How Should a Bathroom or Wet Room Be Waterproofed Before Tiling?

Summary

Waterproofing in bathrooms is one of the most frequently underspecified aspects of tiling work in the UK. Many tilers and bathroom fitters do not routinely apply any waterproofing membrane before tiling, relying instead on the tile and grout to act as the waterproof layer. In low-moisture areas (tiled bath surrounds with a sealed grout joint at the bath rim), this can work adequately for many years. In high-moisture areas — shower enclosures, wet rooms, and spa-style bathrooms — it leads to water penetrating behind tiles, saturating the substrate, and eventually causing tiles to delaminate, plaster to fall away, and structural damage to the floor or ceiling below.

The tile and grout system is not a waterproof membrane. Grout is permeable; tiles crack; silicone sealants at joints eventually fail. The waterproofing must be under the tiles, applied to the substrate, before any tile adhesive is applied. This is the approach recommended by BS 5385-1 and is the standard across Europe and Australasia.

This article covers the main waterproofing systems available for domestic bathrooms and shower rooms, their application requirements, and the key details that make the difference between a lasting installation and a premature failure.

Key Facts

• BS 5385-1:2009 — wall and floor tiling; code of practice for the design and installation of internal ceramic and natural stone wall tiling in normal conditions; recommends tanking of wet areas
• BAL WP1 — a ready-mixed acrylic tanking slurry from British Standard products; one of the most widely used domestic tanking systems; applied by brush or roller; two coats required; reinforcement tape at corners and junctions
• Mapei Mapelastic — two-component polymer-modified cementitious tanking membrane; higher specification than BAL WP1; flexible; used in commercial and high-specification domestic projects; applied with notched trowel
• Schluter Kerdi — a fleece-faced waterproofing membrane sheet applied in the tile adhesive layer; widely used in the USA; increasingly common in the UK for shower enclosures; bonds into thin-bed adhesive; joints overlapped 50mm
• Wedi board — rigid waterproof tile backer board with integral facing; the board itself is waterproof (extruded polystyrene core with glass fibre mesh faces); joints between boards taped with Wedi joint sealant; no separate tanking required if installed correctly
• Curing time — most liquid-applied membranes require 24 hours minimum drying at 20°C, 50% RH; cold or damp conditions extend curing time; do not tile before membrane has fully cured (turns from dark grey/blue to light grey/transparent)
• Joint tape — fibreglass reinforcement tape must be embedded in the first coat of membrane at all internal corners, at the wall/floor junction, and at any movement joints; tape prevents cracking at stress concentrations
• Coverage — BAL WP1 applied at 0.3 kg/m² per coat gives approximately 3.3 m²/kg per coat; a 3.5kg tub covers approximately 11m² in two coats; check product data sheet
• Silicone sealant — at the final face (in the tile surface), all internal corners in wet areas and the joint between tiles and the bath or shower tray rim must be sealed with a movement-accommodating silicone (not grout); silicone to BS EN ISO 11600 classification F 25E or equivalent
• Independence of inspection — for high-specification or commercial projects, the waterproofing should be independently inspected (flood tested or inspected before tiling) to confirm continuity before being concealed by tiles

Quick Reference Table

Detailed Guidance

When Is Tanking Required?

Tanking (applying a waterproofing membrane before tiling) is required, or strongly recommended, in the following areas:

Mandatory (where the risk of water damage to the structure or to rooms below is high):

• Wet room floors (no shower tray; direct drainage into the floor)
• Shower enclosures in upstairs bathrooms, particularly where there is a habitable room below
• Any shower or bath area where the substrate is plaster, plasterboard, or timber-based board

Strongly recommended:

• Shower enclosures at ground floor level over concrete slab
• Bath surround tiles adjacent to the bath rim (particularly at the rim/tile junction)
• Entire bathroom floor in a wet room or walk-in shower configuration

Optional (low-risk areas):

• Tiled splashback above a basin, not adjacent to a shower
• Decorative tiles on walls far from the water source

The key risk factor is: what is below and behind the tiles? If the answer is structural timber, a room below, or a neighbour's ceiling, tanking is required. If the answer is an outer brick wall with no consequence for moisture ingress, the risk is lower.

Substrate Preparation

The substrate must be sound, flat, and free of contamination before any waterproofing membrane is applied. Key preparation steps:

1. Inspect and repair — probe plasterboard for soft spots (signs of previous moisture damage); replace any damaged boards before starting
2. Clean — remove any paint, oil, loose plaster, or tile adhesive residue that could reduce adhesion; prime porous substrates (bare plaster, cement board) with the manufacturer's recommended primer
3. Fill and level — fill any holes, cracks, or uneven areas in the substrate; tiled walls require a flat substrate (max deviation 3mm under a 2m straight edge for a wall to be tiled with standard adhesive); use a tile levelling compound if needed
4. Tile backer board — where the substrate is not suitable for tanking (flexible timber-framed wall, shower stud wall), install a rigid tile backer board (Wedi, Marmox, Hardiebacker) first; these are dimensionally stable, water-resistant, and provide a reliable substrate for liquid membranes

For wet room floors, the subfloor must be structurally rigid — a deflecting floor will crack the waterproof membrane and tiles regardless of the system used. Maximum recommended deflection is L/360 (where L is the span in mm) or approximately 3mm over a 1-metre span. On timber floors, add a layer of 18mm WBP ply and increase joist centres or add noggins if deflection is likely.

Applying BAL WP1 / Liquid Tanking Systems

The application process for a standard domestic liquid tanking membrane (BAL WP1, Mapei Aquadefense, Sika Membrane):

1. Prime the substrate if required by the manufacturer's data sheet; allow to dry
2. Apply first coat — use a brush for corners and edges first, then a roller for the main areas; apply in one direction; ensure full coverage with no missed areas; pay particular attention to pipe penetrations (apply around them with a brush and work the membrane into the joint)
3. Apply joint tape at corners — while the first coat is still slightly wet (check manufacturer's guidance — some require first coat to dry before tape), press fibreglass reinforcement tape into all internal wall corners, at the wall/floor junction, and at any expansion joints; overlap tape ends by 50mm; apply a second coat of membrane over the tape, fully embedding it
4. Apply second coat — perpendicular to the first coat to ensure full coverage; the second coat is the waterproofing coat; ensure an even film without holidays (gaps or thin spots)
5. Allow to cure — minimum 24 hours at 20°C, 50% RH; longer in cold or humid conditions; the membrane is cured when it changes from dark (wet) to light (dry) throughout; do not tile until fully cured
6. Pipe penetrations — all pipe penetrations through the membrane must be sealed with the pipe at least 10mm from the substrate; apply membrane material around the pipe and allow to cure before any fitting collar or rose plate is applied

Schluter Kerdi System

The Kerdi system uses a polyethylene fleece-faced sheet membrane applied in a layer of unmodified tile adhesive. The fleece face of the Kerdi membrane bonds into the adhesive, embedding the sheet within the adhesive layer. Tiles are then applied over the cured Kerdi in a second layer of adhesive.

Installation:

1. Apply unmodified thin-bed tile adhesive to the substrate using a 4mm square notch trowel
2. Press the Kerdi membrane into the adhesive; smooth out bubbles with a float or trowel
3. Kerdi-Band (the perforated corner and seam tape) is applied at all junctions using the same adhesive technique; the Kerdi-Band overlaps 50mm onto adjacent sheets
4. At the wall/floor junction, Kerdi-Band is laid across the angle and embedded in adhesive on both the wall and floor
5. Allow 24 hours for the adhesive to cure before tiling over the Kerdi

Kerdi is particularly effective on tile backer board substrates (Wedi, Hardiebacker) where the combination of waterproof board and Kerdi membrane provides redundant protection.

Wet Room Floor Waterproofing

Wet room floor waterproofing is the most demanding application. The floor must be waterproofed and must have a continuous fall to the waste outlet (minimum 1:80 gradient). Several approaches are used:

Pre-formed wet room formers — Marmox Multiboard Shower Formers, Wedi Shower Elements, Schluter Kerdi-Shower — are rigid, pre-sloped, waterproof elements that provide the fall and the waterproof substrate in one product; liquid membrane is applied over the joints and to the walls; these are strongly recommended over site-formed screeds as the pre-formed slope is accurately manufactured

Site-formed screed slope — a sand/cement screed formed on site at the correct fall; the screed must be fully cured before the liquid membrane is applied; BAL Hydrobond or similar can be applied during the screed pour (the compound is poured into the fresh screed and floated in); this method requires skilled screeding to achieve a consistent gradient

Deck-level waste — the waste outlet must be at the lowest point; the floor must slope uniformly to it from all sides; a linear waste (slot drain) along one wall simplifies the slope geometry to a single plane

Sealing around the waste — the waste outlet is the most vulnerable point; proprietary drain bodies (Schluter Kerdi-Drain, Wedi Fundo drain) with integral flanges that embed into the waterproof system provide a sealed connection between the membrane and the drain body

Frequently Asked Questions

Can I tile over existing tiles without waterproofing?

If the existing tiles are sound, well-bonded, and the existing installation has no history of damp or water ingress, tiling over existing tiles in dry areas is sometimes acceptable. For wet areas (shower enclosures, wet rooms), tiling over existing tiles without waterproofing is not recommended — any water that penetrates grout joints in the new tiles finds the original substrate with no protection. Hacking off and starting fresh with proper waterproofing is always better practice.

How do I know the tanking has cured sufficiently?

Most liquid tanking membranes change colour on curing — from dark grey or blue when wet to light grey or off-white when dry. The full depth of the membrane must show the light colour. Touch testing (firm and non-tacky throughout) is the on-site check. In cold or humid conditions (below 10°C, above 80% RH), curing is significantly slowed; allow longer or apply temporary heating. Never tile before the membrane is fully cured — tile adhesive will pull moisture from the uncured membrane, causing adhesion failure.

Does grout need to be waterproof if the substrate is tanked?

Grout in a wet area should be water-resistant and stain-resistant — standard Portland cement grout is porous and will stain and grow mould. Epoxy grout is fully waterproof but is difficult to apply and very expensive; it is used in commercial kitchens and healthcare. Polymer-modified cement grout (Mapei Ultracolor, BAL Micromax2) is the domestic standard — it is water-resistant when cured and much easier to apply than epoxy. All grout joints, regardless of type, should be sealed at internal corners and at the bath/tray rim with silicone, not grout — grout is rigid and will crack at movement joints.

Can I use standard plasterboard as a substrate for a shower?

Standard plasterboard (gypsum drywall) is not suitable as a direct substrate for wet area tiling — it will absorb water, swell, delaminate, and fail relatively quickly in a shower. Water-resistant plasterboard (green board, Mould-stop board) is better but still not ideal for shower enclosures. The correct substrate for a shower in a timber-framed stud wall is a rigid cement-based backer board (Hardiebacker, Marmox, Wedi) or waterproof extruded foam board (Wedi). These are dimensionally stable in wet conditions and provide a reliable substrate for the waterproofing membrane and tiles.

Regulations & Standards

• BS 5385-1:2009 — wall and floor tiling; code of practice for internal tiling; wet area waterproofing recommendations

• BS 5385-3:2014 — wall and floor tiling; code of practice for internal and external tiling of timber-framed construction and substrates

• BS EN ISO 11600:2003 — building construction; jointing products; classification and requirements for sealants; relevant to silicone specification for movement joints

• BS 8000-11 — workmanship on building sites; code of practice for wall and floor tiling; installation quality standards

• BAL (British Standard Products) — Tiling Specification and System Guides — WP1 and system data sheets

• Mapei — Mapelastic and Aquadefense Technical Data — two-component and single-component tanking membranes

• Schluter Systems — Kerdi Waterproofing Guide — Kerdi sheet membrane and Kerdi-Drain installation

• Wedi — Waterproof Board System Technical Manual — Wedi board and wet room element installation

• RIBA Product Selector — Bathroom Tanking — specification guidance and manufacturer selection

• bathroom planning guide — regulatory overview for bathroom work

• freestanding bath installation — floor structural considerations for heavy baths

• bathroom heating options — UFH compatibility with waterproofing systems

• wetroom construction — full wet room design from gradient to drainage