Bathroom Renovations

Quick Answer: Waterproofing a bathroom requires tanking systems (liquid membranes or sheet membranes) applied to walls and floors in wet areas before tiling. For shower enclosures, you’ll need British Standard BS 5385-compliant waterproofing extending at least 1,200mm high around showers and 300mm beyond the splash zone. Quality tanking kits cost £80-£200 for an average bathroom, and the membrane must cure for 24-48 hours before tiling. Get it wrong and you’ll face water damage, mould, and expensive repairs within months. 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 Pro Tip: Don’t cut corners on coverage. I always tank 300mm beyond the “wet zone” because water travels further than people expect—especially when children splash about or showerheads spray at awkward angles. 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. Membrane Type Best For Typical Cost Cure Time Liquid Applied Membrane Complex shapes, DIY application, retrofit jobs £60-£150 (10L kit) 24-48 hours Sheet Membrane Large flat areas, commercial projects, wetrooms £100-£250 (10m² kit) Immediate (after adhesive sets) Tape & Membrane System Joints, penetrations, guaranteed coverage £120-£200 (complete kit) 24-48 hours 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 ⚠️ Warning: Standard plasterboard is NOT suitable for direct tanking in showers or wetrooms. The gypsum core remains vulnerable to long-term moisture penetration even with tanking applied. Always use tile backer board or moisture-resistant alternatives in wet zones. 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