Wet Room Installation in London Victorian Terraces: Tanking, Drainage, and Shower Specification

THE MOST IMPORTANT TECHNICAL DECISION IN A WET ROOM INSTALLATION IN A LONDON VICTORIAN TERRACE IS THE CHOICE OF TANKING AND DRAINAGE SYSTEM — WHICH IS DETERMINED BY THE FLOOR STRUCTURE BENEATH THE WET ROOM.

TIMBER SUSPENDED FLOOR (THE MOST COMMON FLOOR STRUCTURE IN LONDON VICTORIAN TERRACE BATHROOMS — FIRST FLOOR AND SOME GROUND FLOOR BATHROOMS WHERE THE ORIGINAL TIMBER FLOOR STRUCTURE IS RETAINED): THE CHALLENGE: TIMBER FLOORS MOVE — THEY DEFLECT UNDER LOAD AND EXPAND AND CONTRACT WITH CHANGES IN TEMPERATURE AND HUMIDITY — THIS MOVEMENT IS INCOMPATIBLE WITH RIGID CERAMIC OR PORCELAIN TILES ON A STANDARD SCREED OVERLAY, BECAUSE THE SCREED WILL CRACK (BREAKING THE WATERPROOF LAYER) AT THE POINTS OF MAXIMUM DEFLECTION (MIDSPAN OF THE JOISTS). THE SOLUTION: A SCREED-FREE TANKING AND TILE SYSTEM DESIGNED FOR MOVEMENT — USING RIGID SHEET CARRIER BOARDS (WEDI BUILDING BOARD, SCHLUTER KERDI-BOARD, OR HARDIE BACKER BOARD) AS THE BASE LAYER — THESE BOARDS ARE FIXED DIRECTLY TO THE EXISTING FLOOR BOARDING (OR TO ADDITIONAL STRUCTURAL PLY IF THE FLOOR BOARDING IS THIN OR UNEVEN) WITHOUT A SCREED — THE BOARDS ARE JOINTED WITH TAPE AND A FLEXIBLE WATERPROOFING COMPOUND — AND TILES ARE FIXED DIRECTLY TO THE BOARD SURFACE WITH FLEXIBLE TILE ADHESIVE.

CONCRETE SOLID FLOOR (GROUND FLOOR REAR EXTENSION — NEW BUILD SLAB — OR AN EXISTING GROUND FLOOR WHERE THE ORIGINAL TIMBER FLOOR HAS BEEN REPLACED WITH A CONCRETE SLAB): THE CONCRETE SUBSTRATE IS MUCH MORE STABLE AND RIGID THAN A TIMBER FLOOR — ALLOWING A CONVENTIONAL SAND-CEMENT SCREED OR SELF-LEVELLING ANHYDRITE SCREED OVERLAY WITH A SURFACE-APPLIED WATERPROOF SLURRY TANKING LAYER (BAL TANKING COMPOUND, MAPEI MAPELASTIC, OR SIMILAR) APPLIED OVER THE SCREED. THE SCREED IS LAID WITH A FALL BUILT IN TO DIRECT WATER TO THE DRAIN — TYPICALLY 1:50 FALL (20MM FALL PER METRE OF DISTANCE FROM THE DRAIN — GIVING GOOD DRAINAGE WITHOUT AN EXCESSIVE SLOPE UNDERFOOT). ADVANTAGE OVER TIMBER FLOOR: HEAVIER DRAINS (ACO LINEAR CHANNEL — HEAVY STAINLESS STEEL — REQUIRING A MORTISED CHANNEL IN THE SCREED) ARE EASILY INSTALLED IN A CONCRETE SUBSTRATE — MORE DIFFICULT IN A TIMBER FLOOR CONTEXT WHERE THE DRAIN MUST BE FIXED WITHOUT COMPROMISING THE FLOOR STRUCTURE.

THE TANKING LAYER IS THE CONTINUOUS WATERPROOF MEMBRANE APPLIED TO THE FLOOR AND LOWER WALL SURFACES OF THE WET ROOM — AND IT IS THE MOST CRITICAL ELEMENT OF THE WET ROOM CONSTRUCTION. A FAILED TANKING LAYER WILL ALLOW WATER TO PENETRATE THE FLOOR OR WALL STRUCTURE — CAUSING STRUCTURAL DAMAGE, DRY ROT IN TIMBER FLOORS, AND WATER DAMAGE TO THE FLOOR BELOW.

SYSTEM 1 — WEDI BUILDING BOARD (WEDI GMBH — EMSDETTEN, GERMANY — THE LEADING FOAM CARRIER BOARD SYSTEM FOR WET ROOM INSTALLATION ON TIMBER FLOORS IN THE UK): WEDI BUILDING BOARD IS A RIGID EXTRUDED POLYSTYRENE FOAM BOARD (XPS — LAMBDA 0.034 W/MK — COMPRESSIVE STRENGTH 200 KPA — SUITABLE AS A SUBSTRATE FOR TILING WITHOUT DEFLECTION UNDER THE WEIGHT OF TILES AND GROUT) WITH A REINFORCED MORTAR COATING ON BOTH FACES (WHICH PROVIDES THE KEY FOR TILE ADHESIVE). WATERPROOFING: THE WEDI BUILDING BOARD ITSELF IS WATERPROOF — THE XPS FOAM CORE IS IMPERMEABLE TO WATER — BUT THE JOINTS BETWEEN BOARDS AND THE BOARD-TO-WALL JUNCTION MUST BE SEALED WITH WEDI SEALANT (POLYURETHANE SEALANT — APPLIED IN A BEAD AT ALL JOINTS BEFORE COVERING WITH WEDI TAPE — FIBREGLASS MESH TAPE EMBEDDED IN A SECOND BEAD OF SEALANT). THE WEDI FUNDO PRIMO (THE PRE-FABRICATED WET ROOM SHOWER DRAIN UNIT DESIGNED SPECIFICALLY FOR WEDI SYSTEMS): A FACTORY-MANUFACTURED LIGHTWEIGHT XPS DRAIN UNIT WITH A BUILT-IN CENTRAL DRAIN CHANNEL — SUPPLIED IN SIZES 800X800MM TO 1600X900MM — THE FUNDO PRIMO IS INSTALLED AS A COMPLETE UNIT WITHIN THE WEDI BOARD FIELD — PROVIDING THE FALL TO DRAIN WITHIN THE SHOWER AREA WITHOUT REQUIRING SCREEDING. PRICE (WEDI FUNDO PRIMO 900X900MM): APPROXIMATELY £200-350 (SUPPLY — UK TILE SUPPLIER OR WEDI DISTRIBUTOR).

SYSTEM 2 — SCHLUTER KERDI MEMBRANE (SCHLUTER SYSTEMS LTD — COALVILLE, LEICESTERSHIRE — THE LEADING BONDED WATERPROOF MEMBRANE SYSTEM FOR WET ROOMS IN THE UK): SCHLUTER KERDI IS A FLEXIBLE POLYETHYLENE MEMBRANE (0.2MM THICK — SUPPLIED IN ROLLS 750MM OR 1000MM WIDE) WITH A FLEECE BONDING LAYER ON BOTH FACES — THE MEMBRANE IS EMBEDDED IN UNMODIFIED TILE ADHESIVE ON THE WALL AND FLOOR SURFACES — THE FLEECE ANCHORS THE MEMBRANE TO THE ADHESIVE BED ON BOTH THE SUBSTRATE SIDE AND THE TILE SIDE. THE KERDI MEMBRANE IS APPLIED BEFORE TILING — IT IS NOT A SELF-ADHESIVE PRODUCT — IT REQUIRES AN ADHESIVE BED ON THE SUBSTRATE SURFACE AS THE KEY FOR THE FLEECE ON THE BACK FACE. KERDI DRAIN (THE SCHLUTER DRAIN SYSTEM): KERDI-DRAIN D90 (90MM SQUARE — THE POINT DRAIN FOR A CENTRAL DRAIN IN A SMALL SHOWER AREA) OR KERDI-LINE (THE LINEAR DRAIN — FOR A WALL-SIDE LINEAR CHANNEL IN A LARGER SHOWER AREA). THE KERDI MEMBRANE IS OVERLAPPED ONTO THE DRAIN FLANGE AND BONDED WITH UNMODIFIED ADHESIVE — CREATING A CONTINUOUS WATERPROOF MEMBRANE FROM THE WALL SURFACE DOWN THE FLOOR AND INTO THE DRAIN.

SYSTEM 3 — SLURRY TANKING (BAL TANKING COMPOUND / MAPEI MAPELASTIC / WEBER WEBERFLOOR FLEX — TWO-COAT LIQUID WATERPROOFING SLURRY APPLIED BY BRUSH OR ROLLER TO A SOLID SUBSTRATE — MOST APPROPRIATE FOR CONCRETE GROUND FLOOR SUBSTRATES): BAL TANKING COMPOUND (BAL — BUILDING ADHESIVES LTD — WOLSTANTON, STOKE-ON-TRENT — THE MOST WIDELY USED TANKING SLURRY IN THE UK RESIDENTIAL WET ROOM MARKET): A TWO-COMPONENT SLURRY (POWDER + LIQUID MIXED TO A CONSISTENCY SIMILAR TO THICK PAINT — APPLIED IN TWO COATS BY BRUSH — EACH COAT 0.5-1MM THICK — DRYING TO A FLEXIBLE RUBBERY MEMBRANE). BAL CORNER TAPE (POLYESTER FABRIC TAPE — EMBEDDED IN THE WET TANKING SLURRY AT ALL FLOOR-WALL JUNCTIONS AND INTERNAL CORNERS — REINFORCING THE JOINT WHERE THE GREATEST MOVEMENT OCCURS). TOTAL DRY FILM THICKNESS: 2 COATS = APPROXIMATELY 1-2MM TOTAL. PRICE: BAL TANKING COMPOUND 15KG TUB: APPROXIMATELY £50-70 (COVERS APPROXIMATELY 6-8M² IN 2 COATS).

THE DRAIN IS THE MOST VISIBLE ELEMENT OF THE WET ROOM FLOOR AND THE MOST IMPORTANT FUNCTIONAL COMPONENT — THE DRAIN MUST BE HYDRAULICALLY ADEQUATE (LARGE ENOUGH OUTLET TO HANDLE THE SHOWER FLOW RATE WITHOUT FLOODING THE FLOOR), PHYSICALLY ROBUST, AND AESTHETICALLY COMPATIBLE WITH THE TILE SPECIFICATION.

IMPEY AQUA-DEC LINEAR EVO (IMPEY — SHEPTON MALLET, SOMERSET — THE LEADING UK MANUFACTURER OF ACCESSIBLE AND WET ROOM SHOWER DRAINAGE PRODUCTS): THE AQUA-DEC LINEAR EVO IS A PACKAGED SYSTEM DESIGNED FOR TIMBER FLOOR WET ROOMS — IT INCLUDES A PRE-MANUFACTURED SLOPED TILE INSERT FORMER (A LIGHTWEIGHT POLYMER TRAY WITH A BUILT-IN 1:60 FALL TO THE LINEAR DRAIN CHANNEL — PRE-GRADED TO ELIMINATE THE NEED FOR A FALL SCREED IN A TIMBER FLOOR WET ROOM) IN SIZES: 800X800MM (AQUA-DEC EVO 800), 900X900MM (AQUA-DEC EVO 900), 1200X900MM, AND 1400X900MM. THE FORMER FITS WITHIN THE TIMBER FLOOR STRUCTURE (FIXED BETWEEN JOISTS WITH THE JOIST DEPTH USED AS THE DOWNSTAND DIMENSION — THE FORMER IS NOTCHED OVER THE JOISTS) AND SUPPORTS THE TILE LAYER DIRECTLY — THE LINEAR CHANNEL RUNNING ALONG ONE SIDE OF THE SHOWER AREA CONNECTS TO THE FLOOR WASTE (110MM DIAMETER SVP — SOIL, VENT, AND PIPE — BRANCH BELOW THE FLOOR). PRICE (IMPEY AQUA-DEC EVO 900X900MM WITH LINEAR DRAIN CHANNEL AND GRATING): APPROXIMATELY £350-550 (SUPPLY — IMPEY DISTRIBUTOR OR BATHROOM TRADE SUPPLIER).

ACO LINEAR DRAIN CHANNEL (ACO TECHNOLOGIES PLC — SHEFFORD, BEDFORDSHIRE — THE LEADING UK MANUFACTURER OF LINEAR CHANNEL DRAINAGE): ACO LINEAR SHOWER CHANNELS FOR WET ROOMS (THE ACO SHOWERDRAIN — NOT THE EXTERNAL ACO DRAINAGE CHANNEL): STAINLESS STEEL BODY (304 GRADE — BRUSHED OR POLISHED FINISH) WITH TILE INSERT OPTION (A STAINLESS STEEL FRAME WITH A CENTRAL RECESS INTO WHICH A TILE INSERT THE SAME AS THE SURROUNDING FLOOR TILE IS CUT AND DROPPED — MAKING THE DRAIN ALMOST INVISIBLE — THE MOST POPULAR PREMIUM WET ROOM DRAIN OPTION IN LONDON RESIDENTIAL WET ROOMS 2024-2025). SIZES: 700MM, 900MM, 1200MM STANDARD LENGTHS — OR BESPOKE. OUTLET: 50MM (D50) — CONNECTS TO THE FLOOR WASTE BELOW THE SCREED OR FLOOR STRUCTURE. PRICE (ACO SHOWERDRAIN 900MM WITH TILE INSERT — STAINLESS BODY): £300-500 (SUPPLY ONLY). NOTE: THE ACO TILE-INSERT DRAIN REQUIRES CAREFUL COORDINATION WITH THE TILE SPECIFICATION — THE TILE INSERT IS CUT FROM THE SAME TILE MATERIAL AS THE FLOOR (SO MATCHING CUTS MUST BE PLANNED IN THE TILE LAYOUT).

THE FALL-TO-DRAIN (THE SLOPED FLOOR SURFACE IN THE SHOWER AREA THAT DIRECTS WATER TO THE DRAIN) IS THE CRITICAL CONSTRUCTION DETAIL IN A WET ROOM — AN INSUFFICIENT FALL (LESS THAN 1:80) WILL CAUSE WATER TO POND ON THE FLOOR SURFACE — AN EXCESSIVE FALL (MORE THAN 1:40 IN THE SHOWER AREA) WILL FEEL UNCOMFORTABLE UNDERFOOT AND MAY CAUSE SLIP RISK.

FALL SPECIFICATION: THE STANDARD RECOMMENDED FALL FOR A WET ROOM SHOWER AREA IS 1:50 (20MM FALL PER METRE — E.G., A 1M X 1M SHOWER AREA NEEDS 20MM TOTAL HEIGHT DIFFERENCE BETWEEN THE HIGHEST CORNER AND THE DRAIN). FOR A LINEAR DRAIN ALONG ONE WALL OF A 1.2M WIDE SHOWER AREA: THE FLOOR MUST FALL 1.2 / 50 = 24MM FROM THE OPPOSITE WALL TO THE DRAIN. THE FALL IS IN ONE DIRECTION ONLY (NOT A FOUR-WAY FALL TO A CENTRAL POINT DRAIN — A FOUR-WAY FALL REQUIRES 1.5-2X THE SCREED DEPTH AND IS MORE COMPLEX TO FORM ACCURATELY). FOR A CENTRAL POINT DRAIN (OLDER APPROACH): THE FALL IS IN FOUR DIRECTIONS FROM THE FOUR CORNERS TO THE CENTRAL DRAIN — THE TILES MUST BE CUT AS WEDGE SHAPES TO ACCOMMODATE THE FOUR-WAY FALL WITHOUT LIPPING AT TILE JUNCTIONS.

FALL SCREED (CONCRETE SOLID FLOOR): A SAND-CEMENT SCREED (1:4 MIX — CEMENT:SHARP SAND BY VOLUME — MINIMUM 50MM DEPTH AT THE THIN EDGE OVER A SOLID CONCRETE SUBSTRATE) IS LAID WITH THE FALL BUILT IN. THE FALL IS FORMED BY: FIXING A SCREED RAIL AT THE CORRECT HEIGHT ON EACH SIDE OF THE SHOWER AREA — THE SCREEDER THEN LEVELS THE SCREED TO THE RAILS — ACHIEVING THE REQUIRED FALL BETWEEN RAILS. ALTERNATIVELY: A SELF-LEVELLING ANHYDRITE SCREED (HANSON CEMFLOOR, LAFARGE ARDIEX) CAN BE USED FOR THE BASE LAYER AND A FALL FORMER (E.G., IMPEY FREEFLOW SYSTEM — A LIGHTWEIGHT POLYMER FORMER WITH THE FALL PRE-BUILT) PLACED ON TOP. FALL FORMER (TIMBER FLOOR): THE IMPEY AQUA-DEC LINEAR EVO OR WEDI FUNDO PRIMO ARE THE PRINCIPAL FALL FORMER SYSTEMS FOR TIMBER FLOOR SUBSTRATES — BOTH SUPPLY A PRE-MANUFACTURED TRAY WITH THE FALL BUILT IN — AVOIDING THE NEED FOR A WET SCREED IN A TIMBER FLOOR CONTEXT (WHICH WOULD ADD EXCESSIVE DEAD LOAD TO THE TIMBER JOISTS AND WOULD BE PRONE TO CRACKING AS THE TIMBER FLOOR MOVES). THE IMPEY AQUA-DEC EVO IS THE MOST WIDELY SPECIFIED SYSTEM FOR UK TIMBER FLOOR WET ROOMS — SUPPLIER TECHNICAL SUPPORT AND INSTALLER GUIDES ARE WELL-ESTABLISHED.

THE TILE SPECIFICATION FOR A WET ROOM MUST ADDRESS THREE SPECIFIC REQUIREMENTS NOT APPLICABLE TO STANDARD DRY BATHROOM TILING: SLIP RESISTANCE (THE FLOOR TILE MUST HAVE ADEQUATE SLIP RESISTANCE WHEN WET — R10 OR R11 CLASSIFICATION UNDER DIN 51130 — OR CLASS B/C UNDER DIN 51097 FOR BAREFOOT AREAS), JOINT WIDTH (NARROW GROUT JOINTS ARE MORE DIFFICULT TO KEEP CLEAN AND MORE PRONE TO MOULD IN A WET ROOM ENVIRONMENT — MINIMUM JOINT WIDTH 2-3MM FOR WALL TILES IN A WET ROOM), AND TILE SIZE (LARGER FORMAT TILES REQUIRE FEWER GROUT JOINTS BUT REQUIRE A FLATTER SUBSTRATE — THE FLATNESS TOLERANCE FOR 600MM+ FORMAT TILES IS MORE CRITICAL THAN FOR 300MM TILES).

FLOOR TILE SPECIFICATION FOR WET ROOM: PORCELANOSA URBATEK XLIGHT 600X600MM (PORCELANOSA GROUP — CASTELLON, SPAIN — THE MOST WIDELY DISTRIBUTED PREMIUM PORCELAIN TILE BRAND IN UK RESIDENTIAL RENOVATION): THE URBATEK XLIGHT RANGE IS A LARGE-FORMAT (600X600MM AND 600X1200MM) FULL-BODY PORCELAIN TILE IN LARGE-FORMAT SLABS (5.6MM THICK — ALSO AVAILABLE IN 12MM THICK FOR STANDARD INSTALLATION). SLIP RESISTANCE: URBATEK XLIGHT IS AVAILABLE IN NATURAL (MATT) FINISH — R10 CLASSIFICATION (SUITABLE FOR WET ROOM FLOORS). FOR HIGHER SLIP RESISTANCE: R11 CLASSIFICATION TILES (TEXTURED OR STRUCTURED SURFACE) — ALSO AVAILABLE IN THE URBATEK RANGE. GROUT RECOMMENDATION: MAPEI ULTRACOLOR PLUS FA (ANTI-MOULD, ANTI-EFFLORESCENCE — THE STANDARD SPECIFICATION FOR WET ROOM JOINTS IN LONDON RESIDENTIAL WET ROOMS). 2-3MM JOINT WIDTH (RECOMMENDED FOR 600X600MM FLOOR TILES). ALTERNATIVE FLOOR TILE: PORCELAIN MOSAIC (50X50MM OR 25X25MM — THE TRADITIONAL WET ROOM FLOOR TILE — THE SMALL FORMAT ALLOWS THE TILES TO FOLLOW THE FLOOR FALL WITHOUT LIPPING — HOWEVER, THE HIGH NUMBER OF GROUT JOINTS IS MORE DIFFICULT TO KEEP CLEAN). ADHESIVE: BAL RAPIDAFLEX (C2FTE CLASSIFICATION — FLEXIBLE, FAST-SET, FOR LARGE FORMAT TILES ON TIMBER AND SCREED SUBSTRATES IN WET ROOMS). WALL TILE SPECIFICATION: PORCELANOSA URBATEK XLIGHT 600X1200MM (5.6MM THICK — MAXIMUM FORMAT FOR WALL TILES — REQUIRES A FLAT, SOUND WALL SUBSTRATE — LEVEL TO WITHIN 3MM/2M UNDER A 2M STRAIGHTEDGE — TYPICALLY WEDI BUILDING BOARD OR CEMENT BOARD SUBSTRATE). ADHESIVE: BAL FLEX SINGLE PART S1 (POLYMER-MODIFIED FLEXIBLE — NOTCHED TROWEL 10MM NOTCH FOR LARGE FORMAT — BACK-BUTTER THE TILE AS WELL AS THE SUBSTRATE FOR LARGE FORMAT). GROUT: MAPEI ULTRACOLOR PLUS FA 3MM JOINT WIDTH.

WET ROOM INSTALLATION COSTS IN LONDON (2025) FOR A STANDARD LOFT CONVERSION EN-SUITE WET ROOM (APPROXIMATELY 2.0M X 1.5M — 3M² FLOOR AREA — TIMBER FLOOR SUBSTRATE — WEDI BOARD SYSTEM — LINEAR DRAIN — 600X1200MM PORCELAIN WALL TILES — 600X600MM FLOOR TILES — GROHE THERMOSTATIC SHOWER SYSTEM — SUPPLY AND FIT).

SUPPLY COSTS (ILLUSTRATIVE — QUALITY MID-RANGE SPECIFICATION): WEDI BUILDING BOARD (FLOOR AND WALLS — APPROXIMATELY 15M² TOTAL AT £20-30/M²): £300-450. WEDI FUNDO PRIMO DRAIN UNIT (900X900MM): £200-350. WEDI SEALANT AND TAPE: £50-80. PORCELANOSA URBATEK 600X600MM FLOOR TILES (3M² AT £50-90/M²): £150-270. PORCELANOSA URBATEK 600X1200MM WALL TILES (15M² AT £50-90/M²): £750-1,350. GROHE RAPID SL CONCEALED CISTERN FRAME + GROHE EURO CERAMIC WC: £400-600. GROHE TEMPESTA 210 CUBE THERMOSTATIC SHOWER SYSTEM (GROHE 34741000 — GROHE UK, BERKSHIRE — CONCEALED THERMOSTATIC VALVE + 300MM SQUARE HEAD + HAND SHOWER): £600-900. BAL RAPIDAFLEX ADHESIVE + BAL TANKING COMPOUND: £100-150. MAPEI ULTRACOLOR PLUS FA GROUT: £50-80. ACO SHOWERDRAIN 900MM WITH TILE INSERT: £300-500. TOTAL SUPPLY: £2,900-4,730.

LABOUR (SPECIALIST BATHROOM INSTALLER — LONDON — 2025): WEDI BOARD FIX TO TIMBER FLOOR (FLOOR AND WALLS): £400-600. DRAIN AND PIPEWORK INSTALLATION (PLUMBER — CONNECTION TO WASTE): £200-400. TILING (FLOOR AND WALLS — 18M² TOTAL — AT £40-60/M²): £720-1,080. GROUTING AND SEALING: £100-150. SANITARY WARE INSTALLATION (WC, SHOWER FIXTURES): £300-500. TOTAL LABOUR: £1,720-2,730. TOTAL SUPPLY AND FIT (STANDARD EN-SUITE WET ROOM 2.0X1.5M): £4,620-7,460. FOR A LARGER FAMILY BATHROOM WET ROOM (3.0M X 2.0M — 6M² FLOOR): TOTAL SUPPLY AND FIT: £7,000-12,000. NOTE: WATERPROOFING FAILURES ARE THE MOST EXPENSIVE PROBLEM IN WET ROOM CONSTRUCTION — THE COST OF STRIPPING AND RE-TANKING A FAILED WET ROOM (REMOVING TILES, SUBSTRATE, AND TANKING LAYER DOWN TO THE FLOOR STRUCTURE AND STARTING AGAIN) IS TYPICALLY £3,000-6,000 — SPECIFICATION OF PROPER TANKING SYSTEM (WEDI BOARD OR SCHLUTER KERDI — NOT INFERIOR TANKING COMPOUNDS APPLIED TO AN UNSUITABLE SUBSTRATE) AND USE OF A SPECIALIST WET ROOM INSTALLER (RATHER THAN A GENERAL TILER WITHOUT WET ROOM EXPERIENCE) IS THE MOST IMPORTANT COST-AVOIDANCE DECISION.