Waterproofing wet areas — before a tile touches the wall
Tiles and grout are not waterproofing: cementitious joints pass water slowly, and from beneath them it finds the downstairs neighbour. The flexible membrane under the tiles is the real waterproofing — the tiling above it is protection and finish. The difference between a bathroom that lasts and one that leaks is two hours of work and two crosswise coats.
Where, and how high
The whole bathroom floor, always, turned 10–15 cm up every wall as a general tank. The shower zone: walls to two metres or 20 cm above the head. Around basins and behind tubs: one metre above the working edge. Kitchens: the wall strip behind the sink and the floor in front of it.
Preparation and priming
Everything in the substrate guide applies: sound, clean, flat, and the floor falls to the drain formed before membraning, not after — the membrane follows the shape beneath it, it does not create one. Acrylic primer on absorbent substrates, dry 2–4 hours.
Corners first: tape before coating
Every floor-wall angle, every wall-wall corner, every drain and pipe penetration — these are where waterproofing fails, because the building's movement concentrates there. Bed rubberised sealing tape into a fresh stripe of membrane at each one before the general coats: brush a band of membrane on, press the tape in, coat over it. Preformed collars for drains and pipes.
Two coats, crosswise
First coat by brush or roller in one direction. Let it dry to an even dark tone that does not lift on a finger — typically 4–6 hours. Second coat perpendicular to the first: the cross guarantees what the first pass missed, the second cuts across. Total consumption across both coats 1.2–1.5 kg/m² — less means a film too thin to bridge a hairline crack.
Tiling over the membrane
After a final 24 hours tile straight onto the membrane with a flexible C2 adhesive — no primer on the membrane, no scoring it "for grip". No drilling: any mechanical fixing (shower seat, grab rail) is planned before membraning and collar-sealed, or adhesive-fixed instead of screwed.
Boundary warning: a wet-area membrane is for splash and humidity — not permanent immersion. Pools and water tanks are a different system entirely: a two-component cementitious waterproofing (powder + polymer liquid) built for permanent positive water pressure.
The recurring waterproofing mistakes
- Treating tile and grout as the waterproofing.
- Coating corners without tape — the first settlement crack opens them.
- One thick coat instead of two crosswise: it mud-cracks as it dries and leaves open tracks.
- Tiling before full dry-through, sealing water beneath the membrane.
- Drilling the membrane after tiling to hang an accessory.
- A wet-area membrane inside a swimming pool.
Frequently asked
Should I membrane every bathroom wall to the ceiling?
Not necessary: the shower zone to two metres and other walls 10–15 cm above the floor cover the function. More does no harm but buys nothing in an ordinary domestic bathroom.
How do I test the membrane before tiling?
A flood test: plug the drain, pond a few centimetres of water, leave 24 hours while watching the ceiling below. The test costs a day — a leak after tiling costs the bathroom.
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Last updated: 2026-08-09