Which tile adhesive and grout should I use in a wet area?
The substrate decides the adhesive, the joint width decides the grout, and the tanking that has to happen before either.
- Difficulty
- beginner
- Time
- 30 min
- Read
- 4 min
- Safety
- caution
Short answer
In a shower, use a flexible cement-based adhesive rated S1 or better, over a tanked and board substrate. Use cement grout with an additive for normal joints, or epoxy where staining and water resistance really matter. Grout is not waterproof — the tanking behind it is.
Most tile failures in bathrooms are not the tiles. They are the wrong adhesive on the wrong board, or an assumption that grout keeps water out.
Safety
Step by step
- Start with the substrate.Standard plasterboard is not suitable inside a shower. Use tile backer board, or moisture-resistant board that has been tanked. The substrate choice constrains everything after it.
- Tank the wet area properly.A liquid or sheet waterproof membrane over the boards, with sealing tape in the corners and around pipe penetrations. This is the actual waterproof layer.
- Understand that grout is not waterproof.Cement grout is porous and always lets some moisture through. It is a filler, not a barrier. Anyone relying on grout to keep a shower dry has a leak coming.
- Choose a flexible adhesive for wet areas.Look for S1 or S2 deformability on the bag. Flexibility absorbs the movement from temperature and substrate changes, and rigid adhesive in a shower cracks and debonds.
- Match the adhesive to the tile.Porcelain and large-format tiles need a stronger, more flexible adhesive than small ceramic ones. Natural stone often needs a white, low-water adhesive to avoid staining through the tile.
- Use white adhesive under light or translucent tiles.Grey adhesive shows through marble, glass and pale porcelain. It is a permanent mistake and costs nothing to avoid.
- Consider ready-mixed only for dry areas.Tubbed adhesive is convenient but generally not suitable for showers or floors, because it cures by drying and cannot dry behind a tile in a wet area. Powder mixed on site for anything wet.
- Match the trowel notch to the tile size.Bigger tiles need a deeper notch to get full coverage. Wet areas should have close to full adhesive coverage behind the tile, with no voids to hold water.
- Choose grout by joint width.Unsanded grout for narrow joints up to around 3mm; sanded grout for wider ones. Sanded grout in a narrow joint will not pack in; unsanded in a wide one shrinks and cracks.
- Use a flexible or additive-improved grout.Standard cement grout with a latex additive, or a ready-formulated flexible grout. It resists cracking where there is movement and reduces water absorption.
- Consider epoxy grout for the demanding places.Effectively non-porous, highly stain-resistant, and the right answer for a shower floor or a kitchen splashback behind a hob. It is more expensive and much less forgiving to work with.
- Use silicone at every movement joint.Internal corners, the wall-to-tray junction, and around the bath. Grout in these joints cracks within months because they move. Silicone flexes and stays sealed.
- Choose the right silicone.Sanitary-grade with a fungicide for bathrooms and kitchens. Ordinary silicone goes black with mould, and painter's acrylic is not waterproof at all.
- Respect the curing times.Do not grout until the adhesive has set, usually 24 hours, and do not use a shower until the grout and silicone have fully cured. Rushing traps moisture behind the tiles.
- Seal natural stone before grouting.Porous stone absorbs grout pigment and stains permanently. A sealer applied before grouting prevents it, and it cannot be corrected afterwards.
Tips
- Grout is not the waterproofing. The tanking behind the tiles is.
- S1 or better flexible adhesive in any wet area, mixed from powder rather than tubbed.
- Silicone in every internal corner. Grout there always cracks, because those joints move.
Common mistakes
- Tiling a shower straight onto ordinary plasterboard — It absorbs moisture, softens and loses its grip on the adhesive, so the tiles eventually come away and water gets into the structure behind.
- Grouting internal corners instead of siliconing them — Corners are movement joints. Rigid grout in them cracks within months, which then lets water straight through into the wall.
If it doesn't work
Grout cracking in the corners
Cause: Grout used where silicone is needed — Fix: Rake it out and reseal with sanitary-grade silicone.
Tiles sounding hollow when tapped
Cause: Poor adhesive coverage — Fix: In a wet area this needs redoing. Voids behind tiles hold water and the tiles will eventually debond.
Grey shadow showing through pale tiles
Cause: Grey adhesive under a translucent tile — Fix: Cannot be corrected without removing the tiles. Use white adhesive under light stone, glass and pale porcelain.
Grout staining badly on a shower floor
Cause: Porous cement grout — Fix: Seal it, or replace with epoxy grout, which is effectively non-porous.
Stone tiles stained by grout
Cause: Not sealed before grouting — Fix: Largely permanent. Seal porous stone before grouting next time.
Questions people ask
Is grout waterproof?
No. Cement grout is porous and always passes some moisture. The waterproofing is the tanking membrane behind the tiles, and relying on grout instead is the usual cause of shower leaks.
When should I use epoxy grout?
Shower floors, wet rooms, and kitchen splashbacks where staining matters. It is effectively non-porous and highly stain-resistant, but it costs more and is much less forgiving to apply.
Can I use ready-mixed adhesive in a bathroom?
Only in dry areas such as a splashback away from direct water. Tubbed adhesive cures by drying, which it cannot do behind a tile in a wet area. Use a powder adhesive there.
What does S1 mean on an adhesive bag?
It is a deformability classification — how much the cured adhesive can flex without cracking. S1 is flexible and S2 highly flexible. Wet areas and large tiles want at least S1.