Wet Room Installation Guide

Quick Answer: A wet room is a fully waterproofed, level-access shower area with a floor drain — no tray, no screen frame, just a tanked floor and walls. The critical elements are: a rigid subfloor (minimum 18mm ply on timber; min 65mm screed on concrete), a tanking system applied to all surfaces before tiling, a linear or point waste positioned to create a 1:80 minimum fall to the drain, and a waterproof construction membrane at all joints. BS 5385-4 governs wall and floor tiling in wet areas.

Summary

Wet rooms have moved from a niche specification to one of the most commonly requested bathroom installations in the UK. When executed correctly, they are durable, accessible, and genuinely maintenance-free beyond grout cleaning. When executed poorly, they are expensive disasters: water migrating through inadequate tanking into the subfloor, joists, and the room below; floor drains flooding because the falls are wrong; tile delamination from flexible movement in an inadequately prepared subfloor.

Getting wet room installation right demands attention at every stage, but especially at three points: subfloor preparation (the hardest to fix later), tanking (the most commonly skimped on), and falls setting (the most commonly misjudged). This guide covers each stage in the sequence they must be executed.

For homeowners searching for information on wet rooms: expect to pay £3,000–£8,000 for a professionally installed wet room in a typical UK bathroom, depending on the tanking system, tile specification, and whether the subfloor requires significant reinforcement.

Key Facts

• Level access: the hallmark of a wet room is the absence of a threshold or shower tray lip; provides accessibility for elderly and disabled users; complies with Part M of Building Regulations for accessible bathrooms
• Subfloor requirement: timber floors need 18mm exterior-grade plywood (minimum) to provide a rigid, dimensionally stable base; concrete floors need a flat, sound screed minimum 65mm unbonded
• Tanking systems: liquid tanking (polyurethane or cementitious), sheet membrane systems (Wedi, Schlüter Kerdi), and tile backer board systems; all must be waterproof behind tiles and 500mm up each wall minimum
• Drain position: the drain defines the falls configuration; point drain (circle) allows 1:80 fall from all four sides; linear drain (slot) allows a single-plane fall from one direction; position must be agreed before any screed or backer board work starts
• Fall gradient: minimum 1:80 (12.5mm per metre) to point drains; 1:60 preferred; insufficient fall causes standing water and poor drain performance
• Floor movement joint: a movement joint (silicone, not grout) must be installed at all wall-to-floor junctions, and at intermediate points on floors larger than 3m²; rigid grout here will crack
• BS 5385-4 — British Standard for wall and floor tiling in wet areas; specifies substrate preparation, adhesive class, and movement joint requirements
• Adhesive classification: S2 (highly deformable) adhesive is specified for all wet room tiles; C2 (improved adhesive) minimum; combination C2S2 is standard
• Tile specification: slip resistance minimum R9 for wet room floors (R10 preferred for commercial-adjacent domestic use); frost-proof not required internally but low water absorption tiles preferred (vitrified porcelain <0.5% absorption)
• Drainage: linear drains require a minimum 50mm water seal; some linear drains require a special trap adapter; confirm trap specification matches drain depth available in floor build-up
• Ventilation: Approved Document F requires minimum 15 l/s intermittent extraction or 8 l/s continuous; wet rooms require higher ventilation than standard bathrooms; PIV or mechanical extract essential
• Heating: underfloor heating is the preferred specification under wet room tiles; avoids radiator on wet wall; ensure tile adhesive and grout are rated for UFH service temperatures

Quick Reference Table

Detailed Guidance

Subfloor Preparation — the Foundation of Everything

On a timber floor, the single biggest risk is flex. Any movement in the subfloor — from joist deflection under load — will crack tile adhesive and grout, eventually allowing water ingress below the tanking. The cure is rigidity: add intermediate joists (sister joists) if existing joists are inadequate for the span, fit the ply at 90° to the joists with staggered joints, screw at 150mm centres, and glue the ply to the joists with construction adhesive.

Specific requirements for timber subfloor wet rooms:

• Joists must not deflect more than span/360 under a uniform load; most 50×100mm joists at 400mm centres in a typical bathroom span will pass this test, but check if a heavy tile specification or large wet room is proposed
• 18mm exterior-grade ply (WBP bonded) minimum; 22mm where joist spacing is 600mm
• All joints must be rigid; countersunk screws every 150mm; PVA glue between ply and joist
• Fill any joint gaps flush with a flexible floor levelling compound
• Do not use OSB in wet areas — it is not dimensionally stable under repeated wetting

On a concrete floor, the key concern is levelness and surface contamination. Any oil, paint, or old adhesive must be removed by grinding or shot-blasting. High spots above 5mm must be ground down; low spots filled with floor levelling compound.

Setting Out the Falls

The position of the drain determines everything. Once the drain position is agreed:

1. Identify the high point of the floor (usually the corners farthest from the drain)
2. Calculate the height differential: for a 1.5m × 2m wet room with a central point drain, the corners must be 18.75mm above drain level at minimum (1500mm × 1:80 = 18.75mm)
3. Mark the fall gradient on the subfloor using screed dabs or pre-formed wet room fall inserts (these are now widely available and simplify the screeding process considerably)

If the floor build-up allows it, use a pre-formed wet room former (Wedi, Jackoboard, or similar) — these are manufactured with the correct fall moulded in, eliminating the screeding and levelling stage and providing a waterproof substrate in a single product. They are more expensive than screeding but much faster and more reliable.

Tanking — Applying the Waterproof Membrane

The most critical application rule for all tanking systems: continuous coverage with no pinholes, bridging all junctions with reinforcing tape.

Cementitious tanking slurry:

1. Prime the substrate (use manufacturer's primer; wait for full tack)
2. Apply first coat at 1.5–2mm wet film; work into corners with a brush
3. Apply reinforcing mesh or tape at all joints — floor/wall, wall/wall, around drain surround
4. Allow to dry fully (typically 24 hours; touch dry in 2–4 hours at 20°C)
5. Apply second coat at 1.0–1.5mm wet film; check all corners have double coverage
6. Allow to cure before tiling (typically 24–48 hours; check manufacturer guidance)

Never mix brands of tanking system components — primers, membranes, and tile adhesives from different manufacturers may be chemically incompatible.

Critical junction details:

• Pipe penetrations through the floor: seal with a liquid rubber flashing collar around the pipe; the pipe must pass through the collar without a gap before tiles are applied
• Drain collar: the drain body incorporates a tanking flange; the tanking membrane must bond to this flange with waterproof tape or liquid sealant; this is the most common failure point
• All wall/floor joints: tanking band tape embedded in the membrane at the junction; do not tile into an un-taped joint

Tiling in Wet Rooms

Adhesive: C2S2 adhesive is the specification (C2 = improved adhesive, S2 = highly deformable). This is not optional — standard C1 adhesive on a wet room floor will cause tile delamination within months. Major UK brands (BAL, Mapei, Ardex) all produce a C2S2 wet room adhesive.

Grouting: use a waterproof grout (or apply a grout sealer to cementitious grout after application). Unsanded grout for joints ≤2mm; fine sanded grout for 3–5mm joints; never use standard sanded grout in wet room floor joints wider than 5mm.

Movement joints: apply silicone (not grout) at all wall/floor junctions and at intermediate points in floors larger than 3m². Use a colour-matched silicone — most major tile adhesive suppliers provide matching colours. Apply silicone over fully dried grout, not over uncured adhesive.

Floor tiles: minimum R9 slip rating. Smaller format tiles (20×20 mosaic to 200×200 standard) handle the fall geometry more easily than large format tiles (which require precise levelling to avoid lipping on the fall gradient). Large format tiles (600×600+) can be used but require a skilled tiler with experience in wet room falls — any lipping in the fall direction causes standing water.

Frequently Asked Questions

How long does a wet room installation take?

A typical wet room in an existing bathroom conversion: 5–8 days. Day 1–2: strip-out, subfloor, screed or backer board; Day 2–3: tanking, allow curing; Day 4–6: tiling, grouting; Day 7: sanitaryware, silicone, screen or frame. Allowing adequate curing time between stages — which many installations rush — is the most common shortcut that causes later failures.

Can I waterproof a wet room with just the tiles?

No. Tiles are not waterproof — they absorb moisture at joints and through grout over time. The tanking membrane behind the tiles is the waterproofing. Tiles are a protective and decorative surface layer. Even fully vitrified porcelain grout joints absorb moisture slowly; without a continuous membrane behind, this moisture enters the substrate and structure. Any installation that relies on tiles alone for waterproofing will fail over time.

What is the minimum wet room size?

There is no statutory minimum for a wet room specifically. For practical use, 800mm × 800mm is the functional minimum for an able-bodied adult; 1200mm × 900mm is comfortable. For accessible/wheelchair wet rooms under Part M, a clear floor area of at least 1500mm × 1500mm outside the wet zone is required; the wet zone itself should accommodate a folding shower seat and a 750mm turning space within it. Disabled Facilities Grant (DFG) wet rooms for disabled access are typically 1500mm × 1500mm or larger.

How much does a wet room cost?

For a typical 3m² bathroom converted to a wet room in the UK: £3,000–£5,000 for a mid-spec installation (ceramic tiles, cementitious tanking, standard linear drain, chrome fitments). £5,000–£8,000 for a high-spec installation (large format porcelain, sheet membrane tanking system, flush linear drain, concealed cistern, designer fitments). The subfloor reinforcement and screed/former are the most variable costs — a concrete floor costs £200–£400 for levelling compound; a timber floor requiring joist sistering and ply can cost £600–£1,200 for the structural preparation alone.

Regulations & Standards

• Building Regulations Approved Document F (Ventilation) — minimum 15 l/s intermittent extract for bathrooms; wet rooms warrant mechanical ventilation

• Building Regulations Approved Document M (Access and use) — level-access shower requirements for accessible wet rooms; wheelchair-accessible turning circles

• BS 5385-4 (Wall and floor tiling — Design and installation of ceramic and mosaic tiling in special conditions including swimming pools) — covers wet rooms, shower areas, and swimming pools; specifies adhesive class, tanking, and movement joint requirements

• BS EN ISO 10545-17 — slip resistance classification for ceramic floor tiles; R9/R10 ratings

• Building Regulations Part P — any new electrical circuits (underfloor heating thermostats, fan circuits) in the bathroom are notifiable under Part P

• BS 5385-4 (BSI Group) — the defining technical standard for wet area tiling

• BAL Technical Services — tanking and adhesive specification guidance; free technical helpline

• Schlüter Systems UK — Kerdi membrane and linear drain technical documentation

• Wedi UK — tile backer board and wet room former technical data

• wet room construction including drainage falls and substrate requirements — the complementary tiling-focused reference

• tanking and waterproofing methods for bathroom and wet room tile installations — detailed tanking system comparison

• bathroom floor preparation including levelling compound and backer boards — subfloor preparation steps

• accessible bathroom design including wet room specifications for disabled users — Part M compliance and DFG grants