Waterproofing and tanking: why bathrooms leak — and how to prevent it
Almost every "leaking bathroom" we are asked to fix traces back to the same thing: waterproofing that was skipped or done badly behind the tiles. Here is what good looks like.
In short
- ✓Tiles and grout are porous — water passes through them over time. They are not a waterproof layer.
- ✓Wet areas need a continuous tanking (waterproof) membrane behind the tiles, with reinforced tape at all wall-to-floor junctions.
- ✓Heavy tiles must be set on a full bed of adhesive (BS 5385), not dabs — dot-and-dab leaves voids that trap water and fail.
- ✓Wet rooms need a graded former/tray and tanking across the whole zone — they are not just a tiled bathroom.
Are tiles and grout waterproof?
No. Ceramic and porcelain tiles and the grout between them are porous, so moisture passes through them over time. A separate waterproofing layer — a tanking membrane — must be installed behind the tiles in all wet zones. Relying on tiles and grout alone is the single most common cause of slow leaks and rotten timber.
What is tanking and where is it needed?
Tanking is a continuous waterproof barrier applied to walls and floor before tiling — typically a liquid membrane plus reinforcing tape bonded into every wall-to-floor and corner junction. It is essential across shower enclosures, wet rooms and bath splash zones, extending well up the walls so water can never reach the structure behind.
Why do wet rooms leak more often than normal bathrooms?
Because a true wet room is harder to build than it looks. It needs a structurally supported, graded floor (a former or pre-formed tray with the correct fall to the drain) and continuous tanking across the entire zone. Where installers cut corners — no proper fall, tanking tape missed at junctions, tray over an uneven subfloor — water escapes and rots the floor below.
The myth that causes most leaks
Homeowners often assume tiles and grout keep water out. They don’t — both are porous, and steam and water migrate through grout lines into the wall behind. Without a waterproof barrier underneath, that moisture degrades plasterboard and rots timber, often invisibly for months before it shows.
What a proper tanking system involves
In wet zones, the standard we expect is:
- •A continuous liquid tanking membrane across walls and floor in the wet area.
- •Joint-reinforcing tape bonded into every wall-to-floor junction, internal corner and around the waste.
- •A moisture-resistant board substrate (cement backer board) rather than standard plasterboard in wet areas.
- •For wet rooms: a structurally supported former/tray with the correct gradient to the drain before tanking and tiling.
Dot-and-dab vs full-bed tiling
Large porcelain and stone tiles are heavy and must be bedded on a full, solid layer of adhesive (BS 5385) — "back-buttered" and troweled for 100% contact. The "dot-and-dab" shortcut (blobs of adhesive) leaves hollow voids that collect moisture, weaken the bond and lead to cracked or loose tiles. We require full-contact bedding in all wet zones.
Where corners get cut — and what we insist on
The failures we see on other people’s bathrooms are almost always invisible on handover day: tiling straight onto untreated plasterboard in a shower, dabs of adhesive behind heavy tiles, or tanking tape missed at the wall-to-floor junction. None of it shows for months — then it shows as damp and movement. Getting these fundamentals right first time is exactly what we hold our installers to.
Frequently asked
Do I need tanking if I’m only replacing the suite?+
If the wet areas are being re-tiled, yes — that is the moment to put proper waterproofing in. Re-using an old, untanked wall behind a new shower is a false economy. We treat wet-zone tanking as standard, not an optional extra.
Is a wet room a good idea in an older terraced house?+
It can be excellent for space and accessibility, but it depends on the floor. Suspended timber floors need checking and preparing so the tray and tanking sit on a solid, level base. We assess this at survey before recommending a wet room.