What Is Tanking?
Written by James Walker · Last reviewed 15 August 2026
James Walker has 2 degrees in construction (BSc (Hons) Building Surveying · MSc Environmental Design of Buildings) and an extensive background in issue resolution, resolving over £4 billion of consumer issues.
Tanking is a waterproof barrier applied to walls and floors in wet areas, behind the tiles, to stop water reaching the structure. It is also used in basements and cellars.
Why it is necessary
Because tiles and grout are not waterproof. Grout is porous, and water passing through it reaches whatever is behind.
In a shower enclosure or wet room, that means the substrate gets wet repeatedly. Without a barrier, tiles eventually debond, plasterboard degrades, and water reaches timber.
What it looks like
Usually a liquid membrane painted on, or a boarded system, applied before tiling and extending across the whole wet area and up the walls to a defined height.
The practical point
Once tiled over, you cannot tell whether it was done. That makes it the classic hidden shortcut.
Ask what system will be used, where it extends to, and ask to see it photographed before tiling starts. A competent fitter will not mind. Someone planning to tile straight onto plasterboard will become evasive, which is useful information while it is still fixable.
How it works
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