Water Gets Where You Least Expect It

Most shower leaks have nothing to do with the tiles. Tiles are a decorative, hard-wearing surface, but they are not waterproof — and neither is the grout between them. Grout is porous and slightly flexible, and silicone will eventually fail if it is asked to do a job it was never meant to do. When you stand under a shower every morning, water does not just run down the wall; it creeps sideways behind the tile edge, sits in the corners, and soaks into whatever is behind there.

If that "whatever" is plasterboard or bare plaster, you will know about it in a year or two. You will get damp patches on the other side of the wall, a soft, crumbly patch behind the tiles, or that unmistakable musty smell. The fix is the part nobody sees: tanking. Get that right and the tile becomes a nice finish rather than the flood defence.

Start With the Right Substrate

Before any waterproofing goes on, the wall itself has to be sound. Work through a quick checklist:

• Strip out anything loose. Old tiles, flaking paint, distemper and wallpaper all have to come off. Adhesive will not grip to paint, and no tanking membrane likes a dusty, friable surface.
• Check the weight limit. Bare plaster skim can usually take around 20kg per square metre, plasterboard around 32kg in dry areas, while cement-based tile backer board will comfortably take 60kg or more. Heavy porcelain on a skimmed stud wall is a recipe for tiles on the floor.
• Go for backer board where you can. In a shower, moisture-resistant plasterboard is only a starting point — it is water-resistant, not waterproof. A tile backer board screwed back to the studs at 300mm centres is far more forgiving, and it gives you a flat, stable surface to tank.
• Prime before you tank. Gypsum surfaces generally need a primer so the tanking bonds properly. Read the instructions on the tin and follow them — this is not the moment to improvise.

Also deal with the plumbing first. Fit the shower valve, the pipework and the tray before the tiles go on, because once the walls are sealed, nobody wants to start cutting holes in them.

Tanking: The Step That Saves You Thousands

Tanking is simply a continuous waterproof layer over the whole wet area, painted or sheeted on before tiling. In a shower, that means the walls up to at least 1800mm — preferably to the ceiling — plus the full shower enclosure, and out to about 1500mm beyond the tray on any adjoining wall.

Liquid tanking membranes are the easiest to apply yourself. Roll or brush on two coats at right angles to one another, letting the first cure properly, and aim for the dry film thickness the manufacturer specifies. It goes on a lurid colour, usually blue or green, so you can see exactly where you have and have not covered.

Then reinforce the weak points — and there are more than you would think:

• Internal corners, where wall meets wall, using a flexible jointing tape bedded into the first coat.
• The wall-to-tray junction, including the tray upstand if it has one.
• Around pipe penetrations, with a purpose-made collar or a good overlap of tape.
• Any board joints, taped and skimmed with membrane.

Twenty minutes with tape and a brush here saves you a strip-out later.

Fix With the Right Adhesive and Allow for Movement

In a wet area, use a cement-based powder adhesive rather than the ready-mixed tub stuff. Water-resistant and flexible categories are what you want, particularly on any background that may move slightly. Mix small batches, comb it out evenly, and back-butter large format tiles so you get full contact — voids behind tiles become little reservoirs.

Leave a 2–3mm joint between tiles. Tight-butt jointing leaves nowhere for movement to go, and the first thing to crack will be the grout.

Seal Corners and Joints Properly

This is where most showers fail, and it is almost always because silicone is being used to bridge two surfaces that move independently. Grout the flat field of tiles; seal everything that changes direction or material.

• Use silicone, not grout, in all internal corners, and at the wall-to-tray junction.
• Fill the joint first. Push a foam backer rod or a bead of silicone into the gap so the sealant forms a hourglass shape rather than a thin skin stretched across a void. Silicone stuck only to two edges peels away as soon as the building moves.
• Use a mould-resistant sanitary silicone, and smooth it with a tool or a soapy finger before it skins over.
• Never seal over old silicone. Cut it out completely, clean with a suitable solvent, let it dry, and start again.
• If your tray has an upstand, tile down over it and seal where the tile face meets it, not through it.

Leave silicone at least 24 hours to cure before the shower is used, even if the grout says it is fine to walk on.

A Few Finishing Details That Matter

Grout choice makes a difference. Cement-based grout with a polymer additive, or an epoxy grout in a busy family bathroom, holds up better than basic wall grout. Keep a small tub of spare tiles and grout — matching a discontinued tile in three years' time is far harder than storing a few now.

Finally, treat the sealant as a service item. Check the silicone around the tray and in the corners twice a year. If it has gone black, lifted at an edge or feels spongy, cut it out and redo it — half an hour's work rather than a weekend of drying out a ceiling downstairs. Do that, and the tiles are simply the pretty part of a shower that stays watertight for years.