How to Build a Wet Room: Tanking, Falls, Drainage & BS 5385

Summary

A wet room (open shower area with no enclosure) requires more careful construction than a conventional tray shower because there is no physical barrier containing the water. If the waterproofing fails, water tracks into the substrate, into adjoining walls, or through to the room below — expensive damage that is entirely preventable with correct installation from the outset.

The most common failure mode is inadequate tanking: either the wrong product, insufficient coverage, or a failure at a joint or transition (wall-to-floor junction, pipe penetrations, drain perimeter). The second most common failure is inadequate fall — water that does not drain promptly creates standing pools, slip hazards, and extended contact time that defeats even correctly applied waterproofing.

This guide covers the complete wet room construction sequence for a ground-floor or first-floor domestic bathroom, from substrate preparation through to grouting.

Key Facts

• BS 5385-4 — code of practice for tiling; Part 4 covers waterproof underlays and membranes for tiled areas including wet rooms
• BS 5385-1 — design and installation of natural stone flooring in internal areas; relevant for stone-tiled wet rooms
• Tanking membrane types — brush-applied liquid (BAL, Ardex, Wedi); sheet membrane (Schluter Kerdi, Laticrete); foam board systems (Wedi, Marmox); each has different installation requirements
• Minimum tanking height above floor — 150mm up all walls adjacent to the shower area; full wall height preferred on shower walls
• Floor fall minimum — 1:80 (12.5mm per metre); 1:60 recommended (16.7mm per metre) for comfortable drainage
• Linear drain — allows a single-direction fall across the full floor; easier to achieve than a four-way fall to a central point drain
• Point drain — requires a four-way fall to the central point; more difficult to tile uniformly around the drain
• Pre-made shower trays in wet room — an alternative to a fully tanked screed floor; more reliable waterproofing but less design flexibility
• Substrate suitability — plywood or solid floor boards must be de-flexed (replaced or overlaid with plywood) before tanking; movement in the substrate will crack any rigid tile system
• Underfloor heating in wet room — compatible with wet rooms but requires an electric UFH mat below the tile adhesive; the tanking must be applied over the substrate before the UFH mat
• Slip resistance — floor tiles in a wet area must achieve slip resistance minimum R10 (or B rating per pendulum test); R11 or better for commercial or elderly care settings
• Drain specification — specify a drain rated for the flow rate; typical domestic shower drain 25-40 l/min; verify against actual shower output

Quick Reference Table

Detailed Guidance

Substrate Preparation

The substrate must be:

1. Solid and rigid — no flex; if on a timber floor, the substrate must be solid enough to prevent deflection. Fix plywood subfloor (18-22mm marine or structural grade) securely to joists at 150mm centres. Total thickness above joists should allow the finished floor level to match adjacent rooms.
2. Level in all directions — or deliberately set to the required fall before tanking
3. Clean — free of dust, grease, and loose material

On a timber floor: the floor must be de-flexed. Lay 18mm plywood on top of the existing boards (or replace boards with plywood). Fasten securely. The assembly should not move or creak. Any movement will eventually crack the tile adhesive, grout, and membrane.

On concrete: less of a problem, but check for cracks (inject or fill cracks before tanking) and any surface contamination.

Creating the Fall

The floor must fall consistently to the drain at a gradient of 1:80 minimum (1:60 preferred). Options:

Wet room formers / shower bases: Manufactured polystyrene or foam wet room formers (Wedi Subloor, Schluter Kerdi-Shower-ST, Impey pre-graded trays) have the fall pre-formed. These are by far the easiest way to achieve a consistent, correct fall. Available in standard sizes; can be cut to fit. The former acts as the substrate and waterproofing base simultaneously (if the foam board system) or as a base for the membrane (if separate membrane system).

Sand/cement screed: A dry-packed sand/cement screed (4:1 sand:cement) can be laid to the required fall by an experienced plasterer or tiler. The fall is set using formwork or careful floats from the drain outward. This is traditional but requires skill to achieve a consistent, accurate fall. Allow to cure fully (28 days) before tanking.

Self-levelling compound with integral fall: Some systems use a proprietary levelling compound poured to a former, which creates the fall as it levels. Follow manufacturer's system precisely.

Drain Installation

Point drain (central drain):

• Requires a four-way fall from all directions to the centre
• Tile pattern radiates or steps from the drain outward
• Drain fitting must be incorporated into the tanking membrane before tiling — all connections waterproofed

Linear drain (edge or mid-floor):

• Allows a single-plane fall across the full width of the shower area
• Tiles run parallel and uniform — no cutting around a central drain
• Drain positioned at one edge (usually the far wall from the entrance) or across the middle
• Easier to tile cleanly; increasingly standard in contemporary wet rooms

Drain connection: the drain body connects to the 40mm or 50mm waste pipe before the screed or former is installed. The pipe must be correctly positioned in plan before pour or former placement — it cannot be easily moved afterwards. Ensure the top of the drain body is at the correct height to finish at or just below the finished tile surface.

Tanking System Application

Brush-applied liquid membrane (BAL Waterproof Plus, Ardex S1, Schluter Kerdi-Band + liquid):

1. Ensure substrate is clean and dust-free
2. Apply a primer if specified by the manufacturer
3. Apply reinforcement tape (fibreglass mesh tape) into the first coat at all wall-floor junctions and internal corners — this prevents membrane cracking at movement-prone junctions
4. Apply first coat of membrane with a brush or roller; ensure even coverage; allow to cure as per data sheet (typically 4-6 hours)
5. Apply reinforcement tape at all pipe penetrations (around the pipe) and at the drain perimeter
6. Apply second coat; allow to cure
7. Carry the membrane minimum 150mm up all walls; ideally to the full height of the tile zone on shower walls
8. Allow full cure before tiling (typically 24 hours)

Sheet membrane (Schluter Kerdi):

The Schluter Kerdi system uses a polyethylene membrane with fleece bonding layers. It is embedded in unmodified thin-set tile adhesive and acts as both waterproofing and tile substrate.

1. Apply thin-set to the wall or floor using a notched trowel
2. Embed the Kerdi membrane into the wet thin-set; press firmly and smooth out air pockets
3. Overlap membrane joints by minimum 50mm; embed overlap in thin-set
4. Use Kerdi-Band for internal corners and joints; Kerdi-Seal for drain connection
5. Allow to cure before tiling (24 hours)

Foam board systems (Wedi, Marmox, Jackoboard):

These boards are extruded polystyrene with a fibre-reinforced surface. They are both the substrate and the waterproofing base. Fix boards to the wall or floor with the manufacturer's adhesive/fixing system. Tape all joints with mesh tape and seal with the system sealer. Tile directly onto the boards.

Tiling the Wet Room Floor

Tile selection:

• Minimum R10 slip resistance (DIN 51130); specify in writing to the customer that slip resistance is required
• Maximum 600×600mm tiles on a wet room floor — larger tiles are harder to follow the fall uniformly; 300×300mm or 600×300mm are practical sizes
• Mosaics (50×50mm or 25×25mm): follow the fall naturally; very forgiving; good for irregular shapes
• Grout joint width: 2-5mm on floor; use flexible grout (polymer-modified) in wet areas

Adhesive: use S1 or S2 flexible tile adhesive (BS EN 12004:2007 Type CG2 S1 or S2) in wet rooms; rigid adhesives crack with any movement or thermal cycling.

Grouting: use a waterproof/flexible grout (polymer-modified or epoxy for heavy-use areas). Apply sealer to porous or natural stone tiles. Apply silicone (colour-matched) at all changes of plane (floor-to-wall) — no rigid grout at these junctions.

Frequently Asked Questions

Does my wet room need to be notified to Building Control?

A wet room installation typically does not require Building Control notification in an existing property for the tile and waterproofing work itself. However, if the work involves: moving a soil stack or waste pipe (structural element, potentially notifiable), moving or adding electrical circuits (Part P notifiable), or as part of a larger extension or conversion — then Building Control may be involved. Check with the local authority if in doubt.

Can I tile directly onto plasterboard in a wet room?

Standard plasterboard is not suitable as a tile substrate in a wet room. You must use: cement board (Knauf Aquapanel, HardieBacker), or waterproof foam board (Wedi, Jackoboard), or moisture-resistant plasterboard with a full waterproofing membrane applied. Never tile directly onto standard grey plasterboard in a wet zone.

How long does a wet room last before resealing is needed?

A well-constructed wet room with quality tanking and flexible silicone should not need major resealing for 10-15 years. However, the silicone at floor-to-wall junctions and around the drain does deteriorate and should be inspected and replaced every 5-7 years as preventive maintenance. Grout joints can be re-grouted if staining or deterioration occurs.

What's the minimum size for a wet room shower area?

There is no statutory minimum size, but practically: 900×900mm is the absolute minimum for comfortable use; 1200×900mm is more comfortable; 1200×1200mm or larger is the design standard. The larger the shower area, the more floor area falls in the wet zone and the more critical precise fall geometry becomes.

Regulations & Standards

• BS 5385-4 — wall and floor tiling: design and installation of ceramic and mosaic tiling in specific conditions (includes wet rooms)

• BS 5385-1 — code of practice for the design and installation of ceramic flooring and wall tiling in internal areas

• Building Regulations Approved Document H — drainage connections from the wet room drain

• BS EN 12004 — adhesives for tiles: requirements and classification (S1/S2 flexibility classes)

• DIN 51130 — slip resistance of floors; R9-R13 classification for floor tiles

• Building Regulations Approved Document M — accessible wet rooms; turning circle, threshold, and fitting requirements

• Schluter Systems Technical Library — Kerdi waterproofing system installation guides

• Wedi Technical Documentation — foam board system specifications

• BAL Adhesives Technical Support — liquid membrane application guides

• Tile Association — Wet Room Design Guide — industry best practice for wet room installation

• bathroom ventilation — extract ventilation essential in wet rooms

• waterproofing — full guide to bathroom tanking and waterproofing

• shower types — shower types and their flow rate implications for drain sizing

• bathroom zones — IP ratings and electrical requirements in wet room zones