Wet room tanking and waterproofing explained
Tanking is the single component that decides whether a wet room lasts. This page explains what the membrane is, how far it has to reach, and why installations fail.
What is wet room tanking?
Tanking is a continuous waterproof membrane bonded to the floor and walls beneath the tiles. It reinforces every corner, junction and pipe penetration, and it — not the tile or the grout — is what keeps water out of the floor structure.
What sits between the water and your floor joists
A wet room floor is a layered system. Remove or interrupt any one layer and the system stops working.
- 1
1. Structural deck
Concrete slab, or joists with a rigid tile backer board replacing the original timber deck. It must not flex under load.
- 2
2. Falls and former
A prefabricated sloping former or a screeded gradient directing water to the drain, typically between 1:80 and 1:40.
- 3
3. Primer
Manufacturer-matched primer that gives the membrane something to bond to and controls substrate absorbency.
- 4
4. Reinforced junctions
Waterproof tape bedded into the membrane at every internal corner, wall-to-floor junction, drain flange and pipe penetration.
- 5
5. The membrane
Liquid-applied in two coats or a bonded sheet, continuous across the whole floor and up the wet-zone walls.
- 6
6. Tile, adhesive and grout
Flexible adhesive and grout over the cured membrane. This layer is the wearing surface, not the waterproofing.
How far the membrane has to go
Coverage is where good and bad installations separate. A wet room has no tray edge to contain water, so the water goes wherever the floor lets it. Our standard specification is:
- The entire floor, wall to wall, not just the shower zone.
- A minimum 100mm upstand onto every wall, lapped into the floor membrane.
- Full-height tanking to 1,800mm or more on all walls inside the shower zone.
- Reinforcing tape at every internal corner and every change of plane.
- Collars at every pipe penetration and around the drain flange.
- Sealed threshold at the door so water cannot track under the frame.
The membrane is left to cure for the full time the manufacturer states before tiling starts. Rushing that cure is a silent failure — the tiles go on, the room looks finished, and the membrane never reaches its rated performance.
The five reasons wet rooms leak
- Partial tanking: the shower corner is sealed and the rest of the floor is bare substrate.
- Unreinforced corners: the membrane spans an internal angle with no tape, then cracks as the building moves.
- Flexing deck: tiles laid over a timber floor that was never stiffened, so the adhesive bed and membrane crack.
- Wrong or absent falls: water sits on the floor instead of draining, and standing water finds every weakness.
- Mixed systems: primer from one brand, membrane from another, tape from a third, and no valid warranty on any of it.
Waterproofing in Glasgow properties
In a Glasgow tenement, a floor leak does not damage your property first — it damages the flat below. That changes the risk calculation entirely, and it is why we tank tenement wet rooms to a higher upstand than a ground-floor bungalow needs and flood-test before handover.
Glasgow's humidity also means surfaces stay damp for longer between uses. Constant low-level moisture attacks grout and silicone before it attacks the membrane, so extraction and membrane specification are part of the same conversation. See the ventilation guide.