Waterproofing & grout
Here’s a fact that surprises almost everyone: tiles and grout are not waterproof. They shed most of the water, but grout is mildly porous, and over years of daily showering moisture works its way through the joints. In a splash zone that’s fine; behind a shower or a bath, where water hits the same patch of wall thousands of times a year, it isn’t. The real waterproofing in a well-built bathroom is hidden — a tanking membrane behind the tiles, the right grout in the joints, flexible silicone where surfaces move, and sealing where it’s needed. This guide explains how those layers work together, and why getting them right is the difference between a bathroom that stays dry for twenty years and one that leaks into the ceiling below.
Why tiles alone don’t keep water out
It’s an understandable assumption — tiles look solid and water runs straight off them, so surely they keep the wall dry? The catch is the grout between them. Grout is a cement-based filler, and even the best grout is mildly porous: it absorbs a little water, and in a shower that gets used every day, that constant wetting eventually lets moisture creep through to the surface behind. Add in the joints that inevitably move — where a wall meets a floor, or around a bath — and you have paths for water to travel that the tiles themselves can’t block.
That’s why a properly built wet area is waterproofed behind the tiles, not by them. The tiles are the raincoat; the tanking underneath is the skin that actually keeps you dry. When this hidden layer is skipped — which is depressingly common on quick, cheap jobs — the first sign of trouble is often a damp patch, a musty smell, or a brown stain spreading across the ceiling of the room below. By then the damage is done. Getting waterproofing right from the start is the single most important part of any bathroom tiling and design job, and it’s the part you can least afford to have done badly.
The layers that actually keep a bathroom dry
Tile backer board
Behind wet areas we fit waterproof tile backer board — a cement-based or foam board — rather than standard plasterboard, which softens and swells when repeatedly wetted. It gives a stable, water-resistant base for the tanking and tiles to bond to.
Tanking membrane
A continuous waterproof layer applied to the substrate before tiling — either a flexible paint-on coating in two or more coats, or bonded waterproof matting — with reinforcing tape worked into every internal corner and around every pipe. This is the true waterproof skin.
The right grout
Quality, water-resistant grout, packed fully and finished properly, is the first line of defence at the surface. For the wettest areas or the lowest maintenance, epoxy or premium grout is far more stain- and water-resistant — pricier and more skilled to lay, but close to bulletproof.
Flexible silicone
Where one plane meets another — wall-to-floor, internal corners, around the bath and basin — we seal with flexible silicone, not rigid grout, because those joints move and grout would crack. A neat, unbroken silicone line is a quiet hallmark of a proper job.
Where full tanking matters most
- Shower enclosures — the most heavily and repeatedly wetted surfaces in any bathroom.
- Behind a shower-bath — that wall takes far more water than people assume.
- Wet rooms — the entire wet zone, floor and walls, gets a full tanking system; see our wet room installation service.
- Niches and shelves — recessed shelves are a classic leak point, so we tank them fully before tiling.
How we waterproof and grout a bathroom, step by step
Here’s the order the work runs in, and why each stage sits where it does — because the sequence is as important as the materials.
1. Prepare and board the substrate
Old finishes come off, surfaces are made flat and stable, and waterproof tile backer board goes up behind the wet areas in place of plasterboard — the sound, water-resistant base everything else relies on.
2. Tank the wet zones
A continuous tanking membrane is applied to the shower, the wall behind a shower-bath and any niches, with reinforcing tape into every corner and around every pipe, and left to cure. Water can’t get past this layer.
3. Fix the tiles with full coverage
Tiles are bedded in the right adhesive — flexible on floors — with full coverage behind every tile, so there are no hollow voids where water could collect or the tile could crack under load.
4. Grout the joints
Once the adhesive has cured, the joints are grouted in your chosen colour with a quality water-resistant grout, packed fully and finished smooth, then the tiles are cleaned off before any haze sets.
5. Silicone the movement joints
Flexible silicone — not grout — goes into the wall-to-floor and perimeter joints and around the bath and basin, because these move and rigid grout would crack. Neat, continuous lines, properly tooled.
6. Seal where needed and hand over
Natural stone is sealed, grout can be sealed for extra stain resistance, and the room gets a final clean. We walk you round it and leave simple care notes to keep it looking its best.
The discipline that matters most here is not rushing — letting the tanking cure before tiling, and the adhesive cure before grouting. Grouting onto uncured adhesive, or tiling over half-set tanking, is a classic cause of trouble later. It’s the same unhurried care we bring to every full bathroom installation, and it’s exactly why proper prep underpins the whole tiling and design job.
Keeping grout and silicone looking new
Even a perfectly waterproofed bathroom needs a little looking after to stay looking fresh. The good news is that if the tanking and fixing are right, maintenance is genuinely light — you’re just keeping the surface presentable, not holding back water.
Simple habits that keep it fresh
- Run the extractor. Cutting the damp that lingers after a shower is the single best thing you can do to stop mould taking hold in grout and silicone.
- Wipe the shower down. A quick squeegee or wipe of the wet walls after use removes the moisture that discolours grout over time.
- Avoid harsh acidic cleaners. Aggressive cleaners wear grout away and can damage stone and silicone — gentle, appropriate products last far longer.
- Re-seal stone as advised. Natural stone and its grout need periodic re-sealing to stay stain-resistant; we’ll tell you how often for your particular stone.
When to refresh, and when to worry
Silicone is the one part of a bathroom that has a finite life — over years it can discolour or start to peel at the edges, and periodically raking it out and re-applying a fresh bead is normal maintenance, not a fault. Grout that’s simply discoloured can usually be cleaned or refreshed. What you should not ignore is grout that’s cracked or crumbling, a persistent musty smell, or any staining appearing on a ceiling below — those can signal that water is getting past the surface, and they’re worth having looked at promptly before damage spreads. If you inherit a bathroom you’re unsure about, we’re happy to inspect it as part of a bathroom renovation and tell you honestly what’s going on behind the tiles.
Why cutting this corner costs the most
Of everything in a bathroom, waterproofing is the worst possible place to save money — because when it fails, it fails expensively and invisibly. A skipped tank behind a shower doesn’t announce itself; it quietly wets the wall and the timbers behind it for months or years until rot, damp or a stained ceiling forces the issue. By then you’re not just redoing tiling — you may be repairing structure, replastering, and redecorating the room below. The few pounds “saved” by skipping the membrane can turn into thousands. Grout and silicone are the same story on a smaller scale: cracked, failed joints let water in slowly and steadily.
Our default is always to waterproof properly, tank every wet area, use the right grout and silicone in the right places, and never rush the curing. It costs a little more up front and removes the single biggest cause of bathroom failure. For a broader look at where a bathroom budget goes and where it’s worth spending, see the cost of a bathroom in Plymouth — waterproofing is one line we’d never encourage anyone to trim.
Frequently asked questions
Do bathroom tiles need waterproofing behind them?
Wet areas do. Tiles and grout shed most water but aren’t fully waterproof — grout is mildly porous, so over years of showering moisture works through the joints. That’s why we tank (apply a continuous waterproof membrane to) shower enclosures, the wall behind a shower-bath, niches, and the whole wet zone of a wet room before tiling. Splashback areas that only get the odd splash don’t need a full tank.
What is tanking, and why does it matter?
Tanking is a continuous waterproof membrane applied to the substrate before tiling, in the areas that get genuinely wet. It’s put on as a flexible paint-on coating in two or more coats, or as bonded waterproof matting, with reinforcing tape into every corner and around every pipe. It’s the true waterproof skin behind the tiles — the tiles shed water, but the tanking is what actually keeps the wall and the room below dry.
Why does grout crack, and how do you prevent it?
Grout cracks when the surface beneath it moves — a flexing timber floor, a shrinking new screed, or heat expansion from underfloor heating — and rigid grout can’t cope. We prevent it by using flexible adhesives and grouts, decoupling floors prone to movement, and crucially filling movement joints (wall-to-floor, internal corners, around the bath) with flexible silicone rather than grout, because those joints are designed to move.
Should I use grout or silicone in the corners?
Silicone. Anywhere one plane meets another — wall-to-floor, internal corners, around the bath and basin — the joint moves slightly as the building and fittings shift, so it needs flexible silicone rather than rigid grout, which would crack. Grout is for the joints between tiles on the same flat plane; silicone is for the joints where surfaces meet and move.
How do I keep bathroom grout and silicone looking new?
Run the extractor to cut post-shower damp, wipe the wet walls down after use, and avoid harsh acidic cleaners that wear grout and silicone away. Natural stone and its grout need periodic re-sealing. Silicone has a finite life and raking it out and re-applying a fresh bead every few years is normal maintenance — but cracked grout, a musty smell or ceiling stains below can signal water getting past the surface and are worth checking promptly.
Proud of every bathroom we fit
Get a bathroom that stays dry for the long run
Proper waterproofing is invisible when it’s right and disastrous when it’s skipped. We tank every wet area, use the right grout and silicone in the right places, and never rush the curing. Tell us about your bathroom for one clear written quote — built to keep water where it belongs.