Bathroom Waterproofing and Tanking: When Is It Required and How to Do It

Summary

Tanking is the process of applying a continuous waterproof membrane to walls and floors before tiling, creating a sealed envelope that prevents water reaching the substrate. BS 5385-1:2018 moved tanking from a recommendation to a requirement for all wet areas in both domestic and commercial settings, recognising that cementitious adhesives and grouts are only water-resistant -- they slow moisture penetration but do not stop it. Two main system types exist: liquid-applied membranes (painted or rolled on in two coats) and sheet membranes (bonded to the substrate with tile adhesive). The choice between them depends on the substrate, project timeline, and installation environment. Getting waterproofing wrong is one of the most expensive mistakes in bathroom fitting -- failures typically remain hidden until water damage to joists, ceilings, and adjacent rooms forces a complete strip-out.

Key Facts

• BS 5385-1:2018 clause 6.1.1.3 requires all substrates in wet areas to be waterproofed before tiling -- this applies to domestic as well as commercial installations
• NHBC Standards 2025 (effective 1 January 2025) mandate EAD-certified tanking for wet rooms and showers with flow rates above 12 litres/min in new-build homes
• Liquid-applied systems must be UKCA/UKNI/CE marked to EAD 030352-00-0503; sheet membranes to EAD 030436-00-0503 or EAD 030400-00-0605
• Tiles and grout are not waterproof -- most cementitious adhesives and grouts are water-resistant only, meaning moisture will eventually pass through to the substrate
• Wet room floor falls should be between 1:50 and 1:80 gradient, with a minimum 12mm fall to the drain
• Two coats minimum for liquid membranes, achieving a combined dry film thickness of at least 1mm
• Tanking tape must be applied to all internal corners, floor-wall junctions, pipe penetrations, and drain surrounds before membrane application
• Flood testing (24-hour water retention with drain sealed) is best practice for wet room floors before tiling begins

Detailed Guidance

When is tanking required vs recommended?

BS 5385-1:2018 makes waterproofing a requirement for all wet areas. The NHBC Standards 2025 add specific enforcement tiers for new-build homes:

Key distinction: The NHBC 2025 revision allows a lighter-touch approach for low-flow showers (≤12 L/min) in new builds -- waterproof tile adhesive and grout on a water-resistant backing board, with a preformed or adhered upstand at the perimeter. However, BS 5385-1:2018 itself does not make this distinction and recommends full tanking for all wet areas. Professional best practice remains full tanking for any area that will receive regular water exposure.

What tanking systems are available?

There are two fundamental categories: liquid-applied membranes and sheet membranes. Many professional installers use a combination -- liquid on walls, sheet on floors -- to get the best of both systems.

Liquid-Applied Membranes

Applied by brush, roller, or trowel in two coats at right angles to each other. The combined dry film thickness must reach at least 1mm. Reinforcing tape is embedded in the first coat at all junctions, corners, and penetrations.

Sheet Membranes

Pre-manufactured waterproof sheets bonded to the substrate with tile adhesive or proprietary bonding compound. Tiles can typically be fixed directly over the membrane once the bonding adhesive has set.

Comparison: Liquid vs Sheet

How do I tank a wet room?

Wet rooms demand the highest standard of waterproofing because the entire floor is the shower tray. Every surface that could receive water must be sealed into a continuous, bonded envelope.

Prerequisites

• Structural floor must be rigid enough to support tiled finishes without deflection. Timber floors typically need 18mm WBP plywood over joists at max 400mm centres, with 12.5mm or 6mm cement backer board on top
• Floor falls must be formed in the screed or using a pre-formed gradient tray. Target gradient: 1:60 to 1:80 (approximately 12-20mm fall per metre towards the drain)
• Linear drains require a single-direction fall; point drains require a four-way fall
• All plumbing and drainage must be complete and tested before tanking begins
• Substrate must be clean, dry, and dust-free

Step-by-Step Procedure (Liquid Membrane System)

Step 1: Prime the substrate Apply the manufacturer's recommended primer to all surfaces. This ensures proper adhesion of the membrane to the substrate. Allow primer to dry fully (typically 1 hour). Both absorbent and non-absorbent substrates need priming -- do not skip this step.

Step 2: Apply tanking tape to all junctions Bed reinforcing tanking tape (e.g. Mapeband PE 120, BAL Waterproof 1C Tape) into a coat of membrane at:

• All floor-to-wall junctions
• All internal wall corners
• Around the drain flange/collar
• Around all pipe penetrations
• Any change of substrate (e.g. backer board to plasterboard)
• Soap niches and recesses

The tape must overlap at corners by a minimum of 50mm. Use manufacturer-specific drain collars where available.

Step 3: Apply the first coat of membrane Using a brush or long-pile roller, apply the first coat with horizontal strokes across all primed surfaces -- floor and walls. Ensure full, even coverage with no pinholes or thin spots. The coat should be approximately 0.5mm thick.

Coverage area for walls: apply membrane to all walls within the wet room to a minimum height of 1800mm from finished floor level. Best practice for a wet room is full floor-to-ceiling coverage on all walls.

Step 4: Allow first coat to dry Drying times vary by product:

• ARDEX WPC: approximately 45 minutes
• BAL Waterproof 1C: single coat, skip to Step 6
• Mapei Mapelastic AquaDefense: approximately 2 hours (turns from blue to green when ready)
• Sika Aquaseal: minimum 6 hours

Do not force-dry with heat guns. Maintain adequate ventilation. Do not allow foot traffic during cure.

Step 5: Apply the second coat Apply the second coat with vertical strokes (perpendicular to the first coat). This cross-hatch pattern ensures complete coverage and eliminates any missed areas. The combined dry film thickness of both coats must be at least 1mm.

Step 6: Allow full cure before tiling

• ARDEX WPC: 90 minutes total
• BAL Waterproof 1C: 2 hours
• Mapei Mapelastic AquaDefense: 4 hours total
• Sika Aquaseal: 12 hours after second coat

Step 7: Flood test (best practice) Temporarily seal the drain and fill the wet room floor with 10-20mm of water. Leave for 24 hours. Check the ceiling below and all perimeter edges for any signs of moisture. If the test fails, identify and repair the breach before proceeding.

Step 8: Tile Use a C2 S1 (minimum) adhesive applied with full bed coverage -- no dot-and-dab. Fix tiles working from the drain outwards. Grout with CG2 WA minimum for wet room floors.

How do I waterproof a standard shower area?

Standard shower enclosures with a tray require less intensive treatment than wet rooms, but tanking is still required under BS 5385-1:2018.

Height and Coverage Requirements

• Shower walls: Tank to a minimum of 1800mm above the shower tray, or 200mm above the highest point of water outlet (showerhead), whichever is greater. Best practice is to take tanking to the full tile height
• Bath surrounds: Tank the full tiled area behind and around the bath, typically 3 tile courses above the bath rim as an absolute minimum, but ideally to the full tiled height
• Overlaps: Where membrane meets at corners or junctions, overlap by minimum 50mm
• Returns: Extend tanking minimum 100mm beyond the edge of the shower enclosure or bath on adjacent walls

Procedure

1. Prepare substrate -- remove any loose material, fill voids, ensure surfaces are flat and dry. Cement backer board (e.g. Hardiebacker 12mm, Wedi board 12.5mm) is the preferred substrate for shower walls. Plasterboard is acceptable if tanked, but moisture-resistant (green) board is strongly recommended
2. Prime as per manufacturer's instructions
3. Apply tanking tape to all internal corners, floor-wall junction within the shower zone, around the shower valve penetration, and any pipe entries
4. Apply membrane in two coats as described above. One coat if using BAL Waterproof 1C
5. Allow full cure before tiling
6. Tile with full-bed adhesive -- C2 S1 minimum. Never dot-and-dab in wet areas
7. Grout with CG2 WA and seal the perimeter joint between tiles and shower tray/bath with a flexible silicone sealant (not grout)

Critical Detail: The Perimeter Seal

The junction between the shower tray and wall tiles must be sealed with a flexible silicone sealant, not grout. Grout at this junction will crack as the tray flexes under load, breaking the waterproof envelope. Use a mould-resistant sanitary silicone (e.g. Mapei Mapesil AC, Sika Sanisil, Dow Corning 785).

What are the common failures from inadequate waterproofing?

Waterproofing failures are among the most costly defects in bathroom installations because the damage is often extensive before it becomes visible. Here are the most common failure modes:

1. Water tracking through grout joints Cementitious grout is porous. In a shower used twice daily, moisture steadily penetrates the grout, saturates the adhesive bed, and reaches the substrate. On plasterboard, this causes swelling, delamination, and mould growth. On timber, it causes rot. This process can take 6-18 months to become visible, by which time structural damage may be significant.

2. Dot-and-dab adhesive voids Tiles fixed with adhesive dabs rather than full-bed coverage create voids behind the tile. Water entering through grout collects in these voids, creating reservoirs of standing water against the substrate. This accelerates substrate degradation and provides an ideal environment for mould.

3. Failed corner and junction seals If tanking tape is omitted or poorly applied at internal corners and floor-wall junctions, these high-stress points become the first breach in the waterproof envelope. Water follows gravity to the lowest point of failure, often appearing as damp on the ceiling below -- sometimes metres from the actual breach.

4. Insufficient membrane thickness Applying liquid membrane too thinly (under 1mm combined dry film thickness) results in pinholes and weak spots. A single pinhole in a shower floor can allow enough water through to cause mould on the ceiling below within weeks.

5. Tiling before full cure Tiling over membrane that has not fully cured traps moisture and prevents the membrane from achieving its full bond and waterproofing performance. This is especially common with liquid systems that have longer cure times (e.g. Sika Aquaseal at 12 hours).

6. No tanking behind bath A common shortcut. The bath surround gets tiled but not tanked, on the assumption that the bath itself catches all water. In practice, splash from showering over the bath, condensation, and water running down tiles behind the bath lip all reach the substrate. Over years, this rots timber framing and causes damp in adjacent rooms.

7. Timber substrate movement cracking the membrane Liquid membranes on insufficiently rigid timber floors can crack as the timber deflects under foot traffic. Sheet membranes (e.g. Tilesure, Schluter KERDI) are better at bridging movement on timber substrates.

Typical remediation cost: A failed shower waterproofing in a first-floor bathroom can easily cost GBP 3,000-8,000+ to remediate once you factor in stripping tiles, replacing damaged plasterboard or timber, re-tanking, re-tiling, and repairing the ceiling below. Prevention (proper tanking) adds GBP 150-400 in materials to the original installation.

Does a wet room need Building Control approval?

The answer depends on the scope of work:

Building Control notification is NOT typically required when:

• Converting an existing bathroom into a wet room in an existing dwelling (like-for-like plumbing, no structural changes)
• The waste pipe connects to existing drainage without altering drain runs or access chambers
• No structural alterations are needed (floor strengthening within existing structure is usually acceptable)

Building Control notification IS required when:

• Building a new extension to house a wet room (Planning Permission may also be needed)
• Altering existing drain runs or access chambers -- Part H of the Building Regulations (foul water drainage) applies
• Adding a new soil stack or waste connection point
• Structural work to the floor (e.g. lowering floor level for a flush-threshold wet room in an upper storey)
• The property is a flat and the work affects fire compartmentalisation (Part B) or sound insulation (Part E)
• The wet room is part of a new-build (NHBC Standards apply in addition to Building Regulations)

Drainage Requirements (Building Regulations Part H)

• The wet room drain must connect to the foul drainage system via a suitable trapped waste
• Linear drains typically require a 50mm (2-inch) waste pipe; point drains use 40mm or 50mm
• Minimum trap depth is 50mm water seal
• The drainage system must have sufficient capacity to carry the expected flow rate
• Floor gradient must be adequate to prevent ponding -- minimum 1:80 fall, recommended 1:60
• If connecting to an existing soil stack, the connection must comply with Part H distance rules from other connections

Competent Person Schemes

A plumber registered with a competent person scheme (e.g. APHC, CIPHE, or a multi-trade scheme like NAPIT) can self-certify notifiable plumbing work without separate Building Control involvement. This covers most wet room drainage installations in existing dwellings.

Frequently Asked Questions

Can I tile a shower without tanking?

Technically the tiles will go on, but you should not. BS 5385-1:2018 requires waterproofing of all wet area substrates before tiling. Even if you are not bound by NHBC standards (i.e. it is not a new build), omitting tanking in a shower is a failure waiting to happen. Cementitious grout absorbs water -- it is a matter of when, not if, moisture reaches the substrate. The cost of tanking materials (GBP 50-150 for a standard shower) is trivial compared to the cost of failure.

How long does tanking membrane last?

Quality liquid membranes last 25-30 years when properly applied and protected by tiles. Sheet membrane systems such as Schluter KERDI can exceed 40 years. In practice, the tiles, grout, and silicone seals will need maintenance long before the membrane fails.

Can I tank over existing tiles?

Yes, provided the existing tiles are firmly bonded, the surface is clean, and you use a suitable primer. Products like Mapei Eco Prim Grip or BAL Bond will key the membrane to the glazed tile surface. This is a common approach when converting an existing bathroom to a wet room without stripping tiles from walls that will not receive water directly.

Do I need to tank behind a bath if there is no shower over it?

BS 5385-1:2018 defines the area around a bath as a wet area. While the risk is lower without a shower, condensation and splashing still expose the substrate to moisture over time. Best practice is to tank the full tiled area behind and beside the bath. At minimum, use a waterproof adhesive and ensure all junctions are sealed.

What is the difference between tanking and waterproofing?

In practical UK tiling terminology, the two terms are often used interchangeably. Strictly, "tanking" refers to creating a complete sealed envelope (as in a wet room), while "waterproofing" can refer to localised moisture protection (e.g. a shower wall). For BS 5385-1 compliance, both require a proprietary membrane system -- the distinction is in coverage area rather than product type.

Is a tanking kit enough for a wet room?

Standard tanking kits (e.g. Mapei AquaDefense Kit, Weber SYS Protect Kit) typically contain enough membrane for 4-5 m2 and a roll of tanking tape. This is usually sufficient for a standard shower enclosure but will not cover a full wet room. For a wet room, calculate total wall and floor area and purchase membrane and tape accordingly -- you will typically need 2-3x the quantity in a standard kit.

Regulations & Standards

• BS 5385-1:2018 -- Code of practice for the design and installation of internal ceramic, natural stone, and mosaic wall tiling in normal internal conditions. Clause 6.1.1.3 covers tanking requirements for wet areas

• BS 5385-3:2024 -- Code of practice for internal and external floor tiling. Includes wet room floor requirements

• BS 8204-2 -- Screeds -- concrete/polymer modified cementitious screeds. Relevant to wet room floor construction and screed falls

• BS EN 14891 -- Liquid-applied water-impermeable products for use beneath ceramic tiling bonded with adhesives. Classification and test methods for tanking products

• EAD 030352-00-0503 -- European Assessment Document for liquid-applied waterproofing. Required certification for NHBC compliance

• EAD 030436-00-0503 / EAD 030400-00-0605 -- European Assessment Documents for flexible sheet waterproofing membranes

• Building Regulations Part C -- Site preparation and resistance to contaminants and moisture

• Building Regulations Part H -- Drainage and waste disposal. Covers wet room drainage connections and capacity

• Building Regulations Part E -- Resistance to sound (relevant for wet rooms in flats -- sheet membranes can contribute to acoustic performance)

• NHBC Standards 2025, Section 9.2.5 -- Ceramic wall tiling requirements for new-build homes, including tanking mandates effective 1 January 2025

• BS 5385-1 Waterproofing Regulations -- The Wetroom Store

• How Tiling Regulations Affect Wetroom Installations -- CCL Wetrooms

• NHBC Technical Guidance 9.2/06: Tiling Showers and Bathrooms -- Wetroom Experts

• The New Waterproofing Regulations -- ARDEX UK

• NHBC Tanking Requirements -- BAL Adhesives

• Wet Room Building Regulations -- CCL Wetrooms

• How to Tank a Wet Room -- CCL Wetrooms

• Sheet Membrane vs Liquid -- CCL Wetrooms

• NHBC Standards 2025: Ceramic Wall Tiling

• Mapelastic AquaDefense -- Mapei UK

• Aquaseal Wet Room System -- Sika UK

• Schluter KERDI Waterproofing Membrane -- Schluter Systems

• Waterproofing Details for Bathrooms and Wetrooms -- Detail Library

• TTA Technical Support FAQs -- The Tile Association

• Practical Guide to Waterproofing Legislation -- PHAMnews

• index -- Tiling knowledge base overview: adhesive classification, substrate selection, and BS 5385 standards

• waste pipes -- Waste pipe sizing for wet rooms and shower drains

• building regs -- Building Regulations overview including Parts C, H, and notification requirements