Wet Room Installation Guide: Tanking, Falls, Drainage and Compliance

Quick Answer: A wet room is a fully waterproofed bathroom where the shower area drains directly to a floor gully without a tray or enclosure. UK installation requires structural sub-floor reinforcement (typically a marine ply or cement board layer), professional tanking to BS 5385-4 / BS 8000-11 (proprietary system: Mapei Mapelastic, Wedi, Schluter Kerdi, etc.), correctly formed falls of minimum 1:50 to a linear or square gully, electrical compliance with BS 7671 bathroom zones (see bathroom zones), mechanical ventilation per Approved Document F (see bathroom extractor fan guide), and tiling to manufacturer-specified systems. Typical professionally installed cost: £6,000-£14,000 for a 3-5m² wet room.

Summary

Wet rooms are popular for the open-plan aesthetic, ease of cleaning, accessibility (DDA / Approved Document M compliance), and modern look. They are also one of the most failure-prone bathroom installations when done badly — a wet room without proper tanking, fall, or drainage will leak progressively into the structure below, with damage often only visible months or years later. The difference between a competent wet room and a leaking-into-the-kitchen disaster is methodology, materials, and attention to detail.

This article covers structural preparation, tanking system selection and installation, floor falls and drainage, tiling considerations, electrical compliance, ventilation, and the typical sequence of works. It is the technical reference for installers; the customer-facing case for wet rooms (accessibility, aesthetic, value) is covered separately in walk in shower installation and accessible bathrooms.

Key Facts

• Tanking — full waterproof envelope of the wet area; usually entire room floor and shower zone walls to 1.8-2m height
• BS 5385-4 — Code of Practice for tiling: wet areas
• BS 8000-11 — Code of Practice for waterproofing in wet rooms
• BS EN 14891 — Standard for liquid-applied waterproofing for use under tiling
• Fall ratio — minimum 1:50 (20mm/m); 1:35 preferred for accessibility (28.5mm/m)
• Linear gully — typically 600mm-1200mm length; modern wet room standard
• Square gully — 100×100mm typical; cheaper but less aesthetic
• Wet area — entire shower zone; many designers extend to whole-floor for full wet room
• Splash zone — within 1.5m of shower head; walls require full tanking
• Sub-floor — must be solid; typically marine ply (18mm+) or cement-bonded board over timber joists; suspended timber needs adequate stiffness
• Concrete sub-floor — must be sound; powerfloated or screed acceptable
• Tray formers — pre-formed sloping plywood/foam sections; speeds fall formation
• Tile bed — usually flexible cement-based adhesive (S1 or S2 deformability class per BS EN 12004)
• Grout — for wet area: epoxy or proprietary water-resistant cement grout
• Approved Document M — accessibility; wet rooms easily comply with level-access requirement
• Approved Document P — electrical work in bathroom is notifiable
• BS 7671 Section 701 — bathroom zones for electrical installation
• Approved Document F — bathroom ventilation; mechanical extract required

Quick Reference Table

Detailed Guidance

Suitability assessment

Before quoting a wet room conversion, check:

1. Sub-floor type — solid concrete is easiest; timber suspended floors need reinforcement assessment
2. Joist depth and span — for timber floors, joists must be adequate (typically C24 grade, 200mm+ deep for 4m+ spans); a structural engineer can confirm
3. Floor-to-ceiling height — fall formation needs 30-50mm extra height in shower area; check that this doesn't compromise headroom
4. Drain run — gully needs trapped connection to soil stack; access for plumbing must be feasible
5. Position in property — first floor wet rooms over kitchens are highest risk; consider ground floor or above non-critical spaces
6. Building Regulations approval — significant works require Building Notice or Full Plans; structural changes need approval

Sub-floor preparation — timber

Timber suspended floors need stiffening to prevent flex (flex cracks tanking and tile bed):

1. Inspect joists; replace any rotten or split
2. Add noggins between joists at 400-600mm centres for additional stiffness
3. Fix 18mm marine ply or WBP exterior ply on top of joists, glued and screwed
4. Add cement tile backer board (Hardibacker, Aquapanel, NoMorePly) on top, screwed at 200mm centres
5. Or: fit 22mm tile-backer direct on joists with adequate joist depth
6. Tape and seal all joints with manufacturer's reinforcing mesh and sealant

The result: a flex-free, water-resistant substrate.

Sub-floor preparation — concrete

For concrete:

1. Inspect for cracks, hollow areas, dampness
2. Address any DPM issues; concrete must be dry
3. Self-levelling compound if surface is uneven
4. Prime with manufacturer-specified primer
5. Form fall (next section)

Forming the fall

Two methods:

Method 1: Tray former (pre-fabricated)

• Plywood or foam tray-former with built-in fall and gully cut-out
• Set into the floor (recess sub-floor or build up surrounding floor)
• Fit gully through the tray former
• Tanking applied over former and floor

Method 2: Site-formed screed

• Mark fall lines from perimeter to gully
• Lay sand-cement screed (1:3 or 1:4) tapered to gully
• Fall 1:50 minimum (20mm drop per metre)
• Allow screed to cure (28 days for full strength but tile-ready in 7-14 days with rapid-set additive)
• Apply primer
• Tanking applied over screed

Tray former is faster; site-formed screed is more flexible for awkward shapes.

Tanking — sheet membrane method (Schluter Kerdi example)

1. Prime substrate as manufacturer specifies
2. Apply unmodified thinset adhesive in 4-6mm bed
3. Roll Kerdi sheet into adhesive, smoothing with trowel
4. Overlap sheet edges by 50mm minimum; seal with Kerdi-Band tape and thinset
5. At wall-floor junctions, use Kerdi-Band internal corner pieces
6. At gully, use Kerdi-Drain or proprietary drain flange
7. At pipe penetrations, use Kerdi-Seal-MV mixing valve seals
8. At external corners, fold and tape
9. Flood test for 24 hours before tiling

Tanking — liquid applied method (Mapei Mapelastic example)

1. Prime substrate
2. Apply first coat by brush or roller; ensure full coverage, no pinholes
3. While first coat is wet, embed reinforcing mesh (Mapenet 150 or similar) at all junctions and corners
4. Allow first coat to dry (4-12 hours depending on conditions)
5. Apply second coat at 90° to first coat
6. Achieve total dry film thickness specified (typically 2-3mm)
7. Allow full cure before tiling (24-48 hrs typically)
8. Flood test if possible

Drainage and gully selection

Linear gullies (Geberit, Aquatech, Wedi) — 600mm-1200mm length, tile-able tops, modern look. Cost: £200-£600 for the gully + bonding kit.

Square gullies — 100×100mm or 150×150mm, traditional. Cheaper at £40-£150 but less aesthetic.

Both types need:

• 50mm trap minimum
• Vented (open soil vent or air admittance valve)
• Connection to soil stack or appropriate waste route
• Removable strainer for cleaning

Build the gully into the tanking system per manufacturer's flange and tape detail.

Tile selection

For wet room floors:

• Slip resistance — R10 or R11 (BS 7976)
• Size — smaller tiles (100-200mm or mosaic) make falls easier to form; large format possible with careful setting
• Material — porcelain (impermeable) preferred over ceramic; natural stone needs sealing and high maintenance
• Frost resistance — only relevant for external; check porcelain absorption

For wet room walls:

• Any porcelain or ceramic suitable for shower environment
• Large format acceptable
• Match floor / contrast for design

Tile adhesive and grout

• Adhesive — S1 or S2 deformability class flexible cement (BS EN 12004 C2 minimum); typical: Mapei Keraflex Maxi S1, BAL Single Part Flexible, Ardex X 78
• Floor adhesive — must be suitable for wet area and exterior grade if shower
• Grout — water-resistant cement grout (BAL Wide Joint, Mapei Ultracolor Plus) or epoxy (Mapei Kerapoxy, BAL Easypoxy) for premium installations
• Silicone — sanitary-grade silicone with anti-mould additive at all changes of plane (wall-floor, internal corners)

Electrical compliance

Bathroom electrical work is subject to BS 7671 Section 701 (bathroom zones) and notifiable under Part P:

• Zone 0 (inside bath/shower): IPX7; SELV only (12V or 30V)
• Zone 1 (above showering area to 2.25m): IPX4; SELV or low-voltage with RCD
• Zone 2 (around bath/shower): IPX4; suitable shaver socket allowed
• Outside zones: standard installation but always RCD-protected
• 30mA RCD required for all bathroom circuits

Wet room considerations:

• Underfloor heating: separate circuit; matrix or wire system rated for wet area
• Lighting: IPX4 minimum in shower zone; recessed downlights with appropriate IP
• Extractor fan: low-voltage SELV preferred for in-zone; high-IP if in zone 1/2

All electrical work in a wet room is notifiable. Use a registered competent person (NICEIC, NAPIT, ELECSA) or notify Building Control.

Ventilation

Mechanical extract ventilation is mandatory for any room with a bath or shower (Approved Document F):

• Minimum 15 L/s intermittent or 8 L/s continuous extract
• Located high in the room, ideally near the shower
• Ducted to external air (not into roof void)
• Backdraught shutter
• 15-minute overrun timer (most installs)

See bathroom extractor fan guide for detail.

Sequence of works

Typical professional installation order:

1. Strip existing bathroom; remove old finishes; cap pipework
2. Move/install soil stack and waste runs
3. First fix plumbing (hot/cold supplies, shower valve, basin, toilet)
4. First fix electrical (lighting, fan, towel rail, underfloor heating if specified)
5. Sub-floor preparation (reinforcement, tile backer)
6. Form fall (tray former or screed); install gully
7. Plumb gully; test waste runs
8. Apply tanking system
9. Flood test or visual inspection of tanking
10. Tile floor
11. Tile walls
12. Grout
13. Silicone all junctions
14. Second fix plumbing (taps, shower head, basin, toilet, bath if any)
15. Second fix electrical (sockets, switches, lighting fixtures, fan, towel rail)
16. Commission and test all services
17. Hand over with O&M information

Common failure points

• Tanking film thickness too thin
• Tanking pinholes (apply two coats minimum, at 90° crossover)
• Gully flange not properly sealed into tanking
• Movement at joist-supported floor cracking tile bed
• Fall insufficient — water ponds and leaks at perimeter
• Wall-floor junction silicone failure (use sanitary silicone, apply after grout)
• Pipe penetrations not properly sealed
• Tile grout choice (cement grout in heavily wet area without sealing)
• Failure to flood-test before tiling

Frequently Asked Questions

Can a wet room be installed on a first floor over a kitchen?

Possible but high risk. Any tanking failure leaks directly into the ceiling below — typically the kitchen. Strongly recommend ground floor or over non-critical spaces. If on a first floor, insist on highest-quality tanking (sheet membrane), structural reinforcement of sub-floor, flood testing before tiling, and very careful workmanship. Customers should weigh the consequences.

Is a wet room cheaper than a tiled shower with tray?

No — a wet room costs 30-100% more than a tiled shower with quality tray. The waterproofing and falls add labour, materials and risk. Wet rooms are chosen for design and accessibility reasons, not cost.

Do wet rooms add resale value?

Some research suggests modest value addition (£3,000-£7,000 for a quality wet room in a UK home), but the bigger benefit is broader market appeal — easy-clean modern aesthetic. A leaking wet room destroys value. Quality matters.

Can the floor be heated in a wet room?

Yes. Electric underfloor heating matrix (Ahmed, Devi, Warmup) integrated under the tile bed is common. Wet underfloor heating (pipework) is more complex but possible with appropriate manifolds and screed depth. UFH is on a separate circuit, must be Part P compliant, and adds 10-25mm to floor build-up.

What's the warranty on tanking?

Most proprietary systems offer 10-25 year manufacturer warranty on the material, with conditions:

• Installed by a competent installer (often manufacturer-trained/registered)
• Used in the manufacturer's complete system (membrane + adhesives + grouts + sealants)
• Within specified conditions (temperature, exposure)

DIY-installed tanking generally has no warranty. For customer peace of mind, use a registered installer of the chosen system.

Regulations & Standards

• Approved Document A — Structure (sub-floor strengthening)

• Approved Document C — Site preparation and resistance to moisture

• Approved Document F — Ventilation

• Approved Document M — Accessibility (wet rooms for level access)

• Approved Document P — Electrical safety

• Approved Document G — Sanitation, hot water safety, water efficiency

• BS 5385-4 — Code of practice for tile fixing: wet areas

• BS 8000-11 — Code of practice for waterproofing of wet rooms

• BS EN 14891 — Liquid-applied waterproofing

• BS EN 12004 — Tile adhesives classification

• BS EN 13888 — Tile grouts

• BS 7671 — Wiring regulations; Section 701 bathroom zones

• BS 7976 — Pendulum testing for slip resistance

• Schluter UK technical guides — tanking system reference

• Mapei UK product data sheets — Mapelastic tanking

• BAL Adhesives — tanking and adhesives technical guidance

• Wedi system — XPS-panel waterproofing

• Approved Document F (Ventilation)

• BSI BS 5385-4

• walk in shower installation — walk-in shower as alternative

• bathroom waterproofing tanking — tanking specifics

• wetroom construction — wet room construction reference

• bathroom extractor fan guide — mandatory ventilation

• bathroom zones — electrical compliance in bathrooms