocks around Paddington tend to be in this category, and the job is more predictable.
Basements are a different situation entirely, and this is where terminology causes trouble. Tanking a wet room means keeping water in. Tanking a basement means keeping groundwater out — a structural waterproofing problem, often involving cavity drainage systems and sumps. These are not the same discipline and they are not interchangeable.
If you are putting a wet room into a lower-ground or basement flat — common in Little Venice and across the converted stucco stock in Bayswater — you need to know whether the room already has a structural waterproofing system, because you must not compromise it. Fixing into a tanked wall, cutting a drain through a slab that forms part of the waterproofing, or interrupting a cavity drainage membrane can undo a system that was expensive and is now hidden. Establish what is there before anybody drills anything. If the basement has a waterproofing system with a warranty or a designer behind it, involve them.
Where wet rooms actually fail
Rarely the membrane in the middle of the floor. Almost always one of these:
- The drain junction. The membrane must bond to the drain flange properly. This is the single most common failure point.
- Internal corners and the floor-to-wall junction. Movement plus a change of plane. Needs proper reinforcement, not just more liquid.
- Pipe penetrations. Every pipe through the membrane is a hole that has to be sealed with the right collar, not with silicone and hope.
- Fixings through the tanking. A screw for a shower rail, a grab rail, a glass screen. Every one is a penetration. They must be sealed as they go in.
- The threshold. Where the wet room meets the rest of the flat. Water finds level. If the fall does not beat the doorway, water leaves through the door.
- Insufficient height up the walls. Splash goes further than people think, particularly in a small room. Skimping on the vertical extent is a false economy.
- Nobody tested it. A membrane should be inspected and, where the system allows, flood-tested before tiling. This is the step that gets dropped when a programme is tight, and it is the only chance you will ever get to find a fault while it is still cheap.
If you take one thing from this page: insist on seeing the membrane before it is tiled, and photograph it. Once tiles are on, nobody can tell you what is underneath, and the person who finds out is the flat below.
Ventilation, electrics and the regulations
A wet room puts a great deal of moisture into a flat, and moisture that is not extracted ends up in the fabric. Bathrooms engage Part F of the Building Regulations for ventilation and Part G for sanitation and hot water, and mechanical extraction is a normal requirement. In a mansion block the practical problem is where the duct goes — an external wall may not be available, and routing through a shaft or a long duct run needs designing rather than assuming. Where the building is listed, works affecting its special character can require listed building consent, and putting a new vent through an elevation is exactly the kind of thing that can.
Electrical work in dwellings falls under Part P, and installations must comply with BS 7671, with specific zoning requirements around baths and showers. In a wet room the whole floor is effectively a shower zone, which affects what fittings can go where. That is a question for a qualified electrician at design stage, not an afterthought — our electrical page covers how we handle it. Underfloor heating, which is a genuinely good idea in a wet room because it dries the floor between uses, has to sit correctly within the build-up relative to the membrane, and it is both an electrical and a waterproofing detail.
Consent, and asking first
Most leases require freeholder or managing-agent consent for alterations, often through a licence to alter. A wet room commonly triggers several of the usual conditions at once: altering waste and soil connections, changing floor build-up and finishes, sometimes structural work to joists, sometimes new ventilation through an elevation. Many freeholds in this part of London sit with large estates, and their requirements differ — check your lease and ask your managing agent rather than assume.
Two further points worth raising honestly. Some leases restrict hard floor finishes above other flats for noise reasons, and a tiled floor in a wet room is a hard floor. And in listed buildings and conservation areas, the controls above apply. Establish all of this before design, not after. Getting consent late is expensive; getting it refused after you have bought stone is worse. Our guide to shared riser plumbing in mansion blocks covers the drain connection side of the same question.
Sequence, and why it matters
The order is not negotiable. Strip and inspect. Confirm the drain route and the stack connection. Sort out the substrate and any deflection. Form the falls. First-fix the plumbing and electrics. Waterproof continuously. Inspect and test. Then tile, then fit, then seal. Every step where somebody works out of order — a fixing driven after the membrane, a pipe moved after tanking — is a penetration in a finished system.
This is why we treat a wet room as part of a bathroom refurbishment, priced from £2,800, rather than as a tiling job with waterproofing bolted on. The tiling is the visible part and it is priced from £55/m², but the tiling is not what keeps the water out. If a quote for a wet room does not tell you which waterproofing system is being used and how the drain junction and the corners are detailed, it is not a quote for a wet room.
How we approach it
We look at the floor first, not the tiles. What the joists are doing, where the drain can go, what the room below is, and what your lease and your building will allow. If the honest answer is that a level-access tray is the better outcome for your flat, we will say so. If it can be done properly, you get a fixed scope and a fixed price before anything is stripped, and you get to see the membrane before it disappears.
We work across W1, W2, NW8 and W9 — Marylebone, Mayfair and the surrounding areas. Send photos of the room and the floor through the quote form, or reach a plumber in Little Venice or Paddington the same way. If you are dealing with the aftermath of one that failed, our guide to a leak under the bath covers how to find where the water is really coming from.