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Bathroom waterproofing before tiling
Tiles look waterproof, but grout and tile joints let water through. In wet areas, a waterproofing layer under the tiles protects walls and floors from hidden damage.
1. Why waterproof
Water that passes through grout soaks into plasterboard, plaster, timber or screed, leading to mould, loose tiles and rot, often unseen until the damage is serious. A continuous membrane under the tiles keeps water on the surface.
2. Which areas to cover
Cover the whole shower area: floor, and walls to at least the shower head height, and in walk-in showers often the full room floor. Around baths, waterproof the walls above the tub. Many countries set minimum areas in their building rules; check local requirements.
3. Choose a system
Common options are liquid membranes (painted or rolled on in two coats), sheet membranes bonded with tile adhesive, and waterproof backer boards with sealed joints. Use one complete system from one manufacturer, including its tapes, corners and pipe collars.
4. Apply the membrane
Prime the surface as instructed. Embed sealing tape in the first coat at all wall-floor and wall-wall joints and around drains and pipes. Apply two coats crosswise to the specified thickness, overlapping onto the drain flange, and let each coat dry.
5. Check and tile
Inspect for pinholes and thin spots. Where possible, flood-test the shower floor for 24 hours before tiling. Then tile with an adhesive compatible with the membrane, and keep a slight fall to the drain. See How to tile a floor: a step-by-step guide for tiling.
Common mistakes
- Assuming tiles and grout are waterproof.
- Leaving gaps at corners and pipe penetrations.
- Mixing components from different systems.
- Applying the membrane too thin or in one coat.
- Tiling before the membrane is fully dry.
Frequently asked questions
Are tiles waterproof?
Most tiles are, but grout and joints are not, so a membrane is needed underneath in wet areas.
How high should shower walls be waterproofed?
At least to above the shower head height, and follow local rules.
Can I tile directly onto a membrane?
Yes, with an adhesive that the membrane manufacturer approves.
How long does a liquid membrane take to dry?
Usually a few hours per coat; check the product data sheet.
Should I waterproof the whole bathroom floor?
In walk-in showers and wet rooms, usually yes.