Boarding & waterproofing
Here’s a fact that surprises most homeowners: tiles and grout are not waterproof. Water passes straight through grout lines and around the edges of tiles, and the only thing stopping it reaching the wall behind is the boarding and waterproofing underneath — the stage nobody sees and everybody should ask about. This guide explains what proper boarding and tanking involves in a full bathroom installation, why it’s the difference between a bathroom that lasts twenty years and one that rots in two, and how we get it right on every wet area we build.
Why tiles alone don’t keep water out
It’s the single biggest misunderstanding about bathrooms. People assume a tiled wall is a waterproof wall — but it isn’t. Grout is porous; it soaks up water. Silicone perishes over time. Tiny gaps open around fittings. Over months and years, water finds every one of those routes and soaks into whatever’s behind the tiles. If that’s ordinary plasterboard, it swells, crumbles and eventually fails, and the tiles come away with it. Worse, the water can keep going — into timber studs, floor joists and the room below.
That’s why the real waterproofing in a bathroom happens behind the tiles, in two layers. First, the right board — a moisture-resistant or dedicated tile-backer board that won’t rot even if it gets damp. Second, in the wettest areas, a tanking membrane — a continuous waterproof layer applied over the board before tiling, forming a sealed tray that keeps water off the structure completely. Tiles then go on top as the hard-wearing, wipeable surface — but the boarding and tanking are what actually keep the water where it belongs.
This stage sits between the first fix and the tiling in the installation process. It’s quiet, unglamorous work with nothing to photograph — which is precisely why corner-cutting fitters skimp on it, and precisely why we never do.
The right board for the job
Boarding out means lining the walls and floor with a stable, suitable surface to tile onto. The choice of board matters, especially in wet areas — ordinary plasterboard has no place behind a shower. Here’s what we use and why.
Tile-backer board
The gold standard for wet areas. A rigid, waterproof board (often foam- or cement-cored) that won’t absorb water, won’t rot and gives a rock-solid base for tiles. It’s the right choice behind and around showers and baths, and in wet rooms it’s used throughout. More expensive than plasterboard, and worth every penny where water lives.
Moisture-resistant plasterboard
For the drier walls of a bathroom, moisture-resistant board (the green-faced type) is a sound, cost-effective choice. It handles the general humidity of a bathroom well. But it’s not a substitute for tile-backer plus tanking in the splash zones — used in the right places, it’s part of a sensible, properly specified job.
Floors & overboarding
Timber floors often need overboarding with a suitable tile-backer or the right ply to give a stable, deflection-free base — because a floor that flexes cracks tiles and grout. We check the subfloor at strip-out and board it correctly so what goes on top stays sound. On a solid concrete floor the priorities differ, but the goal is the same: a firm, level base the finish can rely on.
Sound existing walls
Where existing walls are solid, flat and suitable, we prepare them properly rather than needlessly ripping everything out. Good practice is matching the surface to the job — solid where it needs to be solid, waterproofed where it needs to be waterproofed. It saves you money where it can be saved, without ever compromising the wet-area protection where it can’t.
How tanking actually works
Tanking is the process of applying a continuous waterproof membrane over the boarding in the wet areas, so water simply cannot reach the structure behind. It’s the same principle whether it’s a shower enclosure or a full wet room — the difference is just how much of the room is treated. Here’s what a proper tanking job involves.
1. Prepare the surface
The boarding is fixed, clean, dry and sound. Any gaps and joints are made good, because the membrane needs a stable surface to bond to. Skipping preparation is where tanking fails before it’s even started.
2. Seal the weak points
Corners, floor-to-wall junctions and around pipe penetrations are the places water attacks first, so they’re reinforced with waterproof tape and sealing collars before the main membrane goes on. The joints, not the flat areas, are where tanking lives or dies.
3. Apply the membrane
A liquid tanking membrane is applied in coats over the wet area, or a sheet membrane is bonded down, forming one continuous waterproof skin. It’s taken up the walls to a sensible height and across the floor in a wet room, creating a sealed tray.
4. Let it cure
The membrane needs to cure fully before tiling. This is one of the reasons the middle of a bathroom job can’t be rushed — tile onto a membrane that isn’t ready and you compromise the very layer you’ve paid for.
5. Tile over the top
Only once the tanking is sound does tiling begin, bonded to the waterproofed surface. The tiles are the visible, wipeable finish; the tanking underneath is the real defence.
6. Seal the finish
At the end, movement joints are siliconed as the final surface seal. Between the tanking below and the sealant above, water has nowhere to get into the structure — which is exactly the point.
The reason we spell all this out is that tanking is impossible to inspect once the tiles are on — so you’re trusting your fitter to have done it, and done it thoroughly, at every joint and penetration. It’s the stage where a quick job and a proper one look identical on completion and only diverge a year or two later, when one bathroom is still perfect and the other is quietly failing behind the tiles. We’d rather take the extra time and put our name on the layer nobody will ever see, because that’s the layer that decides everything.
Where waterproofing goes wrong
We’re called out to put right water-damaged bathrooms more often than any other single fault, and the causes are almost always the same handful of shortcuts. Every one of them was cheaper to do properly the first time.
Tiling straight onto plasterboard
The shortcut: wet-area tiles bonded to ordinary plasterboard with no backer and no tanking. The result: water soaks the board, it fails, and the tiles come off — often within a year or two.
Tanking the flat but not the joints
The shortcut: membrane rolled over the open wall but the corners and pipe penetrations left unsealed. The result: water gets in exactly where the membrane wasn’t taped — at the weak points that needed it most.
Relying on silicone alone
The shortcut: no tanking, just a bead of silicone at the edges doing all the work. The result: the day the silicone perishes — and it always does eventually — water is straight into the wall with nothing behind it.
Ignoring a flexing floor
The shortcut: tiling a bouncy timber floor without stabilising it. The result: cracked tiles and grout, which then let water through to the joists below. Waterproofing starts with a floor that doesn’t move.
Getting this right is invisible on the day and priceless over the years. It’s the clearest example of what “finished properly” actually means — and it’s all built into your fixed price, not an optional extra. For how it fits the wider budget, see the cost of a bathroom in Plymouth.
Special cases that need extra care
Standard boarding and tanking keep a family bathroom sound for decades, but some situations demand more — and getting them right is exactly where experience shows. These are the jobs where we’re most glad we never treat waterproofing as a formality.
Wet rooms and open showers
A wet room has no tray and often no screen to contain the water, so the entire floor and the lower walls become a wet area that has to be tanked as one continuous, sealed surface, falling to a drain. There’s no margin for a weak joint — if it leaks, it leaks into the floor. That’s why a wet room installation puts even more weight on the waterproofing stage than a standard bathroom, and why it’s not a job to hand to someone learning on the day.
Upstairs and timber floors
Waterproofing an upstairs bathroom carries a bigger consequence, because any water that gets past it ends up in the ceiling of the room below. Timber floors also flex, and movement cracks tiles and grout, opening routes for water. So upstairs we pay particular attention to stabilising the floor, overboarding it correctly and tanking wet areas thoroughly — the cost of getting it wrong is a damaged ceiling and a much bigger repair.
Older Plymouth homes bring their own quirks — the 1930s and mid-century stock around Peverell, Mannamead and Plymstock often has walls and floors that aren’t quite true, or that have been patched over the years. We assess all of this at strip-out and prepare each surface for what it actually is, rather than assuming. It’s slower than a blanket approach, but it’s the only way to be sure the waterproofing has a sound base to work from.
Boarding & waterproofing FAQs
Do I really need tanking, or is tiling enough?
In the wet areas — around showers and baths, and throughout a wet room — you genuinely need tanking. Tiles and grout aren’t waterproof; they let water through to whatever’s behind. Tanking is the continuous membrane that keeps that water off the structure. Skipping it is the most common reason bathrooms fail early, so we treat it as essential, not optional, in every splash zone.
Can you tile onto my existing bathroom walls?
Sometimes, if they’re flat, sound and suitable — but never onto bare plasterboard in a wet area without proper backer and tanking. We check the walls at strip-out and prepare or reboard them as needed. We won’t tile over a surface that isn’t right just to save a day, because that’s exactly the shortcut we get called back to fix on other people’s jobs.
How long does the waterproofing stage take?
Boarding and tanking typically fall across days three and four of a full bathroom, once the first fix is done. The membrane then needs time to cure before tiling, which is why this middle phase is paced rather than rushed. It’s short in the scheme of the job, but it’s the stage everything else depends on — so it’s never something we hurry.
How can I tell if my old bathroom was waterproofed properly?
You often can’t see it directly, but the signs of a poor job show up over time: loose or cracked tiles, dark staining in the grout, a soft or spongy feel to the wall behind tiles, or damp appearing on the other side of a bathroom wall or the ceiling below. If you’re seeing any of those, water is very likely getting past inadequate waterproofing — and a refit is the chance to put the hidden layers right for good.
Is waterproofing included in your quote?
Yes, always. Proper boarding and tanking of the wet areas is a standard part of every full installation we carry out, built into your fixed written price rather than offered as an upgrade. We’d never quote a bathroom without it, because it’s not an optional finish — it’s the thing that makes the whole room last.
Proud of every bathroom we fit
The part you can’t see, done right
Boarding and tanking are the difference between a bathroom that lasts and one that rots — and they’re built into every fixed quote we give, never skimped. Tell us about your room and we’ll show you exactly how we’ll keep the water where it belongs.