Wet Rooms in London Renovations: Design, Waterproofing, and Specification

A wet room is a bathroom where the shower area is not enclosed by a screen or contained by a shower tray. The entire floor — or a defined shower zone — is waterproofed and drained, and water falls directly onto the tiled or stone surface. Done correctly, it is the most elegant of all shower specifications: generous, architectural, easy to clean, and visually open. Done incorrectly — with inadequate tanking, poor falls, or an undersized drain — it leaks, floods, and creates one of the most expensive remediation jobs in residential renovation.

This guide covers what a properly specified wet room requires.

The floor: falls and drainage

The single most critical technical requirement of a wet room floor is adequate fall to the drain. Water that does not flow to the drain pools on the floor, creates standing water, and in a carpeted or timber adjacent space, causes water damage.

Required fall: Building regulations and tile industry guidance recommend a minimum fall of 1:80 (12.5mm per metre) in the shower zone. For a comfortable and fast-draining wet room, 1:60 (16mm per metre) is better. In a small shower zone (1m × 1m), this requires only 12–16mm of fall at the perimeter.

Achieving the fall: The fall must be created in the substrate — typically either a wet mortar bed (a mix of sharp sand and cement, floated to the required fall) or a pre-formed shower former (a sloped foam or compressed mineral board base designed to accept tiling). The fall cannot be created in the tile adhesive layer — adhesive beds are typically 6–12mm and cannot reliably carry a drainage fall.

Drain specification: The drain must be sized for the flow rate from the showerhead. A standard shower valve with a flow rate of 15–20 litres per minute requires a drain with a minimum flow capacity exceeding this. Linear drains (a slot drain running across the width of the shower) are typically more hydraulically efficient than point drains and give a cleaner aesthetic.

The drain must be accessible for cleaning. A removable grating or cover over the trap access is essential — hair and debris accumulate in shower drains, and a trap that cannot be cleared becomes an odour problem.

Waterproofing: the non-negotiable requirement

A wet room floor and walls must be fully waterproofed before any tiles or stone are applied. This is not optional and is not substitutable with water-resistant plasterboard or standard tile adhesive. The waterproofing must form a continuous, unbroken membrane across the entire shower zone — floor and walls, into the wall face to at least 1,800mm height (200mm above the showerhead).

Standard waterproofing systems:

*Liquid-applied membranes* (Mapei Mapelastic, Schlüter Kerdi-DS, Wedi): A liquid polymer compound applied by brush or roller in two coats, forming a seamless waterproof layer over the substrate. This is the most commonly specified system. The corners and junctions must be reinforced with waterproof tape bedded into the first coat. The system must extend up the wall and around any penetrations (drain outlet, pipe penetrations).

*Sheet membranes* (Schlüter Kerdi sheet, Wedi board): Pre-formed sheet or board products that create the waterproof layer directly. Wedi board (a foamed glass mineral board with a mortar face) is particularly robust — it is fully waterproof throughout its thickness, not just at the surface. Kerdi sheet is thinner, bonded over plasterboard or concrete substrate.

*Cementitious tanking* (Weber waterproof, Sika Tite): A cementitious powder mixed to a slurry and trowel-applied. More appropriate for larger wet areas (bathrooms, pool surrounds) than as the primary system for a shower zone.

The drain junction: The junction between the drain flange and the waterproof membrane is the most critical detail. The drain must have an integrated bonding flange (Schlüter Kerdi-Drain, ACO ShowerPoint) that allows the waterproof membrane to bond directly to the drain body. A drain without a bonding flange, where the membrane simply laps over the edge of the drain, is a leak waiting to happen.

Substrate requirements

The shower zone substrate must be:

• —Rigid: any deflection in the substrate will crack tile and grout joints, breaking the waterproof layer. Timber floors must be doubled up (two layers of flooring-grade plywood, screws at 150mm centres) to achieve adequate rigidity. In a bathroom on an upper floor, this often means a structural assessment.
• —Flat: large-format tiles (300mm and above) require a substrate flat to SR2 standard (maximum 3mm deviation under 2m straightedge). Tile adhesive can accommodate minor variations but not gross unevenness.
• —Dry: new screed must be dried to the manufacturer's moisture tolerance before waterproofing — typically below 75% RH (sand-cement screed) or 65% RH (calcium sulphate screed). Waterproofing over damp screed traps moisture and causes adhesive failure.

Floor finish in a wet room

Large-format stone or porcelain: The standard specification. The larger the tile, the fewer grout joints, and the cleaner the aesthetic. However, large-format tiles in a shower zone require very precise falls — a 600×600mm tile must be laid perfectly to the required fall, and any lippage between adjacent tiles creates a trip hazard and a water-pooling point.

Mosaic: 50×50mm or 25×25mm mosaic tiles naturally accommodate the drainage fall and create a non-slip texture. The very high number of grout joints makes cleaning more demanding than large-format, but the flexibility makes installation over a shaped former much simpler.

Slip resistance: Any floor tile in a wet room must have a wet slip resistance appropriate for barefoot use. The R-rating system (DIN 51130) rates tiles for slip resistance under wet conditions: R9 is the minimum for wet rooms; R10 or R11 gives better grip. Large-format polished stone (e.g. polished Calacatta marble) in a shower zone is a slip hazard and should be avoided or used only with a non-slip treatment.

The shower valve and controls

A wet room with an open shower zone requires a thermostatic shower valve — not a simple manual pressure valve. A thermostatic valve maintains a constant outlet temperature regardless of fluctuations in supply temperature, preventing scalding when another tap or appliance draws hot water elsewhere in the building.

Position the controls where they can be reached from outside the shower zone — so the water can be started and temperature adjusted before the user steps in.

Realistic costs

These figures include all wet trades (waterproofing, tiling, plumbing) and finishes but exclude sanitary ware, brassware, and mirrors.

ASAAN specifies and builds wet rooms as part of bathroom and whole-property renovation programmes. Our team coordinates substrate preparation, waterproofing system selection, tiling, and plumbing within a single programme.