Wet room knowledge — waterproofing
In a wet room the floor is the shower, so the one thing standing between the water and the structure of your home is the waterproofing underneath — the tanking. It’s the part you never see and the part that matters most. Get it right and you’ll never think about it again; skimp on it and the first sign of trouble is usually a stain on the ceiling below. This guide explains what tanking is, why tiles and grout can’t do the job alone, the systems we use across Plymouth, and how a floor is properly waterproofed step by step.
What tanking actually is
Tanking is a continuous waterproof membrane applied across the whole wet zone — the floor and the lower walls — before a single tile is laid. The name gives it away: you are effectively turning the shower area into a shallow tank that holds water in and channels it to the drain, rather than letting it soak away into timber, plaster and joists. When people say a wet room has been “tanked”, they mean this hidden layer has been built as one unbroken skin, lapped up the walls, sealed around every pipe and bonded into the drain so there is no gap for water to sneak through.
It helps to picture the build-up in layers. At the bottom is the structural floor — a concrete slab or a timber deck. Over that sits the graded build-up that gives the fall toward the drain, which we cover in our guide to floor formers and gradient. On top of the fall goes the tanking membrane. And only then, bonded to the membrane, come the tiles and grout you actually see. Each layer has a job, but the membrane is the one doing the waterproofing.
Why tiles and grout are not waterproof
This is the single biggest misunderstanding we meet on the doorstep in Plymouth. Homeowners assume that once a floor is tiled and grouted it must be watertight — it looks solid, it feels solid. It isn’t. Porcelain tiles shed most water, but grout is porous, and every joint is a slow route for moisture to pass through. Silicone in the corners perishes, and movement in the building opens hairline cracks. Over months and years, water works its way through and reaches whatever is underneath.
The honest way to think about it: the tiles are the raincoat and the tank is your skin. A raincoat keeps most of the weather off, but you wouldn’t stand in a downpour trusting it to keep you bone dry for twenty years. The membrane beneath is what actually keeps water out, catching anything that gets past the tiles and guiding it back to the drain. Leave the tank out, or do it badly, and the tiles are simply decorating a floor that is quietly getting wet inside.
Liquid, sheet and board tanking systems
There isn’t one single “right” way to tank a wet room — there are a few proven systems, and the skill is choosing the one that suits your floor, drain and room shape. Whatever we use, it is always a complete, manufacturer-approved system rather than a mix of odds and ends, because that is what keeps both the watertightness and the product warranty intact.
Liquid (paint-on) membrane
A flexible waterproof coating that is rolled and brushed onto the substrate in two or more coats, usually a bold colour so we can see complete coverage. Reinforcing tape and matting are bedded into the wet coating at every internal corner, every change of plane and around every pipe. Liquid systems are brilliant for awkward shapes, tight en-suites and anywhere the detailing is fiddly, because the membrane simply follows whatever it is painted over.
Sheet & board systems
Pre-formed waterproof boards, and thin sheet membranes, that are bonded down and sealed at the joints with matching tape and sealant. They give a fast, consistent, factory-quality tank and a tough surface to tile onto, and they pair naturally with a pre-formed shower former that already carries the fall. On many Plymouth jobs we combine the two — a board tank across the room with liquid membrane worked into the tricky junctions.
Both approaches share the same non-negotiables: reinforcement at every corner and junction, correct overlaps, and a proper bond into the drain flange. A tank is only ever as good as its weakest joint, which is why the detailing matters far more than the brand name on the tub.
The details that make a tank watertight
- Reinforcing tape at every corner. The floor-to-wall junction is the exact spot that flexes and cracks, so it gets taped and sealed into the membrane, never left to sealant alone.
- Pipe and penetration collars. Every pipe passing through the tank gets a bonded collar or cuff so water can’t track down the outside of it.
- The drain flange bonded in. The membrane is lapped onto and sealed to the drain’s bonding flange, so the tank and the drain become one continuous waterproof path.
- Height up the walls. The tank is carried well up the walls in the shower zone — comfortably above splash height — and across the lower walls elsewhere, so nothing is left exposed.
How a wet room is tanked, step by step
Here is the process we follow on a typical Plymouth wet room once the falls are formed. None of it is glamorous, and that’s rather the point — good tanking is patient, methodical work that happens before the pretty tiling ever starts.
1. Prepare and prime the substrate
The floor and walls are made sound, clean and dry, any dust is removed, and an appropriate primer is applied so the membrane bonds properly. A tank will only ever be as reliable as the surface it grips onto, so nothing is rushed here.
2. Reinforce corners and junctions
Reinforcing tape and jointing collars are worked into every internal corner, wall-to-floor junction and pipe penetration. These are the high-movement, high-risk points, so they are detailed first and detailed carefully.
3. Bond in the drain flange
The membrane is lapped onto the drain’s bonding flange and sealed, so the tank and the drain form one continuous waterproof route. This single detail is where a huge number of cheap wet rooms quietly fail.
4. Apply the membrane across the zone
The waterproof coating or board tank is applied across the whole floor and up the walls, following the fall so water is always guided toward the drain. Liquid systems get the full number of coats; board systems get every joint sealed.
5. Cure and water-test
The tank is left to cure for the time the manufacturer specifies — it cannot be hurried. On suitable jobs we then plug the drain and flood-test the floor, checking the level holds before a single tile goes down.
6. Tile onto the tank
Only now do the tiles go on, bonded directly to the cured membrane with flexible, wet-rated adhesive and grout. The waterproofing is complete before the finish begins, exactly as it should be.
Sequence matters as much as skill. The falls are formed first, the tank goes over the falls, and the tiles go over the tank — never the other way round. Our main wet room installation page walks through survey to handover, and the drainage guide explains how the drain choice shapes the falls the tank has to follow.
Where cheap tanking jobs fail
We get called out to put right other people’s wet rooms often enough to know exactly where the shortcuts happen. The failures almost always trace back to the same handful of corners cut — and every one of them is invisible until water finds it.
Sealant instead of a membrane
A smear of silicone in the corners is treated as if it were waterproofing. It isn’t a tank — it’s a bead that shrinks and perishes, and water walks straight past it into the corner below.
No reinforcing tape
The floor-to-wall junction is left un-taped, so the very spot that flexes most has nothing holding the membrane together. The first hairline crack becomes the first leak.
Drain not bonded in
The membrane stops short of the drain instead of being lapped onto its flange. Water reaching the tank simply runs off the edge and disappears under the floor.
Membrane stopped too low
The tank is carried only a few centimetres up the wall, so splash and run-off get behind the tiling above it. Waterproofing has to go higher than people expect.
The reason these failures are so costly is that the repair is never small. Because the leak sits under the tiles and the floor build-up, putting it right usually means lifting the entire floor and starting the tank again. That is precisely why we treat the waterproofing as the heart of the job and back it with a written workmanship guarantee. Homeowners in Plympton, Plymstock and Peverell who have lived through a botched wet room tell us the same thing: they wish they’d paid for the tank, not the tiles.
Curing, testing and other considerations
Give it time to cure
Every tanking system needs curing time before it can be tiled — anything from a few hours to overnight depending on the product and the temperature. It genuinely cannot be rushed without risking the bond, and it is the main reason a wet room takes a little longer than dropping in a shower tray. We plan the schedule around it, which is one of the quiet advantages of a fixed written quote over a day rate that tempts a fitter to hurry.
Water-testing before tiling
On suitable floors we flood-test the tank once it has cured — plugging the drain, filling the shower zone to a shallow depth and checking the level holds — so any weakness is found while it is still easy to fix. Finding a fault before the tiles go on is a minor adjustment; finding it afterwards is a full re-build.
Ventilation protects the whole room
Tanking keeps liquid water out of the structure, but a wet room also produces a lot of steam, and warm damp air will find any cold surface to condense on. That is why Building Regulations Part F calls for adequate ventilation, and why we fit a properly rated, often humidity-sensing extractor. Good waterproofing and good ventilation are two halves of the same job.
It works with the whole build-up
The tank never works alone. It sits on a correctly formed fall, ties into the right drain, and is tiled with a slip-resistant, flexible finish, so we treat the floor as a single system rather than separate trades. Costs for the whole job are set out on our Plymouth bathroom cost guide.
The waterproofing also sits alongside the electrical work — lighting, the extractor, any underfloor heating — which must be certified to Part P, as covered in our overview of wet room building regulations. Doing it to standard is not the expensive option; it is simply the correct one.
Tanking & waterproofing FAQs
What does “tanking” a wet room actually mean?
Tanking means applying a continuous waterproof membrane across the whole wet zone — the floor and lower walls — before any tiles are laid. It effectively turns the shower area into a shallow tank that channels water to the drain, protecting the timber, plaster and joists underneath. The tiles go on top of this membrane, not instead of it.
Aren’t tiles and grout waterproof on their own?
No. Porcelain tiles shed most water, but grout is porous and every joint lets moisture through slowly, while silicone in the corners perishes and cracks. The tiles act like a raincoat while the tank beneath acts like your skin — the membrane is what genuinely keeps water out of the structure, catching anything that gets past the tiles and returning it to the drain.
Which is better, liquid or sheet tanking?
Neither is universally better — they suit different situations. Liquid paint-on membranes are ideal for awkward shapes and fiddly detailing. Sheet and board systems give a fast, consistent tank and pair well with a pre-formed shower former, and on many jobs we combine the two. What matters far more than the type is that it is a complete, manufacturer-approved system, reinforced at every corner and bonded into the drain.
How far up the walls does the tanking go?
In the shower zone the membrane is carried well up the wall, comfortably above splash height, and the lower walls around the rest of the room are tanked too. Stopping the membrane too low is one of the most common reasons cheap wet rooms leak, because splash and run-off get behind the tiling above the tank.
Why does a wet room leak, and can it be fixed?
Leaks nearly always come from poor tanking — sealant used instead of a membrane, no reinforcing tape in the corners, the drain not bonded in, or the membrane stopped too low. It can be fixed, but because the fault sits under the tiles and floor build-up the repair usually means lifting the whole floor and re-tanking. That is exactly why the waterproofing is worth paying to get right first time, and why ours carries a written guarantee.
How long does the tanking need to cure?
It depends on the system and the temperature, but curing can take anywhere from a few hours to overnight. It cannot be rushed without risking the bond and the watertightness, and it is the main reason a wet room takes a little longer to fit than a standard shower enclosure. We build the curing time into the schedule rather than working against it.
Does the tanking meet building regulations?
We waterproof to recognised, manufacturer-approved standards and follow the system’s own detailing throughout. Alongside the tanking, ventilation must meet Building Regulations Part F and the electrical work — extractor, lighting, underfloor heating — must be certified to Part P. We handle all of it as standard so the finished wet room is both watertight and compliant.
Where tanking fits in the bigger picture
Waterproofing is one part of a properly built wet room, but it’s the part everything else depends on. If you’re planning a project, read it alongside the guides on forming the floor falls and choosing between linear and point drainage, since the tank has to follow the fall and bond into the drain. The wet room hub ties the whole process together, while our full bathroom installation and tiling and flooring pages cover the alternatives. Whichever route you take, the principle holds: pay for the parts you can’t see, because they’re the parts you can’t easily fix.
Proud of every bathroom we fit
Get your wet room tanked properly
If you want a wet room that stays bone dry for decades, it starts with the waterproofing. Tell us about your room and we’ll visit, check the floor and the waste run, and give you one clear written quote — fully tanked, expertly graded, finished properly. No surprises, no pressure.