In short
- Tile adhesive and ordinary grout are not waterproof. A continuous membrane behind the tiles is what keeps water out of the wall and floor.
- Liquid membranes, sheet membranes and waterproof boards all do the job, provided one manufacturer's full system is used and fitted to its instructions.
- The tiling code of practice, BS 5385-1, expects wet areas to be protected, and treats plaster and plasterboard as unsuitable there without it.
- Ask for a 24-hour flood test and photos of every stage. They cost very little and they're the only proof you'll ever have.
Why tiles and grout let water through
A glazed porcelain tile barely absorbs water at all, which is why people assume a tiled wall is a waterproof one. The weak points are the joints. Standard cement grout is porous, so it soaks up water during every shower and slowly passes it inwards. Over time it also develops hairline cracks as the building moves and the room heats up and cools down.
Tile adhesive doesn't stop that water either; it holds tiles in place, it isn't a barrier. So without anything behind the tiles, the wall gets wet several times a day. Plaster and plasterboard soften, tiles turn hollow and lift, and timber floors swell and rot, often noticed only when a mark appears on the ceiling below.
What tanking actually is
Tanking is the trade name for a waterproof layer applied to walls, and to floors in wet rooms, after the surface has been prepared and before any tiles go on. The word comes from basement waterproofing, where the whole room is sealed like the inside of a tank.
The important word is continuous. A membrane that covers 98% of a shower is not 98% effective: water finds the 2% that's missing, usually a corner, a board joint, a pipe or the edge of the tray. So a proper system is more than a tub of membrane. It includes primer, tapes, preformed corners, pipe collars, drain flanges and a compatible adhesive.
The main waterproofing systems compared
There isn't one right product. A good installer will tell you which system they plan to use, and why.
| System | How it goes on | Works well for | Watch for |
|---|---|---|---|
| Liquid membrane | Brushed or rolled on in two coats over a primer, with reinforcing tape bedded into corners and joints | Most shower enclosures and bath surrounds, niches and awkward shapes | Both coats must reach the thickness on the data sheet, and each needs its full drying time |
| Sheet membrane | Flexible fleece-backed sheets bonded to the wall or floor with adhesive, with overlapping seams | Large flat areas and wet room floors, where it can be tiled quickly | Every overlap must be fully bonded, and corners need preformed pieces rather than folded sheet |
| Waterproof boards | Lightweight foam-core panels screwed or bonded to the wall, with joints and fixings sealed using the maker's tape and sealant | Stud walls, boxing-in and rooms that benefit from a little insulation | The board is only waterproof once every joint and screw head is sealed; unsealed joints leak like any other |
| Wet room formers and trays | Pre-sloped floor panels or tray formers with a built-in drain, bonded to the floor and joined to a membrane | Level-access showers and wet rooms, because the falls to the drain are built in | They must be bedded perfectly flat and supported properly; the joists underneath may need work first |
What matters more than the type is that every component comes from one manufacturer's complete system, fitted exactly as its instructions say. Manufacturers test their membranes, tapes and adhesives together. Mix a membrane from one brand with tape from another and nobody can tell you how they'll behave, and most system warranties no longer apply.
What the British Standard expects
BS 5385-1 is the British Standard code of practice for wall and floor tiling. You don't need to read it, but four of its expectations are useful to know when you compare quotes:
- Wet areas should be protected with a suitable proprietary tanking system. Impervious grout or adhesive on their own don't count.
- Gypsum plaster and plasterboard, including moisture-resistant boards, are treated as unsuitable in wet areas unless they're tanked.
- Plywood shouldn't be tiled directly. On timber floors it needs a suitable board or membrane over it first.
- Backgrounds have weight limits. Gypsum plaster is generally taken to carry around 20 kg of tiling per m², which heavy porcelain and its adhesive can exceed. That's one reason many showers are re-boarded with cement board, which costs from £78 per m² as part of a renovation.
A code of practice isn't law. But if a shower leaks and there's a dispute, it's the yardstick an ombudsman, an insurer or a court would use to judge whether the work was done with reasonable care and skill.
The details where leaks usually start
Large flat areas of membrane rarely fail. Leaks start at edges and interruptions:
- Internal corners, where two walls meet and where walls meet the floor. They need tape or preformed corners, not just membrane brushed into the angle.
- Board joints and screw heads, which move slightly as the building moves and need reinforcing.
- Pipes, valves and outlets coming through the wall, each sealed with a collar from the same system.
- The tray or bath edge, where the wall membrane has to be lapped and sealed onto the tray's upstand or edge.
- Drains in wet rooms, which need a flange bonded into the floor membrane.
- Movement joints at every change of plane, filled with flexible silicone rather than grout once tiling is done.
Drying times and the flood test
Liquid membranes need drying time between coats and before tiling: anything from a few hours to a full day, depending on the product, thickness, temperature and ventilation. Tiling over wet membrane to save time ignores the manufacturer's instructions.
The flood test is the simplest quality check in the whole job. The drain is plugged, the shower tray or wet room floor is filled with water, the level is marked and it's left for 24 hours. If the level hasn't dropped and nothing has appeared below, the floor is sound. Walls can't be filled with water, so they're checked visually and photographed instead. Building a day for this into the programme is normal: it's part of the 10 working days a typical family bathroom takes with us.
The photos to ask for
Photos are your only record of what's behind the tiles. Keep them with your guarantee.
- Boards fixed and joints taped. Shows what the membrane is bonded to.
- Corners, collars and the tray edge sealed. The details that cause most leaks.
- Membrane complete, with the product packaging in shot so you know which system was used.
- The flood test, with the water level marked at the start and again after 24 hours.
- First fix plumbing under pressure test, before any pipe disappears into a wall.
Questions to ask before you sign
Waterproofing is often one line on a quote, if it appears at all. These questions show whether it's been thought about:
- Which waterproofing system will you use, by brand and product name?
- Is every component, including tapes and collars, from that one system?
- What will the membrane go onto: existing plaster, plasterboard or new cement board?
- How high will the walls be tanked, and will the floor be done in a walk-in shower?
- Will you flood-test the tray or floor for 24 hours before tiling?
- Will I get photos of each stage?
- Is the waterproofing itemised in the written quote, and covered by your workmanship guarantee?
Insurance, leaks and flats
Many home insurance policies exclude damage caused by failed grout or sealant, and a slow leak is often disputed as gradual damage. That can leave the cost with the homeowner, or with whoever did the work, so the quality of the waterproofing genuinely matters.
It matters even more in a flat. The Association of British Insurers told MPs in April 2026 that escape-of-water claims in blocks of flats rose from £652 million in 2017 to £1.3 billion in 2025, and that a building which has had one claim is likely to have another. Many leases also require bathroom work to be carried out by a competent contractor. A written specification, the system's name and dated photos are exactly what a managing agent or insurer will ask to see.
What waterproofing adds to the cost
Done as part of a renovation, waterproofing is a modest line. Tanking is priced by the square metre, from £64 per m² with us, so a shower enclosure usually adds a few hundred pounds. Rebuilding a shower that leaked because it wasn't tanked almost always costs more than doing it properly the first time.
For a figure for your whole room, use the bathroom renovation cost calculator.
Questions
Is waterproof grout enough on its own?
No. Epoxy and other water-resistant grouts reduce how much water gets into the joints, but they don't seal the wall behind the tiles, and the adhesive isn't a barrier either. BS 5385-1 expects a waterproofing membrane in wet areas whatever grout you choose.
Is moisture-resistant plasterboard OK in a shower?
Not on its own. Moisture-resistant board copes with humid air, not with water running down it every day. The tiling standard treats plasterboard as unsuitable in wet areas unless it's tanked, and many installers replace it with cement board in the shower zone.
How long does tanking take to dry before tiling?
It depends on the product, the number of coats and the conditions in the room, from a few hours to a full day. Allow a day for the membrane and another for the 24-hour flood test, and check the manufacturer's data sheet rather than relying on a rule of thumb.
Does the whole bathroom floor need waterproofing?
In a wet room or a level-access shower, yes: the whole floor is tanked and laid to fall towards the drain. In a bathroom with a tray and a bath, the walls around the shower and bath are the priority, and the floor outside them isn't usually tanked.
Can I check the waterproofing once the tiles are on?
Not directly. Moisture meters and thermal cameras can help find a leak later, but they can't tell you how well a membrane was applied. Your evidence is the system's name, the flood test record and the photos taken before tiling.
Last updated 21 October 2026. Prices are guide prices for London homes, from our published price list.