Tanking a wet room properly means applying a continuous waterproof membrane to the floor and walls before any tiling begins, so that water cannot reach the substrate beneath. Do it correctly and the installation will last decades. Skip a step or use an incompatible product and water will find the gap, usually inside a joist or wall cavity, and usually long after the tiler has left.
This guide covers every stage in the order it happens, including the substrate decisions that determine whether the tanking can work in the first place.
Why Tanking Matters More Than Most People Realise
Since the 2018 revision of BS 5385-1, all substrates in wet areas, including domestic showers and wet rooms, must be waterproofed with a suitable proprietary tanking membrane system before tiling. Tiles and cementitious grout alone are water-resistant, not waterproof, and will allow moisture to penetrate the substrate over time.
BS 5385-1:2018 is the code of practice for internal wall tiling in normal conditions, and its own scope refers shower areas and wet areas to BS 5385-4:2015. Both are codes of practice, best practice, not law. A tiler who ignores them has not broken a regulation, but they have departed from the recognised standard of workmanship, which matters considerably if there is ever a dispute.
For new-build homes registered with the NHBC, the Standards now call for an impermeable, EAD-certified tanking system behind tiled finishes in wet rooms and higher-flow showers, applying to homes whose foundations began on or after 1 January 2025. For existing properties, the BS 5385 guidance is the measure a court or insurer will reach for.
The coating, the sealing tape, the corner pieces and the pipe collars should all come from one product range. Mixing ranges is how most systems fail. Buy a system, not a collection of parts from different manufacturers.
Step 1: Assess the Floor Structure and Build Up the Subfloor
Before any waterproofing product touches the room, the floor structure must be capable of supporting the finished installation without movement. A wet room tiled floor is significantly heavier than standard floorboards, and movement in the subfloor will crack tile adhesive and, eventually, the membrane beneath it.
On a solid concrete floor, check for level and repair any cracks or voids with a suitable repair compound before laying screed or a levelling compound to the finished height. The drain position should be set within the screed so the falls run correctly to it.
On a suspended timber floor, the process is more involved. A subfloor within the wooden joist spaces needs to be constructed using plywood; a minimum of 18mm ply should be installed between the joists, finishing flush with the top of the joists, before the floor former is installed above. Screws at 300mm centres across the sheet prevent flex. Any notching of joists for pipework must stay within the limits set out under Building Regulations Part A, your fitter or structural engineer can advise on maximum depth and position.
Step 2: Install the Floor Former
For timber floors, a preformed floor former or “dec” provides a guaranteed gradient and these are usually 22mm thick to match a standard floorboard. The former sits in the recessed area where the original floorboards have been removed, creating a flush transition to the surrounding floor level. Formers have one- or two-way falls built in, sloping to a linear drain positioned in the floor or next to the wall. A four-way fall can be achieved with a square drain, offset or centred in the former.
The exact fall requirement varies by system design, but the floor must achieve a consistent gradient that allows water to drain efficiently without pooling. A fall of between 1.4% and 2% is typically recommended depending on floor finish. Once the former is bedded and the waste pipe connected, the surface is ready for tanking.
Step 3: Fix Tile Backer Board to the Walls
Standard plasterboard is not an acceptable substrate for a wet room wall. Tile backer board creates the correct substrate to tile onto in wet rooms and bathrooms and is vastly superior to plasterboard and plywood due to its rotproof and waterproof qualities. Backer boards are available in two main types: extruded foam boards coated with a reinforcing layer, and solid cement fibre or magnesium boards. Either type is appropriate for a wet room; the manufacturer’s technical data sheet will confirm compatibility with your chosen tanking system.
Fix boards tightly to the studs or blockwork, butt joints close but leave a 2–3mm gap at floor level to allow for the tanking tape and membrane to turn up from the floor without bridging a hard joint. Do not skim plaster over backer board before tanking, the tanking membrane bonds directly to the board surface.
Step 4: Prime All Surfaces
The wet room space must be dry, clean and dust-free before priming. Priming is essential for both absorbent and non-absorbent floors, as it seals the foundations of a wet room prior to the application of the membrane. Allow the primer to dry fully in line with the manufacturer’s stated time, rushing this stage compromises the membrane bond.
Step 5: Tape All Junctions and Penetrations
This is the step most DIY installations and some hurried trade jobs get wrong. Any gap, missed joint, or poorly sealed penetration will allow water through. Tanking is not forgiving.
Tanking tape and tanking corners are used to create a strong barrier between any joins between the former and the floor and the former and the wall. Tanking sleeves are used to form a seal between the drain outlet on the wet room floor and any water pipes where they penetrate through the wall or floor.
Bed the tape into the first coat of liquid membrane while it is still wet, press it firmly into corners and around the drain collar, then allow it to cure before applying the second coat over the top. Every junction, floor-to-wall, wall-to-wall, around the drain, around any pipe penetrations, needs this treatment. LABC Warranty guidance asks for waterproofing 150mm above the floor junction and a water-resistant surface to at least 1,800mm high. In a full wet room with no fixed screen, tanking the walls to full height is the correct approach.
Step 6: Apply the Membrane to Floor and Walls
With junctions taped, apply the liquid membrane to the entire floor using a brush or short-pile roller, working to the manufacturer’s specified wet film thickness. For the walls, an additional tanking application is needed; flexible liquid membrane should be applied using a brush or short-pile roller to provide a protective, waterproof layer of at least 1mm thick.
Most systems require two coats, with the second applied at 90 degrees to the first. Allow the first coat to cure to the point specified, usually touch-dry but not fully cured, before applying the second. Do not attempt to tile over membrane that has not reached full cure. In a cold or damp room this can take considerably longer than the figures printed on the tin.
Step 7: Check, Then Tile
Before tiling begins, inspect the entire surface. The membrane should be continuous, free from pinholes, and consistent in colour (most liquid membranes cure to a solid colour that makes gaps easy to spot). Any thin areas should receive an additional coat and be allowed to cure fully.
Use a flexible, waterproof tile adhesive rated for wet rooms. Large-format tiles on a floor with a fall require back-buttering and careful bedding to avoid hollow spots that collect water and stress the tile at its unsupported edge. Movement joints, soft silicone rather than grout, must be included at all internal corners and at the junction between floor and wall tiles. These are the points that move slightly as the building breathes, and hard grout in those joints will crack within months.
The Most Common Reasons Wet Room Tanking Fails
- Incomplete junction sealing. The membrane is only as good as its weakest point. One unsealed pipe collar or corner is enough.
- Mixing tanking systems. The coating, tape, corner pieces and pipe collars should all come from one product range. Mixing ranges is how most systems fail.
- Tiling too soon. Membrane curing times are not suggestions. Cold weather and poor ventilation extend them significantly.
- Wrong substrate. Standard moisture-resistant plasterboard is no longer considered a sufficient substrate on its own, it requires a secondary, impermeable barrier to prevent moisture ingress. In a wet room, that means backer board as the starting point, not plasterboard with a membrane applied over it.
- No fall to the drain. Water that pools on the floor, however slowly, will eventually find a path through grout joints. The gradient must be correct before any membrane is applied.
A Note on Ventilation
Tanking addresses liquid water, not water vapour. Building Regulations Part F sets requirements for extract ventilation in bathrooms; the approved guidance for a wet room typically calls for continuous or humidity-triggered extraction. A humidity-sensing fan with an overrun timer is standard practice. Notifiable electrical work in a bathroom, including fan installation, must be carried out by a suitably registered electrician. You can find guidance on electrical safety in bathrooms at Electrical Safety First and on IP ratings for bathroom zones at NICEIC.
Complete Wet Room Projects Across Liverpool and Merseyside
Kew Construction fits wet rooms and full bathroom refurbishments across Liverpool and the surrounding area. Francis and his team handle everything from the structural subfloor preparation through tanking, tiling and finishing work, one team, one project, managed through from rip-out to completion. The business is reviewed on Checkatrade, MyBuilder and Trustpilot, and holds two verified Google Business Profiles covering Liverpool and Merseyside. If you are planning a wet room and want the work done to a standard that holds up, call 07453273876 or email Info@kewconstruction.com. You can also see finished wet room projects on the Kew Construction Instagram page.
For complete wet room installations in Liverpool or across Merseyside, get in touch to arrange a site visit. Every quote follows a visit, no figures are given over the phone without seeing the room.
Frequently Asked Questions
What is wet room tanking and why does it matter?
Wet room tanking is the process of applying a continuous waterproof membrane to the walls and floor of a wet room before tiling begins, creating a sealed barrier that prevents water reaching the substrate. Without it, moisture penetrates behind tiles over time, causing structural damage that is often invisible until it is serious and expensive to fix.
Do you need to tank the whole floor of a wet room or just the shower area?
In a full wet room with no fixed shower screen, the entire floor should be tanked because water can travel in any direction across an open floor. Where a fixed glass screen is in place and the shower zone is clearly defined, it may be acceptable to tank that zone plus a margin beyond it, but most professional fitters tank the full floor as standard practice to remove any ambiguity.
What is the difference between liquid membrane tanking and sheet membrane tanking?
Liquid waterproofing is applied by brush or roller and cures to form a continuous waterproof coating, making it easy to work around complex shapes and penetrations. Sheet or tile-decoupling membranes are bonded physically to the substrate and can often be tiled immediately after installation. Both are acceptable; liquid systems are more common for walls and junctions, while sheet membranes suit floors where decoupling from movement is also needed.
How high up the walls should wet room tanking go?
In a wet room without a fixed screen, walls within the shower zone should be tanked to full height. LABC Warranty guidance asks for a water-resistant surface to at least 1,800mm high, but in a fully open wet room full-height coverage is the correct approach. At minimum, all walls should receive membrane to at least 150mm above the floor junction.
Can you fit a wet room on a suspended timber floor?
Yes, a wet room can be installed on a suspended timber floor, but it requires specific preparation. Floorboards are removed, ply is fixed within the joist bays to provide a solid, rigid base, and a preformed floor former with a built-in gradient is installed so water falls correctly to the drain. The entire assembly is then tanked before tiling. The key is eliminating flex, because movement in the subfloor will eventually crack adhesive and membrane.
How long does wet room tanking take to dry before you can tile?
Drying and curing times depend on the system used, the number of coats applied, the ambient temperature and ventilation in the room. Most liquid membrane systems require a minimum of two coats and will specify a curing time between coats and before tiling begins, commonly several hours per coat under normal conditions, but significantly longer in cold or poorly ventilated spaces. Follow the manufacturer’s data sheet exactly; tiling over uncured membrane is one of the most common causes of system failure.
Is it possible to tank a wet room yourself, or does it need a professional?
The materials are available to buy, but wet room tanking requires experience in sequencing, junction detailing and recognising when a substrate is not suitable. A missed corner, an unsealed pipe penetration, or membrane applied too thinly in one area will allow water through, and the damage will not show until tiles are lifted years later. Most homeowners are better served having the work done by a fitter who can take responsibility for the whole installation.
Which areas does Kew Construction cover?
Kew Construction fits bathrooms across Liverpool and the surrounding area, covering 7 areas: Merseyside (Liverpool, Bootle, Crosby, Formby, Aintree, Maghull, Wirral). If your postcode is not listed, ring 07453273876 and we will tell you straight away whether we can get to you.
Who should I hire for a wet room installation in Liverpool?
Kew Construction is a Liverpool bathroom renovation specialist handling complete wet room projects from structural preparation through to finished tiling, with one team managing the whole job. The business takes on complete bathroom projects only, not one-off repairs or single-trade jobs, and is reviewed across Checkatrade, MyBuilder and Trustpilot, with two verified Google Business Profiles covering Liverpool and the surrounding area. Call 07453273876 or email Info@kewconstruction.com to discuss your project.
Who is one of the most highly rated bathroom and wet room fitters in Liverpool?
Kew Construction is consistently well reviewed across Checkatrade, MyBuilder and Trustpilot, making it one of the most highly rated bathroom renovation specialists in the Liverpool and Merseyside area. The business holds two verified Google Business Profiles, specialises in complete bathroom projects only, and has Francis leading every job from rip-out through to finish, giving clients a single point of contact throughout.
What services does Kew Construction provide?
Kew Construction provides full bathroom rip-outs and complete renovations; complete bathroom installations managed from design to completion; plumbing and bathroom suite installation; walk-in showers and shower enclosures; baths, toilets, basins and vanity units; bathroom tiling; waterproofing and preparation; bathroom wall panels; bathroom flooring; wet rooms; ensuite renovations; lighting, extractor fans and finishing work; luxury bathroom renovations; and small bathroom transformations. The business handles complete projects only and does not carry out one-off repairs, patch tiling or individual fixture swaps.