Module 1 of 2 · 45 minutes
Substrates, deflection and tanking a wet area
By the end of this module you will be able to
- Assess a substrate for flatness, stability and deflection
- Explain why tiles crack over a floor that moves
- Tank a shower or wet room correctly and know where tanking must extend
- Choose adhesive and grout appropriate to the situation
Work through it
1 interactive for this module, built on the WAJD Teach engine. Nothing moves until you ask it to, and every one has a written version if you would rather read it.
Amara Start me with the thing people get most wrong.
Nadia That a tiled shower is waterproof. It is not, and almost nobody knows it.
Amara The tiles are impervious though.
Nadia The tiles are. The grout is not, and neither is the adhesive bed. Water goes through a grout line steadily and invisibly, and it has been doing it behind a lot of showers for years.
Amara So what actually keeps water out?
Nadia The tanking behind the tiles. A liquid membrane or a bonded sheet system applied to the substrate before any tile goes on, with the corners taped and pipe penetrations sealed with collars.
Amara How far does it need to go?
Nadia Beyond the bit that looks wet. Full height of the shower and at least the enclosure footprint plus a margin. In a wet room, the floor and the walls to a defined height across the whole room. Follow BS 5385 and the tanking manufacturer together, and if they disagree, that is a conversation to have before you start.
Amara What about plasterboard in a shower?
Nadia It has no place there. Tile backer board, and then tank it anyway.
Amara Let us do floors. Why do floor tiles crack?
Nadia Because a tile is a rigid brittle plate and the floor underneath it is not. Deflection is what kills floor tiles.
Amara How much movement is too much?
Nadia The rule of thumb is span over 360 for ceramic, and span over 720 for natural stone, which is twice as demanding. Stone is stiffer and much less forgiving.
Amara Is there a site test?
Nadia Walk on it. If you can feel it move, tiles will crack. Put a glass of water on the floor and have somebody walk past: if it ripples, you have a problem you need to fix before you tile, not a problem to mention afterwards.
Amara And the fix?
Nadia Extra noggins, sistering the joists, overboarding with a proper tile backer rather than chipboard, or an uncoupling membrane where you genuinely cannot remove the movement. It is not preparation, it is the job.
Amara Adhesive. I have always just bought a tub.
Nadia Then keep the tub for dry walls with small light tiles, and nothing else. Tub adhesive cures by drying, so behind a large impervious tile it may never fully cure at all.
Amara It just stays soft back there?
Nadia It can, for a very long time. Powder cement based adhesive mixed with water for floors, wet areas, porcelain, stone and anything large or heavy. And flexible adhesive over anything that could move or has underfloor heating.
Amara How do I know I am getting enough adhesive behind the tile?
Nadia Lift one occasionally and look. Everybody trowels the wall, presses the tile on, and assumes. In a wet area or on a floor you need close to solid bed coverage, because a void holds water and is where the tile cracks under a point load.
Amara Back buttering?
Nadia For large format tiles, yes. It is the difference between a floor that lasts and one where three tiles crack in the first winter.
Amara Last thing. Movement joints. I have never fitted one in my life.
Nadia Almost nobody has, and it is the commonest avoidable defect in domestic tiling. Tiled surfaces expand and contract, and BS 5385 wants joints at the perimeter, over structural joints, at changes of background, and at intervals across large areas.
Amara What does that mean in a normal bathroom?
Nadia Something very simple. Silicone every internal corner, silicone around the bath and the tray, leave a gap at the perimeter behind the trim, and do not grout tight into the wall to floor junction.
Amara And if I do grout it tight?
Nadia You get that crack along the bottom of the wall that everybody has and nobody understands. Grout is rigid. Two planes that move relative to each other need something flexible between them, and that is what silicone is for.
The written material
Tiles do not bend
A tile is a rigid brittle plate stuck to something that may not be rigid. Every cracked tile, every hollow one, and almost every failed grout line is that mismatch.
For floors, deflection is the killer. A timber floor that flexes underfoot will crack ceramic tiles, and the industry rule of thumb is that deflection should not exceed span over 360 for ceramic, and span over 720 for natural stone, which is far stiffer in requirement and far less forgiving in practice.
Fixing a bouncy floor is not optional preparation, it is the job. Additional noggins, sistering joists, overboarding with a proper tile backer board rather than chipboard, or an uncoupling membrane where movement cannot be eliminated.
Wet areas: tiles and grout are not waterproof
This is the single most misunderstood thing in domestic tiling. Ceramic tile is impervious but the grout and the adhesive bed are not, and water passes through a grout line steadily and invisibly.
So a tiled shower is not waterproof. What makes it waterproof is the tanking system behind the tiles: a liquid membrane or a bonded sheet system applied to the substrate before any tile goes on, with corners, junctions and pipe penetrations sealed with matching tape and collars.
Tanking must extend beyond the visibly wet area. In a shower enclosure, the full height of the shower and at least the enclosure footprint plus a margin. In a wet room, the floor and the walls to a defined height across the whole room. Standard practice follows BS 5385 and the tanking manufacturer's instructions, and the two must agree.
Standard plasterboard has no place in a shower enclosure. Use a tile backer board, then tank it anyway.
- Tiles and grout are not a waterproof layer, they are a wearing surface
- Tank the substrate before tiling, with taped corners and sealed penetrations
- Tile backer board in wet areas, never standard plasterboard
- Silicone, not grout, at every internal corner and every change of plane
- Follow the tanking manufacturer's instructions and BS 5385 together
Adhesive and grout, chosen rather than defaulted
Powder cement based adhesive mixed with water is the standard for floors, wet areas and anything large or heavy. Ready mixed tub adhesive is convenient and belongs only on dry walls with small light tiles: it cures by drying, so behind a large impervious tile it may never fully cure.
Use a flexible adhesive over any substrate that could move, over underfloor heating, and for porcelain and stone.
Coverage matters more than people think. Solid bed coverage is required in wet areas and on floors, meaning close to full contact behind the tile. Voids hold water, and on a floor they are where a tile cracks under point load. Back butter large format tiles and lift one occasionally to check what coverage you are actually achieving.
Grout joints need to be wide enough to accommodate movement and tile size tolerance. Very narrow joints on large format tiles look precise and concentrate stress. And use silicone at internal corners: grout is rigid and will crack exactly where two planes meet.
Movement joints, the thing nobody puts in
Tiled surfaces expand and contract. BS 5385 requires movement joints at the perimeter of tiled areas, over any structural movement joint, at changes of background material, and at intervals across large areas, typically every eight to ten metres internally and considerably less externally or over underfloor heating.
In a domestic bathroom the practical version is simple: silicone every internal corner, silicone around the bath and tray, leave a gap at the perimeter behind the skirting or trim, and do not grout tight into the junction between wall and floor.
Tiling hard into every corner is why grout cracks in a line along the bottom of a wall, and it is the most common avoidable defect in domestic tiling.
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