Painting a Wet Room in London: Waterproof Systems, Tanking and the Right Finishes

Wet Rooms Are Not the Same as Bathrooms

A wet room — a fully open shower area where water falls on tiled or coated surfaces that slope to a drain, with no shower tray or enclosure — presents waterproofing demands that go well beyond a standard bathroom. In a wet room, water contacts the floor and wall surfaces directly and repeatedly. Any coating or paint system must function as part of a waterproof barrier that prevents water reaching the substrate beneath.

In London properties, where wet rooms are often retrofitted into older buildings without a specifically designed structural deck, getting this right is critical. Water that penetrates through failed coatings in a first-floor wet room will damage the ceiling of the room below, rot structural timbers, and create damp conditions that are expensive to remediate. In a leasehold flat, the consequences extend to disputes with the leaseholder below and potential liability claims.

Tanking Comes First

Before any decorative coating or paint is applied in a wet room, the substrate must be tanked — waterproofed with a continuous membrane system applied to all surfaces that will receive water contact. This is a separate trade process that precedes tiling, coating or painting.

The standard tanking systems used in London wet rooms are:

• Liquid-applied waterproof membranes such as BAL Waterproofing Membrane, Mapei Mapelastic, Schluter Kerdi-Board systems or Wedi board. These are applied to the walls and floor substrate (usually tile backer board, cement board or sand-and-cement screed) and cured before any tile adhesive or surface coating is applied
• Sheet membranes bonded to the substrate at all joints and corners, providing a continuous sealed layer

The membrane must be continuous — any gap or unsealed joint is a potential water pathway. Corners and junctions between floor and wall are the highest-risk areas and must receive reinforcing tape bedded into the liquid membrane system.

Painting directly over a wet room wall without tanking the substrate is not a wet room. It is a painted wall that will eventually leak.

Suitable Decorative Coatings and Paints

Where a wet room is tiled, the paint question does not arise on the tiled surfaces themselves — grout is the critical waterproofing element there, and epoxy grout is the correct choice for fully wetted surfaces. The painting specification applies to:

• Ceilings above the wet zone
• Walls adjacent to but not directly in the spray zone (typically at doorways and dry areas of the room)
• Painted panel systems where a tile-free approach has been chosen

For ceilings in wet rooms, the product must be moisture-resistant and mould-resistant. The condensation load in a wet room with no shower enclosure is significant — steam rises continuously during use. Standard bathroom emulsion is insufficiently robust. The correct products are:

• Zinsser Perma-White (available in matt and satin) — a self-priming water-based paint with a ten-year mould-free guarantee. This is one of the most consistently specified products by London decorators in high-humidity environments
• Crown Trade Clean Extreme — available in a range of finishes with strong mould resistance
• Dulux Trade Diamond Matt — the biocide package in this product gives it good mould resistance in steam-heavy environments

All of these products require proper surface preparation: the substrate must be clean, dry, sound and free from existing mould. Any mould present must be treated with a fungicidal wash and allowed to dry before priming.

What to Do About Existing Mould

Wet rooms in London that have been improperly sealed or painted over the years frequently present with significant black mould growth on ceilings and upper walls. The correct sequence:

1. Do not paint over active mould — the mould will continue to grow through any coating applied on top of it
2. Apply a fungicidal wash (Polycell Mould Remover, Ronseal Mould and Mildew Remover or Wykamol Biocide) to all affected areas; allow the full recommended dwell time
3. Remove dead mould by wiping with a damp cloth; do not dry-sand as this releases spores into the air
4. Allow the surface to dry thoroughly; in London's climate this may require a dehumidifier run in the room for 24–48 hours
5. Apply Zinsser BIN shellac-based primer as a stain-blocking coat to prevent mould pigmentation bleeding through subsequent coats
6. Apply two full coats of a mould-resistant topcoat

Ventilation Is Not Optional

The most technically correct paint system will still fail if the wet room has inadequate ventilation. Building Regulations Part F requires extract ventilation from wet rooms at a minimum rate of 15 litres per second. In practice, a wet room without a high-capacity extractor fan running during and for 20 minutes after use will generate condensation levels that exceed the capacity of any coating system to withstand long-term.

Before specifying any decoration in a London wet room, confirm that existing extract ventilation is adequate. If it is not, the client should be advised to upgrade the fan before any painting work is undertaken.

For wet room painting and waterproofing in London, contact us here to discuss your project, or request a free quote for a correct specification.