Who Is Responsible for Waterproofing a Bathroom Before Tiling?

For installers

If you are the main contractor delivering the finished bathroom, waterproofing is not something I would be comfortable simply assuming the tiler has dealt with.

The tiler may physically install the waterproofing system. A plumber may have fitted the shower tray. Somebody else may have prepared the walls. But somebody needs to take responsibility for making sure all those parts work together. If that somebody is you, because the customer has employed you to deliver the complete bathroom, then you need to understand what has been installed behind those tiles whether you physically installed it yourself or not.

Water is unforgiving. A mistake can sit unnoticed for months, sometimes years, before the first sign appears. And where the water eventually appears is not necessarily where the leak started.

Water doesn't always show you where the problem is

Anyone who has dealt with bathroom leaks knows how frustrating they can be. You might get a mark on the ceiling downstairs, but that does not automatically mean the problem is directly above it. Water can track along joists, pipes, boards and other surfaces before eventually finding somewhere to show itself. By that point, the damage may already be considerably greater than the original leak.

That is why I put so much importance on what happens before the finished surfaces go on. Once everything has been tiled, sealed and handed over, much of the work that is actually protecting the building has disappeared. You can't admire the waterproofing system in the finished photographs. You certainly notice when it fails.

Tiles and grout are not the waterproofing system

This is an important distinction. Tiles may shed water and grout can be water resistant, but that does not mean a tiled surface automatically provides the waterproof protection required behind it. The Tile Association’s guidance treats waterproofing in showers, wet rooms and other relevant wet areas as a system behind the tiled finish. That includes suitable backgrounds and the appropriate treatment of joints, corners and penetrations. Water-resistant adhesive and grout are not a substitute for a waterproofing system where one is required. (tiles.org.uk)

That does not mean every square inch of every bathroom automatically needs to be tanked in exactly the same way. A shower enclosure, wet room and wall beside a washbasin are not necessarily the same exposure. The correct approach depends on the area, the background, the products being installed and the relevant system guidance. That is why installers need to understand what they are actually building rather than simply buying a tub of tanking membrane and assuming the job is done.

Preparation makes all the difference

There is a theme running through almost everything I have said about fitting bathrooms: preparation matters. Waterproofing is no different. The background needs to be suitable. Junctions need treating correctly. Penetrations around pipes need thinking about. The correct thickness or coverage needs to be achieved where a liquid system is used, and drying times need to be respected.

Different systems have different requirements. For example, manufacturers publish specific instructions around application, number of coats, drying time, suitable substrates and compatible products. NHBC’s guidance for its warranty sites also describes an impervious layer behind the tiles, sealed junctions and penetrations, and compatible system components. The requirements vary with the area and substrate; NHBC warranty guidance is not a blanket legal rule for every bathroom renovation. (The Tile Association’s explanation)

This is not an area where I would make up my own version of the instructions because I have used something similar before. Read the technical information for the actual system you are using. Then install it that way.

Understand the product guarantee before you install it

This is another area I think installers should take more seriously. If you are relying on a waterproofing product or system, understand what the manufacturer actually requires. Which products are designed to work together? What substrate is acceptable? What tapes, collars or accessories are required?

How long must it dry? Are there limits on the tile or adhesive being used afterwards? What records might you need if there is a problem? You should not assume that buying a product with a long guarantee means every installation using that product automatically has the same protection. Warranty terms vary between manufacturers, and installation conditions matter. Technical approvals and system guidance commonly require particular components and installation methods to be followed. (tiles.org.uk)

If you ignore the installation instructions and something subsequently fails, trying to argue that the manufacturer should pay for the consequences could become very difficult. And with a bathroom, those consequences can be substantial.

Don't mix a system together without understanding it

There is always a temptation on site to use whatever is in the van. A bit of one manufacturer's system, another manufacturer's tape, a different adhesive and whatever sealant happens to be open. Sometimes products may be perfectly compatible.

Sometimes they may not be. If you are expecting a manufacturer to stand behind a system, understand what that system actually consists of. MAPEI's UK guidance, for example, presents bathroom waterproofing as a series of compatible layers and components rather than one isolated product. NHBC guidance takes the same system-based approach. (mapei.com) The few pounds you save using the wrong leftover product can look fairly ridiculous if the bathroom then has to come apart.

If somebody else does the tanking, don't simply forget about it

This is where being the main contractor becomes important. You may have an excellent tiler who applies the waterproofing system for you. Fine. But I would still want to know what system they use, how they apply it and what guarantee sits behind their work. I would also want the commercial relationship between us to be clear.

If my business has contracted with the customer to deliver the finished bathroom, I cannot simply assume that because a subcontractor physically carried out one part of the work, my responsibility to my customer has disappeared. Under the Consumer Rights Act, a trader supplying a service to a consumer must carry out that service with reasonable care and skill. Exactly how liability falls in a particular dispute depends on the facts and the contracts involved, but your separate arrangement with a subcontractor does not magically remove the contract you have with your customer. (legislation.gov.uk)

That matters when the customer phones two years later. You might still use the same tiler. You might not. They may answer their phone. They may not. Sort that side of the business out before you need it.

Have proper agreements with your subcontractors

Many small installation businesses work on relationships. You've known your plumber for years. You use the same tiler. The electrician has worked alongside you on dozens of jobs. That's great. It still doesn't mean the commercial arrangements should consist entirely of “we've always worked it out between us”. If subcontractors are carrying out work that could expose your business to significant cost later, have sensible written terms between you.

Be clear about responsibility for their workmanship, guarantees, remedial work and what happens if there is a claim. Check that the people you use have appropriate insurance for the work they undertake and keep current evidence where appropriate. Public liability insurance is generally not compulsory by law in the UK in the way employers' liability insurance can be, but it is an important form of protection for trades working in customers' properties and can be required contractually. Government business guidance specifically recommends considering appropriate insurance for the work being undertaken. (business.gov.uk)

This is professionalism. You don't want the first proper discussion about liability with a subcontractor to happen while water is coming through somebody's ceiling.

Keep records of what has gone behind the tiles

I think this is increasingly worthwhile. Record the products used. Keep invoices or proof of purchase where sensible. Keep the relevant technical information. Record batch information where manufacturers may require it. And take photographs before everything disappears. That doesn't need to turn every bathroom into a construction dossier. But if there is a problem eighteen months later, a photograph showing the waterproofing before the room was tiled can be considerably more useful than everybody trying to remember what was done.

Manufacturers investigating potential product failures can ask for things such as proof of purchase, batch numbers, photographs and details of how the product was installed. (ardex.co.uk) Good records protect everybody.

Don't rush the drying and curing times

This comes back to one of my biggest issues with bathrooms generally. Trying to force the programme creates problems. If a product says it needs a particular preparation or drying period, that time is part of the job. It isn't an inconvenience that needs working around because the tiler is arriving at eight o'clock tomorrow morning.

Some modern waterproofing systems are genuinely quick drying. Others have different requirements. The point is not to apply one standard waiting time to everything. Follow the system you are actually using. If your programme only works by ignoring manufacturer instructions, the programme is wrong.

Water damage is not normally a small comeback

This is why I wouldn't cut corners here. A loose handle or a door that needs adjusting is annoying. Water getting into the structure of a customer's home is in another league. You can be dealing with flooring, walls, ceilings, decoration, electrical installations, rooms below and the customer's ability to use the bathroom. There can also be the cost of finding the leak in the first place.

Suddenly, the few pounds saved on materials or the hour somebody tried to shave off the waterproofing work becomes completely irrelevant. That is why some bathroom installers cost more than others. It doesn't automatically mean the most expensive installer is doing everything correctly. But proper products, proper preparation, trained people, appropriate insurance, documentation and enough time to carry out the work correctly all cost money.

Professional installation has a cost. So does getting it wrong.

Who is responsible for waterproofing before tiling?

Someone needs to be clearly responsible. If you are only carrying out one defined part of a larger project, make sure the boundaries of your work are clear. If you are acting as the main contractor and have agreed to provide the complete bathroom, then don't treat waterproofing as somebody else's problem simply because a subcontractor physically applied it.

Make sure the right system is specified. Make sure the background is suitable. Make sure it is installed in accordance with the relevant instructions. Understand the guarantees. Understand your agreement with the subcontractor doing the work.

Keep sensible records. And give the job enough time to be completed properly. Waterproofing is one of those parts of a bathroom that the customer may never see. Done properly, hopefully they never need to think about it again. Done badly, it can become the only thing anybody talks about.