Water Leaking Through Shower Tiles: 6 Causes and Fixes

Water leaking through shower tiles is one of the most misdiagnosed jobs in a bathroom. The tiles get the blame because that is where the water vanishes, but a fired ceramic or porcelain tile is not porous, and in a shower that was built properly the tiles were never the waterproof layer in the first place. Something behind them, or at the edge of them, let water past first.

That distinction decides your whole weekend. Re-grouting a shower that is actually leaking at the tray junction changes nothing, and re-siliconing a shower that has no waterproofing behind the boards just delays the ceiling stain by a few months. Below are the six causes we actually find, how to prove which one you have before you buy anything, and how to tell a repair from a strip-out.

Tiles and grout were never the waterproof layer

Cement-based grout is porous. It takes up water, holds it, and hands it on to whatever sits behind. Tile adhesive is water-resistant, which is not the same as watertight. In a shower that gets daily use, a steady amount of water passes through the grout lines by design, and the layer behind the tiles is what is supposed to catch it and send it back into the tray.

The British Standard covering tiling in wet conditions, BS 5385-4:2015, puts this plainly: in areas subject to frequent wetting, the structure behind the tiles should be watertight, and an impervious grout is no substitute for a tanked installation. The companion standard for ordinary internal tiling, BS 5385-1:2018, points installers to Part 4 the moment the job moves into a wet area.

So a shower tiled straight onto standard plasterboard with nothing behind it does not have a grout problem. It has no waterproofing, and it was always going to leak. Knowing that up front saves people a lot of money on repairs that were never going to work.

The six things that actually leak

1. The silicone at the tray or bath junction

This is the single most common cause, by a wide margin. The joint where a tiled wall meets a shower tray or bath moves. The tray flexes when somebody stands in it, timber floors move seasonally, and tile and acrylic expand at different rates. Rigid grout in that joint cracks, usually inside the first year, which is why the standard detail is a flexible silicone sealant rather than grout.

Stained and discoloured tiled shower wall with water marks running down from a shower valve — Staining and discoloured grout around a shower valve. Water tracking down the wall from a penetration leaves marks like this. | db_1247 by darkbuffet (CC BY 2.0), via Flickr, https://www.flickr.com/photos/16380910@N04/2323767967

Silicone is a consumable. Mould roots into the surface and breaks the bond, bleach and limescale removers attack it, and the bead eventually lifts at one end. The tell is a leak that only shows up when somebody showers standing up, plus a bead that is blackened, curling away from the tray lip, or has a hairline split you can catch with a fingernail.

2. Cracked or missing grout

Grout does fail, just not usually on its own. Hairline cracks along internal corners are normal and are a sign the corners were grouted when they should have been siliconed, because every internal corner is a movement joint. Cracks in the middle of a wall are different: they usually mean the substrate behind is moving, the tiles have debonded, or the adhesive bed was hollow.

Tap along the wall with a knuckle. A solid bedded tile gives a dull thud. A hollow ring means air behind it, water can sit in that void indefinitely, and re-grouting the surface will not touch the problem.

3. No tanking behind the tiles

Older showers, and a lot of rushed refits, were tiled onto plasterboard or even onto plaster. Once water gets through the grout it soaks the board, the board swells and loses strength, the adhesive bond goes, and tiles start sounding hollow and eventually move. By the time water appears on a ceiling below, the board behind is usually soft.

Modern practice is a tanking system: either a liquid-applied membrane to BS EN 14891 painted over the boards with reinforcing tape in every corner, or a cement-based tile backer board with taped and sealed joints. Both need to carry up the walls to above showerhead height and lap into the tray.

4. The shower tray itself, or how it was bedded

Stone resin and acrylic trays need full support underneath. A tray sitting on a few dabs, or on an uneven floor, rocks slightly every time it is used. That movement breaks the silicone joint above it, and on a suspended timber floor it can crack a stone resin tray outright. Low-profile trays are the usual offenders because people fit them without the manufacturer’s support kit.

Look for a tray that gives underfoot, a silicone bead that keeps failing in the same spot no matter how often it is redone, or a visible hairline crack in the tray surface.

5. The waste, trap and their seals

A leak here has nothing to do with the tiles at all, which is why it survives every re-grout. The rubber washer between the waste flange and the tray hardens, the trap connection works loose, or the compression nut on the outlet was never fully tightened. The signature is a leak that appears whenever the shower runs, whether anyone is standing in it or not, and often a damp patch directly under the waste rather than under the wall.

6. The mixer valve and pipe penetrations

Every hole cut through a tiled wall for a valve body, riser rail bracket or shower arm is a hole through the waterproofing. If the tanking was not dressed into the opening and the fixing was not sealed, water runs down the back of the tile and straight into the wall. Chrome escutcheon plates hide this completely. So does a valve union weeping behind the wall, which shows as a damp patch that is worst at the height of the valve.

How to work out which one you have

Do not open anything up until you have narrowed it down. Staged testing takes an evening and tells you far more than guesswork:

Fill the tray only. Block the waste, put a couple of inches of water in the tray, leave it two hours. If it leaks, the problem is the tray, the waste or the trap. No walls involved.
Run the water down the waste. Unblock it and let the shower drain for several minutes without wetting the walls. A leak now points at the trap or the outlet pipe.
Wet the walls only. Spray the tiled walls with a hand shower, keeping water off the tray junction as far as you can. A leak here means grout, tanking or a penetration.
Wet the tray junction only. Direct water along the silicone line. If this is the one that produces the leak, you have found the cheapest fix on the list.
Stand in it. Have somebody stand in the tray while the walls are wetted. A leak that only appears with weight in the tray is movement, so it is the tray support or the joint above it.

Ceiling stains below are worth reading too. A stain that grows slowly over weeks and dries between showers suggests grout or tanking taking water in and letting it out slowly. A stain that appears within minutes of the shower running and drips is a pipe, a waste or a badly split joint.

What a lasting repair involves

Matched to the cause, the work is straightforward:

Failed silicone: cut the old bead out completely rather than skinning over it, scrape the residue, clean the surfaces with a solvent so the new bead can bond, let everything dry properly, then run one continuous bead of a sanitary-grade silicone. Fill the tray with water before you seal, and leave it filled until the silicone has cured, so the joint sets in its loaded position.
Cracked grout in corners: rake it out and replace it with silicone, not more grout. Corners are movement joints and always will be.
Hollow or debonded tiles: the affected area comes off and goes back on a solid bed. Patch repairs over hollow tiles fail again.
No tanking: this one is a strip-out. The tiles and the wet boards come off, the substrate is replaced with tile backer board or tanked with a membrane, then it is re-tiled. There is no shortcut, and anyone offering one is selling you next year’s leak.
Waste or trap: a plumbing repair. Often just a new washer and a proper tighten, sometimes a new waste kit.
Penetrations: the fitting comes off, the opening gets sealed properly, and the fixing goes back with sealant behind the escutcheon.

When to repair and when to strip it out

Repair is the right call when the tiles all ring solid, the substrate behind is dry and firm, and the failure is confined to a joint, a fitting or the waste. That covers most leaks caught early.

Strip out and start again when tiles sound hollow across an area rather than in one spot, when a screwdriver pushed into a grout line finds soft board, when the tray moves underfoot, or when the same bead of silicone has failed three times. Those are all signs the water has already been in the structure for a while, and every further repair is spent money on a wall that is coming off anyway.

If you are weighing that up on a Bromley or south east London bathroom and want a second opinion before committing, the rest of Bromley Tilers has detailed guides on tanking, backer boards and re-tiling costs.

Frequently asked questions

Can water really leak through grout?

Yes. Cement-based grout is porous and passes a small amount of water through by design. That is not a fault, which is why the waterproofing has to sit behind the tiles. Sealing grout slows absorption and helps with staining, but a grout sealer is a surface treatment and does not make a shower watertight.

Why does my shower silicone keep going black and failing?

Black is mould growing in the surface of the sealant, which happens fastest where the bead holds water. Repeat failure in the same place usually means movement in the joint, most often a tray that is not fully supported, or the bead was applied over old silicone or a wet surface so it never bonded properly.

Should shower corners be grouted or siliconed?

Siliconed. Internal corners, and the junctions where tiling meets a tray, bath or a different material, are movement joints. Grout is rigid and cracks there. This is standard practice for tiling in wet areas, not a shortcut.

How do I know if the leak is the tiles or the plumbing?

Block the waste and fill the tray without wetting the walls. If it leaks with the shower switched off, it is the tray, waste or trap. If it only leaks while water is hitting the walls, it is the tiled surface, the sealed joints or the waterproofing behind them.

Can I tank over existing tiles instead of removing them?

Not as a repair for a leaking shower. Tanking has to sit behind the tiled surface to work, and tanking over old tiles means you have sealed the damp substrate in rather than dealt with it. If the boards behind are wet or soft, they have to come out.

How long should a tiled shower last before it needs work?

The tiling itself should last decades if it was tanked and bedded properly. The silicone will not. Treat the bead where the wall meets the tray as a maintenance item, inspect it once a year, and replace it as soon as it splits or lifts rather than waiting for a ceiling stain.

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