How to Waterproof a Bathroom: Tanking and Membrane Guide

Why Bathroom Waterproofing Matters More Than You Think

I’ve seen countless bathroom failures over my years as a plasterer, and 90% of them trace back to inadequate waterproofing. Water finds every weakness—seeping through grout lines, penetrating plasterboard, saturating timber joists, and wreaking havoc on the room below.

The consequences aren’t minor. Water ingress leads to structural timber rot, electrical hazards, mould growth that triggers respiratory issues, and repair bills running into thousands. One client we worked with in Maidstone faced a £7,500 insurance claim after water from an improperly tanked shower destroyed the ceiling below and caused extensive mould damage.

Modern Building Regulations Approved Document C and BS 5385-4:2015 set clear standards for bathroom waterproofing. These aren’t suggestions—they’re requirements that professional installers must follow to ensure your bathroom remains watertight for decades.

Understanding Bathroom Tanking: The Basics

Tanking refers to creating a continuous waterproof barrier behind tiles and around wet areas. Think of it as a bathtub turned inside out—every surface that might encounter water needs complete protection from penetration.

The principle is simple: water should never reach the substrate (the wall or floor structure beneath). Whether you’re tiling over plasterboard, cement board, or render, a waterproof membrane stands between moisture and materials that would otherwise absorb it.

Where Tanking is Essential

• Shower enclosures: All walls to 1,800-2,000mm height (minimum 1,200mm per BS 5385)
• Bath surrounds: 300mm above the bath rim and 300mm beyond on adjacent walls
• Entire wetroom floors: Plus 150mm up all walls, with particular attention to floor waste connections
• Basin splashbacks: At least 300mm around the basin perimeter
• Floor-to-wall junctions: These joints are prime failure points requiring reinforcement tape

Types of Waterproofing Membranes

You’ve got two main options for bathroom tanking: liquid membranes and sheet membranes. Both work when applied correctly, but each has distinct advantages depending on your situation.

Liquid Applied Membranes (Most Popular)

Liquid tanking kits like BAL WP1, Mapei Mapelastic AquaDefense, or Sika Waterproofing Membrane are the go-to choice for most bathroom projects. They’re acrylic or polymer-based liquids that you paint onto surfaces, forming a flexible rubber-like coating once cured.

Advantages:

• Easy to apply with a brush or roller—accessible for competent DIYers
• Excellent for awkward corners, pipe penetrations, and irregular surfaces
• Creates a seamless barrier without joints to fail
• Relatively quick application (2-4 hours for average bathroom)
• Available from Screwfix, Wickes, and Topps Tiles at reasonable prices

Disadvantages:

• Requires multiple coats (typically 2-3) with drying time between
• Application thickness matters—too thin and it fails, too thick wastes product
• Vulnerable to application errors (missed spots, inadequate coverage)
• Must wait 24-48 hours before tiling

Sheet Membranes (Professional Standard)

Products like Schlüter KERDI or Wedi Subliner Dry use thin polyethylene sheets bonded to walls and floors with modified adhesive. The joints overlap and seal with special tape or adhesive to create continuous protection.

These are favoured in wetroom installations where floor-level drainage demands absolute reliability. The Schlüter KERDI system is particularly respected in the trade—it’s what we specify for high-end wetrooms and commercial bathrooms.

Advantages:

• Guaranteed thickness and consistent protection
• Immediate readiness for tiling (once adhesive sets)
• Ideal for floor formers and drainage integration
• Highly durable and puncture-resistant

Disadvantages:

• More expensive than liquid systems
• Requires precise cutting and fitting skills
• Joints must be executed perfectly—failure points if done poorly
• Less forgiving around complex pipe arrangements

Step-by-Step: How to Tank a Bathroom with Liquid Membrane

This is the method we use for standard bathroom refurbishments. It’s proven, cost-effective, and achievable for experienced DIYers with attention to detail. For specific guidance on preparing bathroom surfaces, see our guide on bathroom plastering in wet areas.

Step 1: Prepare the Substrate

Your waterproofing is only as good as what’s underneath. The substrate must be clean, dry, sound, and structurally stable.

Acceptable substrates:

• Cement-based tile backer board (HardieBacker, Wedi Board, Marmox)
• Moisture-resistant plasterboard (green board or Aqua-board)
• Dense concrete blockwork
• Existing ceramic tiles (if firmly bonded)
• Sand/cement render

Preparation checklist:

• Remove all loose material, dust, grease, and old sealant
• Fill holes and gaps with suitable filler (BAL Stopgap for floors, standard filler for walls)
• Sand smooth any proud spots or rough patches
• Apply primer if specified by the membrane manufacturer (most modern systems don’t require it)
• Ensure relative humidity is below 75% and temperature above 5°C

Step 2: Apply Reinforcement Tape to Joints

Every joint, corner, and penetration needs reinforcement tape embedded in the first coat of membrane. This prevents cracking at stress points where different surfaces meet.

Critical areas requiring tape:

• All internal corners (wall-to-wall, wall-to-floor)
• Pipe penetrations (soil pipes, supply pipes, waste outlets)
• Changes of plane (shower tray edges, bath rims)
• Expansion joints in floors
• Edges of drainage outlets

Apply the first coat of membrane, press the tape firmly into it while wet, then immediately coat over the tape with more membrane. The tape should be completely encapsulated with no air bubbles or wrinkles.

Step 3: Apply First Full Coat

Using a 10-12mm synthetic roller or a 100mm brush, apply the tanking membrane in vertical strokes working from top to bottom. Maintain consistent thickness—most manufacturers specify 1mm wet film thickness per coat.

Coverage rates typically achieve 1-1.5m² per litre per coat, meaning a 10L kit covers 10-15m² with two coats. Don’t stretch it thinner to save money—inadequate thickness is the primary cause of tanking failure.

Work methodically around the room. Don’t miss areas behind the toilet, under the bath panel, or in tight corners. Water will find any gap you leave.

Step 4: Allow Proper Curing Time

Most liquid membranes require 4-6 hours between coats and 24-48 hours final cure time before tiling. Temperature and humidity affect this—cold, damp conditions extend curing times significantly.

The membrane should be touch-dry and show a consistent colour change (usually darker when wet, lighter when cured). If you’re unsure, wait longer. Rushing this stage causes adhesive failure and membrane damage during tile fixing.

Step 5: Apply Second (and Third) Coat

The second coat goes on perpendicular to the first—if you rolled vertically initially, roll horizontally now. This ensures complete coverage of any microscopic gaps.

For shower enclosures and wetrooms, I recommend a third coat in high-exposure areas (directly behind the showerhead, around floor wastes). It’s cheap insurance against failure.

Check the cured membrane carefully. Hold a bright torch at an angle—it’ll highlight any thin spots or missed patches that need touching up before you proceed.

Step 6: Test the Seal (Wetroom Floors)

For wetroom floors or shower trays, perform a flood test before tiling. Block the waste, fill the area with 50mm of water, and leave for 24 hours. Check below for any signs of moisture penetration.

If it passes, proceed confidently. If it fails, you’ve caught it early and can repair before expensive tiling work begins.

Common Tanking Mistakes (and How to Avoid Them)

I’ve been called to diagnose and repair dozens of failed tanking jobs. The same errors crop up repeatedly:

1. Inadequate Coverage Area

The mistake: Tanking stops at the visible wet zone—exactly where the shower screen sits or where tiles meet a dado rail.

Why it fails: Water splashes, condensation runs, and moisture migrates beyond obvious boundaries. A shower screen isn’t waterproof—water passes around seals constantly.

The fix: Always extend tanking 300mm beyond visible wet zones. For showers, tank the entire width of the wall plus adjacent returns. The small extra cost prevents catastrophic failure.

2. Skipping Primer or Using Wrong Products

The mistake: Applying membrane directly to dusty or porous surfaces, or mixing incompatible products from different manufacturers.

Why it fails: Poor adhesion causes membrane delamination. Different chemical formulations don’t bond properly and can react adversely.

The fix: Follow manufacturer instructions religiously. If primer is specified, use it. Stick to one product system throughout—don’t use BAL membrane with Mapei tape, for example.

3. Insufficient Thickness

The mistake: Stretching product to save money, applying one thick coat instead of proper multiple coats, or leaving visible substrate through the membrane.

Why it fails: Thin membranes have microscopic pinholes that allow water through under tile pressure. Single thick coats don’t cure properly and crack.

The fix: Apply minimum two coats at specified thickness. If in doubt, check with a wet film thickness gauge (£10 from Screwfix). The membrane should be opaque and completely conceal the substrate colour.

4. Poor Detail Work at Junctions

The mistake: Inadequate reinforcement at corners, gaps around pipe penetrations, or no transition detail at changes of plane.

Why it fails: Differential movement causes cracking. Pipes vibrate, buildings settle, and thermal expansion creates stress—unreinforced membranes split at these points.

The fix: Use proper corner reinforcement tape at all junctions. For pipe penetrations, cut membrane sheets to fit snugly, apply tape around the penetration, and build up multiple membrane coats to create a robust seal.

Tanking Products: What to Buy in 2026

The tanking market has matured significantly. Here are the products we specify regularly and trust for professional installations:

Budget-Friendly Options (£60-£100)

Palace Tanking Kit (Screwfix): Solid performer for standard bathroom refurbs. 7.5kg covers approximately 7.5m² with two coats. Includes reinforcement tape. Cures in 24 hours. Perfect for DIY projects.

Everbuild Aquaseal 250: Reliable liquid membrane available at builders’ merchants nationwide. Good coverage rates and easy application. Slightly longer cure time (48 hours recommended).

Professional Grade (£100-£180)

BAL WP1 Tanking Kit: Industry standard for good reason. Excellent adhesion, reliable performance, available everywhere from Topps Tiles to Travis Perkins. 10kg kit covers 10-12m² with two coats. This is what we use for most residential projects.

Mapei Mapelastic AquaDefense: Premium single-component membrane with excellent crack-bridging properties. Faster curing than competitors (24 hours to ready). Particularly good for retrofits over existing tiles.

Premium Systems (£180-£300+)

Schlüter KERDI: The gold standard sheet membrane system. Used in commercial installations and high-end wetrooms. Comes with matching drainage systems, corner pieces, and pipe seals. Expensive but virtually foolproof when installed correctly.

Wedi Building Systems: Complete insulated tile backer board system with integrated waterproofing. Popular for wetroom constructions where both thermal and waterproofing performance matter. Boards are 100% waterproof throughout—no separate tanking needed.

Wetrooms: Extra Considerations for Floor-Level Drainage

Wetrooms demand absolute waterproofing perfection because the entire floor is a shower tray. There’s no bath or shower tray to contain water—it flows directly across the floor to a drainage gully.

Essential Wetroom Components

• Pre-formed floor former: Creates the fall (gradient) toward the drain. Typically 1:40 to 1:80 slope. Wedi Fundo or Impey Aqua-Dec systems are popular.
• Linear drain or point drain: Collects water efficiently. Linear drains handle higher flow rates and look sleeker.
• Floor-to-wall seal: Proprietary corner seals prevent water tracking under the wall membrane.
• Upstand tanking: Membrane extends 150-200mm up all walls from the floor. No exceptions.
• Drainage gully seal: The connection between membrane and drain must be watertight using manufacturer-specific components.

Wetroom installations are notifiable under Building Regulations in England and Wales. You’ll need Building Control approval or certification via a Competent Persons Scheme. This isn’t optional—it’s a legal requirement and affects property sale later.

For more on damp-related issues that can arise from poor waterproofing, see our comprehensive guide on rising damp causes and fixes.

Building Regulations and British Standards

Professional installations must comply with current regulations. Here’s what matters in 2026:

Key Standards and Regulations

• BS 5385-4:2015: Code of practice for tiling and mosaics in specific conditions. Section covering waterproofing in domestic bathrooms.
• BS EN 14891: Specifications for liquid applied waterproofing products. Look for products with this certification.
• Building Regulations Approved Document C: Resistance to moisture. Requires adequate protection against water penetration in bathrooms.
• Building Regulations Part H: Drainage and waste. Covers floor waste installations in wetrooms.

When you hire a professional, they should provide written certification that the installation meets these standards. This documentation is increasingly important for property sales and insurance claims.

Cost Breakdown: What to Budget for Tanking

Tanking adds to your overall bathroom renovation cost but prevents far more expensive failures. Here’s what to expect in 2026:

These costs sit within a broader bathroom renovation budget. For complete planning guidance including all trades and costs, see our comprehensive bathroom renovation guide.

Should You DIY or Hire a Professional?

Tanking is accessible to competent DIYers with good attention to detail. It’s not complex plastering work requiring years of skill—it’s methodical, careful surface preparation and membrane application.

DIY is Feasible If:

• You’re tackling a straightforward shower cubicle or bath surround (not a wetroom)
• You have time to follow manufacturer instructions precisely
• You’re comfortable with preparation work (fixing backer board, filling gaps)
• You understand the critical importance of proper coverage—no shortcuts
• You’re willing to wait appropriate curing times before tiling

Hire a Professional When:

• Installing a wetroom with floor-level drainage (high-risk installation)
• Working with structural alterations or complex pipe arrangements
• Building Regulations certification is required
• Your property is leasehold with warranty requirements
• You’re doing insurance work requiring trade certification
• Time pressure means you can’t wait for proper curing between coats

Professional labour for tanking costs £200-£500 depending on room size and complexity. That’s minor compared to the £3,000-£8,000 cost of repairing water damage from failed waterproofing.

Maintaining Your Tanked Bathroom

Properly installed tanking should last 25+ years with minimal maintenance. However, the surface protection (tiles and grout) requires attention:

• Reseal grout annually: Quality grout sealer (£12-£18 for 1L) prevents water gradually penetrating grout lines
• Refresh silicone every 2-3 years: Silicone degrades with cleaning chemicals and eventually loses adhesion
• Check waste covers: Hair traps and gully covers should seal properly—gaps allow water to pool incorrectly
• Ventilate properly: Extract fans reduce condensation loading on the tanking system
• Fix leaks immediately: A dripping showerhead or loose pipe connection defeats waterproofing by introducing water above the tanked area

If you spot any warning signs—loose tiles, persistent damp patches, musty smells—investigate immediately. Early intervention saves thousands compared to full strip-outs once structural damage occurs.

Frequently Asked Questions

Can you tank over existing bathroom tiles?

Yes, provided the tiles are firmly bonded and the surface is thoroughly cleaned and degreased. Use a mechanical sander to key the glazed surface, creating a profile the membrane can grip. Products like Mapei Primer G improve adhesion to smooth tiles.

However, this only works for walls. Never tank over floor tiles for wetrooms—the tile bed may not be sound, and you’re adding another failure layer. For floors, strip back to structural substrate and start fresh.

How long does tanking membrane last?

Quality liquid membranes last 25-30 years when installed correctly and protected by tiles. Sheet membrane systems like Schlüter KERDI can exceed 40 years. The limiting factor is usually the tile adhesive or grout failure, not the membrane itself.

The membrane remains flexible throughout its life, accommodating minor substrate movement without cracking—one reason modern membranes vastly outperform old-fashioned methods like cement renders with waterproof additives.

Do you need to tank the ceiling in a bathroom?

No, ceiling tanking isn’t necessary or practical. Ceilings don’t experience direct water contact like walls and floors. Focus waterproofing effort on splash zones and wet areas.

However, ensure adequate ventilation to manage condensation. Persistent condensation causes paint failure, mould growth, and timber rot. An inline extract fan (minimum 15L/s extraction rate) handles moisture effectively. See our guide on bathroom plastering for wet areas for ceiling preparation advice.

What’s the difference between tanking and damp proofing?

Tanking creates a complete water barrier preventing liquid water penetration from the wet side (inside the bathroom). It’s applied to the face of walls and floors before tiling.

Damp proofing prevents moisture migration through walls from external sources (rising damp, penetrating damp). It’s applied to the exterior face or within the wall structure itself.

Bathrooms often need both: tanking on internal wet surfaces and damp-proof membranes on external walls prone to penetrating damp. For comprehensive coverage of damp issues, see our damp proofing and insulation guide.

Can you tile immediately after tanking?

Not immediately—you must respect the curing time specified by the manufacturer. Liquid membranes typically require 24-48 hours before tiling. Ambient temperature and humidity affect this significantly.

Tile adhesive contains water and solvents that can damage uncured membrane. Premature tiling prevents proper membrane cure, resulting in poor adhesion and potential failure. Cold weather (below 10°C) extends curing times—sometimes to 72 hours or more.

Sheet membrane systems like Schlüter KERDI allow tiling as soon as the bonding adhesive has set (usually 24 hours), making them faster for time-sensitive projects.

Is tanking required by law in UK bathrooms?

Building Regulations don’t explicitly mandate tanking systems by name, but Approved Document C requires adequate resistance to moisture penetration. In practice, any professional installation in wet areas uses tanking to achieve compliance.

For wetrooms and floor-level shower installations, Building Control typically requires certification that waterproofing meets BS 5385 and manufacturer specifications. DIY installations may not need formal approval for simple shower cubicles, but following best practice protects your investment.

Leasehold properties often have additional requirements in the lease—check before starting work. Insurance may refuse claims for water damage if you’ve not followed proper waterproofing methods.