Waterproofing and tanking · London

Bathroom Waterproofing in London

Tiles and grout are not waterproof. Water passes through grout lines every time you shower, and what stops it reaching the wall, the floor or the ceiling below is a waterproofing layer behind the tiles, usually called tanking. We build that layer into every bathroom we renovate, test it for 24 hours and photograph it before anything covers it.

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BS 538524-hour flood testPhoto record

Tiled shower niche with a slim metal edge, built into a waterproofed wall

Shower tanking, in plain English

A continuous waterproof skin, under the tiles

Tanking is a membrane applied to the walls, and in wet rooms the floor, before any tiles go on. It has to be continuous: one gap at a corner, a pipe or the edge of the shower tray is enough to let water through.

There are two common types. Sheet membranes are bonded to the wall with adhesive. Liquid membranes are brushed or rolled on in two coats over reinforcing tape. Both work well when they're applied as the manufacturer specifies.

What matters most is using one manufacturer's complete system: the primer, membrane, corner tapes, pipe collars and drain flanges are designed and tested to work together. Mixing products from different brands is where problems start. The British Standard for wall and floor tiling, BS 5385, expects wet areas to be protected this way, and it's the standard we work to.

Every shower we build is waterproofed like this before tiling, then tested and photographed.

Where it's needed

The parts of a bathroom that need waterproofing

Anywhere water is sprayed, splashed or left standing. Some areas are obvious; the small details are where leaks usually begin.

Shower walls

Every wall inside the enclosure, usually to at least 2 metres above the tray, and higher with a ceiling-mounted rainfall head or a steam shower.

Around the bath

The walls along the bath, taken up to above the shower head if there's a shower over it, with the joint between bath and wall sealed into the membrane.

Wet-room floors

The whole floor, turned up the walls at the edges and bonded to the drain, laid to falls so water always runs to the outlet.

Niches, benches and windows

Every recess has extra corners and edges. Each one is taped and sealed, and a window inside a shower needs its reveal and sill treated too.

Our method

How we waterproof a shower, stage by stage

Each stage is checked before the next starts. You get photos of the finished waterproofing before the first tile goes on.

1
Strip back and check
Old tiles come off down to a sound wall. Any damp, rot or soft plasterboard is found now, not later.
Strip-out
2
Board and prime
Cement backer board in the wet zone, joints taped, then primer so the membrane bonds properly.
Preparation
3
Tape and collar
Reinforcing tape in every corner, collars round each pipe, and the tray or bath edge sealed in.
Detailing
4
Membrane
Two coats, or bonded sheets, applied to the manufacturer's coverage and drying times.
Tanking
5
Flood test and photos
Trays and wet-room floors held under water for 24 hours, walls inspected, everything photographed.
Before tiling

Is your shower leaking?

Signs the waterproofing behind your tiles has failed

None of these proves a leak on its own, but two or three together usually mean water is getting behind the tiles.

If water is coming through a ceiling right now, stop using the shower and speak to your home insurer first. A leak detection specialist can confirm the source before any renovation is planned.

Loose or hollow tilesTiles near the shower that move or sound hollow when tapped.
Grout that keeps failingCracking, darkening or crumbling again soon after repair.
Marks on the ceiling belowStaining, bubbling paint or fine cracks under the bathroom.
A musty smellOne that stays even when the room is dry and ventilated.
A soft or springy floorEspecially beside the tray, the bath or the toilet.
Silicone that won't stay cleanBlack mould returning within weeks of resealing.

Why re-grouting rarely fixes it

New grout covers the problem, not the cause

Fresh grout and silicone can slow water down for a while, but they don't rebuild the layer behind the tiles. If that layer was never there, or has split, water keeps reaching the board behind it. Plasterboard softens, timber rots and tiles start to lift.

The lasting fix is to take the shower area back to a sound wall, let it dry, repair whatever has been damaged and rebuild it with tanking. Hidden repairs such as rotten boards or joists are priced from our published extra works list, and you agree them before we do them.

Flats, neighbours and insurance

A leak rarely stays in your bathroom

In a block of flats, water from a failed shower usually ends up in the flat below. The Association of British Insurers has reported that escape-of-water claims in multi-occupancy buildings doubled between 2017 and 2025. Many home insurance policies also exclude damage caused by failed grout or sealant, which leaves the cost with the owner.

Proper tanking, testing and a photographic record protect you, your neighbours and your building. If your lease requires work to be done by a competent contractor, the photos and written specification are what you'll need to show.

Prices

What waterproofing adds to a bathroom

Waterproofing is priced per square metre and included in every bathroom renovation we quote, whether you choose installation only from £6,200 or a complete renovation from approximately £9,500.

It's a small part of the total, and far less than the cost of stripping out and rebuilding a leaking shower. Work out your own figure with the bathroom cost calculator.

Tanking membrane system, per m²£64
Cement backer board, per m²£78
24-hour flood test and photo recordincluded
Wet-room floors, falls and drainafter survey

Questions

Shower tanking and waterproofing: your questions

Straight answers about what goes behind the tiles. Worried about your current shower? Send us photos and we'll tell you what we think.

What is shower tanking?

Tanking is a waterproof membrane applied to the walls, and in wet rooms the floor, before tiling. It forms a continuous skin that stops water passing through the grout and reaching the wall, floor or ceiling below.

Aren't tiles and grout waterproof already?

No. Porcelain tiles resist water, but standard grout lets moisture through and adhesive isn't a waterproof layer. Without tanking behind them, water reaches the wall every time the shower runs.

How high should shower walls be tanked?

Usually to at least 2 metres above the shower tray, covering every wall inside the enclosure. With a ceiling-mounted rainfall head or a steam shower we take it higher, and in a wet room the whole floor is tanked as well.

Can a leaking shower be fixed without removing the tiles?

Sometimes. If the leak comes from a failed seal, a valve or a waste fitting, it can often be repaired on its own. If the waterproofing behind the tiles has failed, re-grouting won't fix it, and the tiles in the shower area need to come off so it can be rebuilt.

How do I know the waterproofing was done properly?

Ask which system was used, see photos of it before tiling, and check it was flood-tested. We give you all three: the manufacturer and product, a photo record of every wet area, and the result of the 24-hour flood test.

Is waterproofing included in your bathroom prices?

Yes. Every shower and bath area in our renovations is boarded and tanked, priced at £64 per square metre, and it's built into the estimates from our cost calculator.