Bathroom Waterproofing for London Homes: Tanking, Wet Rooms, and Tile Backer Systems Guide

THE MOST COMMON WATERPROOFING FAILURE MODE IN LONDON BATHROOMS IS NOT THE MEMBRANE ITSELF BUT THE SUBSTRATE BENEATH IT. VICTORIAN AND EDWARDIAN TERRACES HAVE TIMBER SUSPENDED FLOORS ON THE UPPER STOREYS — THE BATHROOM FLOOR IS TYPICALLY 18MM TO 22MM PLYWOOD OR CHIPBOARD ON 50X200MM JOISTS AT 400MM CENTRES. TIMBER MOVES: IT EXPANDS AND CONTRACTS SEASONALLY WITH HUMIDITY AND TEMPERATURE CHANGES AND DEFLECTS UNDER LOAD. A TILE INSTALLATION ON A MOVING SUBSTRATE WILL CRACK — AND ONCE THE GROUT CRACKS, WATER REACHES THE WATERPROOFING LAYER. IF THAT LAYER HAS INADEQUATE FLEXIBILITY (OLD-FASHIONED BITUMEN TANKING, CEMENT SLURRY, OR NO WATERPROOFING AT ALL), WATER PASSES THROUGH TO THE TIMBER.

THE SECOND FAILURE MODE IS INADEQUATE COVERAGE OF WATERPROOFING AT CHANGE-OF-PLANE DETAILS — THE FLOOR-TO-WALL JUNCTION, THE WALL-TO-WALL INTERNAL CORNER, AND THE PENETRATIONS WHERE WASTE PIPES PASS THROUGH THE FLOOR. THESE ARE THE POINTS OF HIGHEST MOVEMENT AND HIGHEST HYDRAULIC PRESSURE. IN A SHOWER, THESE JUNCTIONS SEE WATER IMPACT DAILY. EVEN A HAIRLINE CRACK IN THE GROUT AT A FLOOR-TO-WALL JUNCTION WILL ALLOW WATER INGRESS OVER TIME.

THE THIRD FAILURE MODE IS INCOMPATIBLE PRODUCT SYSTEMS — USING A MEMBRANE FROM ONE MANUFACTURER WITH TILE ADHESIVE AND GROUT FROM ANOTHER, OR APPLYING WATERPROOFING OVER UNSUITABLE BACKGROUNDS (OLD TILES, PLASTERBOARD NOT RATED FOR WET AREAS, STANDARD PLYWOOD).

THE MODERN STANDARD FOR BATHROOM WALL AND FLOOR SUBSTRATES IN HIGH-USE SHOWER AREAS IS A CEMENT-BASED OR FOAM-CORE TILE BACKER BOARD — NOT PLASTERBOARD AND NOT STANDARD PLYWOOD. TILE BACKER BOARDS ARE DIMENSIONALLY STABLE IN WET CONDITIONS, DO NOT ABSORB WATER, AND PROVIDE A SUITABLE BONDING SURFACE FOR TILE ADHESIVE.

WEDI BUILDING BOARD IS A RIGID EXTRUDED POLYSTYRENE (XPS) FOAM CORE WITH FIBREGLASS-REINFORCED CEMENT SURFACES. WEDI IS WATERPROOF THROUGHOUT ITS ENTIRE BODY — NOT JUST ON THE SURFACE — AND IS COMPATIBLE WITH THE WEDI FLANGE SYSTEM FOR PIPE PENETRATIONS AND THE WEDI FUNDO PRIMO SHOWER TRAY. WEDI BOARDS BOND DIRECTLY TO TIMBER STUDS, JOISTS, OR MASONRY USING WEDI FIX (POLYURETHANE ADHESIVE) AND STAINLESS STEEL SCREWS. WEDI JOINT SEALANT IS APPLIED AT ALL JOINTS BEFORE TILING. THIS CREATES A FULLY WATERPROOF SUBSTRATE WITHOUT REQUIRING A SEPARATE LIQUID MEMBRANE.

SCHLÜTER KERDI BOARD (POLYURETHANE FOAM CORE WITH INORGANIC POLYMER SURFACE) IS AN ALTERNATIVE TO WEDI AND IS FULLY COMPATIBLE WITH THE SCHLÜTER KERDI SYSTEM OF STRIP DRAINS, SHOWER TRAYS (DITRA-DRAIN), AND KERDI-BAND FABRIC SEALING TAPE. KERDI BOARD IS LIGHTER THAN WEDI AND CUTS WITH A UTILITY KNIFE, MAKING INSTALLATION FASTER. SCHLÜTER KERDI DRAINS (8MM PROFILE HEIGHT) ALLOW VERY SHALLOW GRADIENT FLOORS FOR LEVEL-ACCESS WET ROOMS.

HARDIE BACKER 500 (JAMES HARDIE) IS A CEMENT-BASED FIBRE-CEMENT BOARD (6MM OR 12MM THICKNESS) WITH EXCELLENT DIMENSIONAL STABILITY AND COMPATIBILITY WITH ALL TILE ADHESIVES AND LIQUID TANKING SYSTEMS. IT IS HEAVIER THAN WEDI OR KERDI BOARD BUT MORE RIGID AND IMPACT-RESISTANT. HARDIE BACKER 500 IS NOT ITSELF WATERPROOF AND MUST BE USED WITH A SEPARATE LIQUID TANKING MEMBRANE OR MAPEI PRIMER L + MAPELASTIC AQUADEFENSE APPLIED OVER THE BOARD SURFACE BEFORE TILING.

LIQUID TANKING MEMBRANES ARE FLEXIBLE WATERPROOFING COATINGS APPLIED OVER THE SUBSTRATE BEFORE TILING. THEY ARE THE STANDARD APPROACH FOR CONVENTIONAL BATHROOM WALL AND FLOOR TANKING WHERE TILE BACKER BOARDS OR EXISTING CERAMIC TILE ARE USED AS THE SUBSTRATE.

MAPEI MAPELASTIC AQUADEFENSE IS THE MOST WIDELY SPECIFIED LIQUID TANKING PRODUCT FOR UK BATHROOM APPLICATIONS. IT IS A SINGLE-COMPONENT POLYURETHANE-ACRYLIC POLYMER THAT IS APPLIED BY BRUSH OR ROLLER IN TWO COATS (MINIMUM DRY FILM THICKNESS 1.0MM), WITH MAPEI MAPEBAND PE TAPE (THE POLYETHYLENE FABRIC SEALING TAPE) EMBEDDED IN THE FIRST COAT AT ALL JOINTS, INTERNAL CORNERS, AND PIPE PENETRATIONS. THE TOTAL CONSUMPTION IS APPROXIMATELY 0.6 TO 0.9 KG/M' FOR TWO COATS ON WALL AND 1.0 TO 1.5 KG/M' ON FLOOR. MAPELASTIC AQUADEFENSE CURES IN 3 TO 4 HOURS AT 20'C AND CAN BE TILED OVER IMMEDIATELY AFTER CURING.

BAL TANKING KIT (BRITISH BUILDERS MERCHANTS STAPLE PRODUCT) CONSISTS OF BAL WATERPROOFING MEMBRANE LIQUID (GREY FLEXIBLE POLYMER EMULSION) PLUS BAL TANKING TAPE FOR JOINTS AND CORNERS. IT IS MORE WIDELY AVAILABLE THROUGH TRADE BUILDERS' MERCHANTS THAN MAPEI AND IS COMPETITIVELY PRICED. THE BAL WATERPROOFING MEMBRANE IS CERTIFIED TO BS EN ISO 15036 AND IS ACCEPTED BY MAJOR TILE ADHESIVE MANUFACTURERS. HOWEVER, BAL TANKING IS NOT COMPATIBLE WITH ALL TILE ADHESIVE SYSTEMS — ALWAYS USE BAL RAPID-FLEX OR BAL MAX FLEX OVER A BAL-TANKED SURFACE.

REMMERS MB 2K (TWO-COMPONENT MINERAL-POLYMER) IS THE HIGHEST-PERFORMANCE LIQUID TANKING SYSTEM FOR HEAVY-DUTY OR COMMERCIAL APPLICATIONS SUCH AS LARGE WALK-IN SHOWERS, WET ROOMS WITH UNDERFLOOR HEATING, OR WHERE EXTREME WATER PRESSURE IS EXPECTED. IT ACHIEVES CRACK-BRIDGING OF UP TO 2MM AND IS RATED FOR CONTINUOUS WATER IMMERSION. MB 2K IS THE PROFESSIONAL'S CHOICE FOR HIGH-END PROJECTS.

A TRUE LEVEL-ACCESS WET ROOM — WHERE THE SHOWER FLOOR IS FLUSH WITH THE BATHROOM FLOOR AND THERE IS NO RAISED SHOWER TRAY LIP — REQUIRES A DEDICATED WET ROOM FORMER THAT CREATES THE NECESSARY FALL TO THE DRAIN WITHIN THE FLOOR ZONE WITHOUT EXCESSIVE FLOOR LEVEL RAISE.

WEDI FUNDO PRIMO IS A PREFABRICATED XPS SHOWER FORMER WITH A PRE-FORMED FALL (1.5% GRADIENT) TO A CENTRAL DRAIN FLANGE. THE FUNDO PRIMO COMES IN SIZES FROM 800X800MM TO 1200X1200MM (CUSTOM SIZES AVAILABLE) AND IS 50MM TO 65MM DEEP, FITTING BETWEEN OR OVER TIMBER JOISTS ON TIMBER FLOORS WITH A RISER PLATE SYSTEM. THE WEDI FUNDO DRAIN (90MM OR 120MM) IS A SLOT DRAIN THAT INTEGRATES WITH THE WEDI WATERPROOFING SYSTEM AND ELIMINATES THE NEED FOR A CONVENTIONAL WASTE FLANGE.

SCHLÜTER DITRA-DRAIN IS THE COMPETING SYSTEM, COMBINING THE SCHLÜTER KERDI MEMBRANE WITH A FOAM DRAIN FORMER. IT IS TYPICALLY SHALLOWER (40MM MINIMUM) THAN THE WEDI FUNDO PRIMO AND BETTER SUITED TO FLOORS WITH VERY LIMITED DEPTH.

IMPEY AQUA-DEC EVO LINEAR IS A PREFABRICATED WET ROOM TRAY SYSTEM WITH A LINEAR SLOT DRAIN AT ONE WALL — THE IMPEY LINEAR SYSTEM. THE FLOOR FALLS IN ONE DIRECTION ONLY (AWAY FROM THE MAIN SHOWER WALL), WHICH IS ARCHITECTURALLY CLEANER AND ALLOWS FULL-SIZE FORMAT TILES (600X300MM OR LARGER) TO LAY FLAT WITHOUT CUTTING TO A CENTRAL DRAIN. PARTICULARLY SUITABLE FOR LARGER SHOWERS AND EN-SUITE WETROOMS. THE IMPEY AQUA-DEC EVO IS 44MM DEEP AND IS ACO COMPATIBLE FOR THE DRAIN COMPONENT.

BUILDING REGULATIONS PART M (ACCESS TO AND USE OF BUILDINGS) REQUIRES LEVEL-ACCESS SHOWER FACILITIES IN ANY BATHROOM THAT IS BUILT OR RENOVATED WHERE M4(2) OR M4(3) ACCESSIBILITY STANDARDS APPLY. FOR GENERAL MARKET NEW BUILDS AND MAIN BATHROOM REFURBISHMENTS, M4(1) (VISITABLE) APPLIES ONLY — BUT FOR EXTENSIONS WITH GROUND FLOOR BATHROOMS OR PROPERTIES MARKETED AS ACCESSIBLE, THE HIGHER CATEGORY STANDARDS ARE RELEVANT.

THE CHANGE-OF-PLANE DETAILS ARE WHERE WATERPROOFING MOST COMMONLY FAILS. THESE ARE THE SPECIFIC LOCATIONS THAT MUST BE TREATED WITH ADDITIONAL CARE:

FLOOR-TO-WALL JUNCTION (COVE DETAIL): THE INTERNAL CORNER BETWEEN FLOOR AND WALL MUST HAVE A FABRIC SEALING TAPE (MAPEI MAPEBAND PE, SCHLÜTER KERDI-BAND, OR BAL TANKING TAPE) EMBEDDED IN THE FIRST COAT OF TANKING MEMBRANE. THE TAPE MUST BE AT LEAST 100MM WIDE, LAPPED 50MM ONTO EACH SURFACE. ABOVE THE TAPE, THE SECOND COAT OF MEMBRANE MUST OVERLAP THE TAPE FULLY. THE TILE INSTALLATION MUST THEN LEAVE A 5MM MOVEMENT JOINT AT THIS CORNER FILLED WITH MAPESIL AC SILICONE SEALANT (OR EQUIVALENT COLOUR-MATCHED SANITARY SILICONE) RATHER THAN GROUT — GROUT IS RIGID AND WILL CRACK AT THIS MOVEMENT JOINT.

PIPE PENETRATIONS: WHERE A WASTE PIPE PASSES THROUGH THE FLOOR, A WATERPROOF COLLAR MUST BE FORMED. THE WEDI FLANGE SYSTEM AND THE SCHLÜTER KERDI-PIPE ARE PROPRIETARY SOLUTIONS THAT INTEGRATE WITH THEIR RESPECTIVE BOARD SYSTEMS. FOR LIQUID TANKING APPLICATIONS, MAPEI MAPEBAND PIPE COLLAR (PRE-CUT FABRIC RING) EMBEDDED IN MAPELASTIC AQUADEFENSE IS THE STANDARD DETAIL.

THROUGH-WALL FIXINGS: SOAP DISHES, GRAB RAILS, AND SHELVES FIXED INTO A TILED WALL BREACH THE WATERPROOFING MEMBRANE. ANY FIXING MUST BE MADE WITH STAINLESS STEEL SCREWS AND THE PENETRATION SEALED WITH A FLEXIBLE WATERPROOFING SEALANT BEFORE THE FIXING IS INSTALLED. SILICONE SEALANT AROUND THE FIXING PLATE AFTER INSTALLATION IS NOT SUFFICIENT.

GROUT SPECIFICATION: FOR SHOWER AREAS, ONLY FLEXIBLE EPOXY GROUT (MAPEI KERAPOXY DESIGN OR LATICRETE SPECTRALOCK) OR ANTI-MOULD FLEXIBLE CEMENTITIOUS GROUT (MAPEI ULTRACOLOR PLUS FA) SHOULD BE USED. STANDARD RIGID CEMENTITIOUS GROUT WILL CRACK IN A WET-AND-DRY CYCLING ENVIRONMENT AND ADMIT WATER WITHIN 2 TO 3 YEARS.

THE COST OF BATHROOM WATERPROOFING AS A STANDALONE SCOPE (EXCLUDING TILING, SANITARYWARE, AND PLUMBING) FOR A STANDARD 2.5M X 2.0M BATHROOM IN A LONDON TERRACE IS APPROXIMATELY AS FOLLOWS.

STRIP-OUT AND SUBSTRATE PREPARATION (REMOVAL OF OLD TILES, OLD PLASTERBOARD, INSTALLATION OF NEW HARDIE BACKER OR WEDI BOARD TO WALLS AND 22MM PLYWOOD TO FLOOR): £400 TO £900 DEPENDING ON FLOOR AREA AND NUMBER OF WALLS.

WATERPROOFING APPLICATION — LIQUID MEMBRANE (MAPELASTIC AQUADEFENSE OR BAL TANKING KIT, TWO COATS, TAPE AT ALL JOINTS): £200 TO £500 FOR LABOUR AND MATERIALS.

WET ROOM FORMER SYSTEM (WEDI FUNDO PRIMO 900X900MM OR IMPEY AQUA-DEC EVO, SUPPLY AND FIT): £350 TO £650 INCLUDING DRAIN COMPONENT.

FULL BATHROOM WATERPROOFING SCOPE (SUBSTRATE PREP + TANKING + WET ROOM FORMER + ALL CRITICAL DETAILS BUT NOT TILING): £1,000 TO £2,200 FOR A STANDARD BATHROOM. THIS IS TYPICALLY INCLUDED AS PART OF THE BATHROOM RENOVATION PRICE RATHER THAN QUOTED SEPARATELY.

A COMPLETE BATHROOM RENOVATION INCLUDING ALL WATERPROOFING, TILING, SANITARYWARE SUPPLY AND FIT, PLUMBING ALTERATIONS, AND DECORATION TYPICALLY COSTS £7,000 TO £16,000 IN LONDON DEPENDING ON SPECIFICATION LEVEL.