Wet rooms
Wet room floors and drainage: the part that decides everything
Written by Martin, M24/7 Bathrooms
Last updated:
A wet room succeeds or fails in the floor. Gradients, drains and build-up decide whether water goes where it should, and they are settled before a single tile is chosen.
Ask what makes a wet room and most people point at the open shower. Ask a fitter and the answer is the floor: the falls that move water to the drain, the drain itself and the build-up that hides all of it under a flat, tiled surface.
This guide covers that hidden half of the job. It sits alongside the wet room tanking guide, which covers waterproofing, and the wet room cost guide, which explains what the work means for the budget. Our wet room installation service covers the whole build.
The gradient: how water finds the drain
A wet room floor is not level. It falls towards the drain at a gentle, deliberate gradient, typically around one in sixty, so water moves away without the floor feeling sloped underfoot. Getting that fall right, in the right direction, across the whole showering area, is the core skill of the job.
The fall is formed in a screed or a pre-formed tray former, then tanked and tiled over. A fall that is too shallow leaves puddles; too steep and the floor feels wrong and tiles become hard to cut cleanly.
Linear drains versus centre drains
A centre drain, the traditional point waste, sits in the middle of the showering area and needs the floor to fall towards it from all sides. That four-way fall means smaller tiles or careful cuts, which is why classic wet rooms often use mosaics.
A linear drain runs as a channel along one edge, usually the wall side. The floor then falls in a single direction, which suits large format tiles and gives a cleaner, more modern floor. Linear drains cost more and need their position planned early, but they simplify everything above them.
Floor build-up on timber and concrete
On a suspended timber floor, the waste can usually run between or through the joists, and the former or screed sits within the floor depth. The joists are checked for strength and any notching is done properly, because a bouncy floor cracks grout and breaks seals.
On a concrete floor the waste route is harder. Either the floor is chased out for new pipework, or the showering area is raised slightly to create the depth the drain and falls need. Neither is a problem, but both are decided at the survey rather than discovered mid-job.
Drainage capacity and where the water goes
The drain has to clear water as fast as the shower delivers it. A pumped or rainfall shower with a high flow rate needs a drain rated to match, and the waste pipe needs a clear run to the soil stack with the right fall of its own.
This is why the waste route is one of the first things checked at a wet room survey. A beautiful drain connected to a struggling waste pipe is a puddle waiting to happen, and the shower drain smell guide shows what happens when waste water sits where it should not.
Tiling over the falls
The finished floor follows the falls exactly, so tile choice and drainage choice are linked. Large tiles suit a single fall to a linear drain; smaller tiles or mosaics follow a four-way fall to a centre drain. Either way the floor is tanked before tiling, and the drain is sealed into the tanking system rather than just the tiles.
A slip rated floor tile is worth specifying for the wet area. Smooth polished porcelain looks sharp in photographs and is treacherous under wet feet; a rated tile grips without feeling rough.
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Questions
Frequently asked
Does a wet room floor have to slope?
Yes, but gently. The fall towards the drain is around one in sixty, enough to move water without being noticeable underfoot. The rest of the bathroom floor stays level.
Is a linear drain better than a centre drain?
It depends on the room. A linear drain allows a single fall and large format tiles, which gives a cleaner look. A centre drain is simpler and cheaper but needs a four-way fall and usually smaller tiles.
Can a wet room go on a concrete floor?
Yes. Either the floor is chased for new waste pipework or the showering area is raised slightly to create the depth needed. The survey establishes which approach suits your floor.
Why does my wet room hold water in one corner?
A persistent puddle means the fall is wrong or the floor has moved. It is worth having it looked at, because standing water eventually finds its way past grout and sealant.