Wet Room Tanking and Waterproofing Explained

Tanking is a continuous waterproof membrane system applied to the floor and walls before tiling, designed to stop water reaching the substrate beneath. It usually takes the form of either a liquid membrane, painted on in coats, or a sheet membrane, bonded down with adhesive, sometimes a combination of both depending on the detail.

The tiles and grout you see afterwards are not the waterproofing, they're a decorative and hard-wearing surface. Grout in particular is porous and will let water through over time, so if the membrane beneath it has gaps, water will eventually find them.

The overwhelming majority of wet room leaks come from a small number of predictable spots: internal corners where the membrane wasn't properly reinforced with tape or a corner fillet, the join between floor and wall, pipe penetrations for the shower valve and waste, and the upstand around the drain itself.

Flat, uninterrupted areas of floor and wall rarely fail, it's always the joints, corners and penetrations. This is why a rushed tanking job, done by someone unfamiliar with the specific product being used, is a false economy that can cost far more to fix later than it saved at the time.

Most systems start with a primer to help the membrane bond to the substrate, whether that's plywood, tile backer board, or screed. The membrane itself is then applied in at least two coats for liquid systems, with a reinforcing tape or fabric bedded into internal corners, around the drain, and at any wall-floor junctions.

Manufacturers specify exact coverage rates and drying times between coats, and cutting these corners, applying a second coat before the first has cured, for instance, weakens the finished membrane even though it looks fine on the day. This is one of the reasons DIY tanking jobs, or ones rushed to hit a tight schedule, tend to be where problems show up.

The floor-to-wall junction needs the membrane taken up the wall to at least 200mm generally, and considerably higher, often 1800mm, around the shower area itself to cope with splash. At doorways and thresholds, the tanking needs to be detailed so water can't track underneath the door frame or skirting into the adjoining room, particularly important where a wet room sits next to a bedroom or landing on a suspended timber floor.

Getting this threshold detail right is one of the trickier parts of the job and is where a lot of DIY and inexperienced-fitter tanking comes unstuck, because it's easy to focus on the visible floor area and treat the doorway as an afterthought.

A trustworthy quote will name the tanking system being used, whether liquid or sheet, and should include reinforcing tape at corners and joints as standard, not as an extra. Ask specifically how the shower valve penetration and drain upstand will be detailed, since a fitter who can answer this clearly has done it properly many times before.

A written guarantee covering the waterproofing installation gives you recourse if anything does go wrong, which is one reason to work with a team that carries out plumbing, electrics and tiling in house, so there's a single point of accountability rather than several trades each blaming another.

Once the membrane has fully cured, a proper installation includes a flood test, filling the floor area with a shallow depth of water and leaving it for a set period, usually 24 hours, to check for any drop in level that would indicate a leak. This happens before tiling begins, because it's vastly cheaper to fix a fault at this stage than to lift tiles later.

Not every fitter does this, and it takes a day out of the programme, but it's a genuinely important checkpoint. If a quote's timescale doesn't seem to allow any time for this, it's worth asking directly whether a flood test is included.