Tanking is a continuous waterproof membrane applied to walls and floor before tiling, with the system’s own tapes at corners and collars at penetrations. It must be a complete single-manufacturer system, applied to a suitable substrate and allowed to cure.
Why tiles are not the waterproofing
This surprises people, and it is the foundation of everything else.
- Grout is porous. Cementitious grout absorbs water. It is designed to, and sealers reduce it rather than eliminate it.
- Silicone has a finite life. It shrinks, discolours, loses adhesion and eventually lets water past. It is a maintenance item, not a permanent seal.
- Tile adhesive is not a membrane. It bonds; it does not waterproof.
- Movement happens. Buildings move, floors deflect, and any rigid surface eventually develops hairline cracks.
So in a normal shower enclosure, water that gets past the surface reaches the substrate. In a conventional bathroom with a tray, the tray contains most of it. In a wet room, there is no tray – the floor is the tray – so the membrane underneath is the only thing standing between the shower and the structure.
What a tanking system is
A complete, single-manufacturer kit. The components are designed to work together and mixing brands invalidates the lot.
| Component | Purpose |
|---|---|
| Primer | Prepares and stabilises the substrate so the membrane bonds |
| Membrane | The waterproof layer – liquid-applied in coats, or a bonded sheet |
| Reinforcing tape | At every internal and external corner and every wall-floor junction, where movement concentrates |
| Preformed corners | Where tape alone cannot reliably form a waterproof corner |
| Pipe collars | Sealed around every pipe penetration |
| Drain flange or bonding collar | Ties the membrane into the drain body itself |
| Sealant for specific junctions | As the system specifies |
Two types dominate. Liquid-applied systems are painted or rolled on in coats, typically two, with tapes bedded into the first. They are flexible around awkward shapes. Sheet membranes are bonded to the substrate with adhesive and their thickness is guaranteed by manufacture rather than by application. Both work; sheet removes the risk of an under-applied coat.
Substrate matters as much as membrane
A membrane is only as good as what it is stuck to.
- Tile backer board – cement-based or foam-cored – is the right substrate for a wet area. It is dimensionally stable when wet and does not degrade.
- Standard plasterboard is not suitable for a wet room, even moisture-resistant grades. It softens if it ever gets wet and it is not a stable base for tile in that environment.
- Plaster is not a wet room substrate. Tanking over plaster relies on the plaster’s bond to the wall, which is not designed for it.
- Existing tiles can sometimes be tanked over with the right system, and frequently should not be – if the tiles are sound and well bonded it can work, but you are relying on a bond nobody can inspect.
- Timber floors need overboarding or a purpose-made former, and need to be stiff enough not to deflect.
- Every substrate has to be sound, dry, clean and flat before priming.
Liquid membranes need to cure fully before tiling. Tiling over uncured membrane traps solvent, prevents proper curing and compromises the membrane permanently. The waiting time depends on the product, the number of coats, the temperature and the humidity, and it is specified by the manufacturer. It is invisible in the finished room, which is why it gets compressed when a job is running late.
Where failures actually occur
Almost never in the middle of a wall. Failures happen at the discontinuities.
- Wall-to-floor junctions, where movement concentrates and where tape is essential.
- Internal and external corners, for the same reason.
- Pipe penetrations, where the membrane has to seal around a round object that moves slightly with temperature. This needs the system’s own collar, not a bead of silicone.
- The drain flange, where the membrane has to tie into the drain body. Getting this wrong means water running behind the membrane straight into the floor.
- Screw fixings through the membrane, for screens, rails and fittings. Every one is a hole, and each must be sealed as it is made.
- The edge of the tanked area, where it stops. Water does not respect the boundary somebody drew, and the tanked zone should extend well beyond the obviously wet area.
- Thin spots in a liquid-applied membrane, where coverage was skimped. Which is why sheet systems are attractive.
How to tell whether yours was done properly
Once it is tiled, you cannot. That is precisely why it is worth doing this at the time.
- Ask to see it before tiling. Any contractor confident in their work will be pleased to show you a complete, continuous membrane with taped corners and sealed penetrations.
- Photograph it, or ask for photographs. Wide shots and close shots of every corner, junction and penetration.
- Ask which system was used, and get the manufacturer and product name. Keep it – it matters for any future warranty conversation.
- Check everything is one manufacturer. Membrane, tapes, corners, collars and flange should all be from the same system.
- Ask about cure time, and when tiling started relative to when the membrane was finished.
If you are having a wet room built and nobody offers to show you the tanking, ask. It is the single most important thing in the room and the only one you will never be able to inspect again.
Conventional bathrooms
Tanking is mandatory in a wet room and it is still worth doing in a standard bathroom with a tray and enclosure.
Tanking the shower area – the tray upstand and the walls of the enclosure to above shower head height – costs comparatively little at the time and removes the dependence on silicone as the only barrier. Given that silicone is a maintenance item that will eventually fail, having a membrane behind it converts a future failure from a structural problem into a cosmetic one.
It is not required, and we would recommend it in any shower area as a matter of course.
Having a wet room built? Ask to see the tanking before it is tiled, and take photographs. It is the one thing you cannot check later.
Questions we get asked about this
Do I need to tank a normal bathroom?
It is not required for a standard bathroom with a tray and enclosure, but tanking the shower area is worth doing. It removes the reliance on silicone, which will eventually fail, as the only barrier.
Can you tank over existing tiles?
Some systems allow it where the existing tiles are sound and well bonded. It is workable and it means relying on a bond nobody can inspect. Removing them is safer where practical.
How long does tanking take to cure?
It depends on the product, the number of coats, the temperature and humidity, and it is specified by the manufacturer. Tiling before it has cured compromises the membrane permanently and is invisible afterwards.
Is sheet or liquid membrane better?
Both work. Sheet guarantees thickness by manufacture, which removes the risk of a thin coat; liquid is more adaptable around awkward shapes. The important thing is that it is a complete system properly applied.
What if water gets behind the tanking?
That is the failure mode, and it is why penetrations, corners and the drain flange matter so much. Water behind the membrane goes into the structure with nothing to stop it, and it usually shows up as damage somewhere unexpected.
Can I tank it myself?
The materials are available and the application is not difficult. The parts that go wrong are the details – taped corners, pipe collars and the drain flange – and the consequence of getting them wrong is invisible until it is expensive.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.