Bathroom Waterproofing Explained
Key takeaways
- Water gets through tiles and grout, slowly and always. The membrane under them is what keeps it out of the wall and the floor.
- The system is the membrane plus the detailing: corners, thresholds, the drain collar, every pipe.
- The fall is part of it: water that cannot reach the drain stands, and standing water finds a way.
- It is visible for a day or two between application and tiling. Ask to see it; the photograph is the record.
Every bathroom has two layers: the finish you choose and the system under it that you never see again. This guide is the second layer, explained for someone who is about to read a quote: what waterproofing is, where it goes, where it fails, and what to ask. The waterproofing page is where it becomes a request.
Why tiles are not enough
A tiled wall looks impervious and is not: grout is porous, sealant ages, and the joints around a bath or a tray open with use. Water gets through, slowly and always, and where there is nothing behind the tiles to stop it, it goes into the wall, the floor, and eventually the room below. Nothing looks wrong for a long time, and then everything does at once.
What the system is
- A membrane – liquid-applied or sheet – across the shower floor and up the walls, on a substrate that is sound and dry
- The corners and junctions, where floor meets wall and wall meets wall, reinforced and detailed
- The drain collar, where the membrane meets the drain body, sealed rather than hoped
- The threshold, where the wet area ends and the dry floor begins
- Every pipe and fixture that pierces the membrane, collared
- The fall, formed in the floor before the membrane, so water has somewhere to go
- In a wet room, all of the above across the whole floor
Where it fails
At the details, almost always: a corner left to the tile adhesive, a threshold that stops short, a drain collar that was never fitted, a pipe hole cut after the membrane went on. And at the fall – a floor laid flat under a shower, so water stands in a corner and works at the grout there every day. The membrane itself rarely fails; the edges of it do, which is why a company that talks about corners and collars has done this before.
The four questions
- What waterproofing system are you using, and is it named on the quote?
- How far up the walls does it go, in the shower and outside it?
- How are the corners, the threshold and the drain detailed?
- Will you show me the membrane before the tiles go on?
The island, specifically
Humidity that hangs, coastal air that reaches into the room, old houses whose floors sit on the ground and whose walls hold moisture: none of it changes what waterproofing is, all of it changes how much of the rest of the room the system has to think about – ventilation that moves air, a floor separated from what is under it, a wall finish that lets the stone dry. In a block, the floor of your bathroom is somebody else’s ceiling, and the waterproofing is the difference between a renovation and a dispute.
Is microcement waterproof?
It is sealed, which sheds water; it is not the waterproofing system, which goes under it like under tiles. An applicator who says the sealer is enough has not done many bathrooms.
My bathroom has never leaked – does it need waterproofing when it is renovated?
If the wet area is being opened, yes: whatever was or was not behind the old tiles, the new shower gets a system under it. A renovation that tiles over the old wet area has tiled over the one thing it cannot see.
Can I check it was done?
Ask to see it, and ask for photographs: the membrane, the corners, the drain collar, the height on the walls. A company that expects the question will offer.
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