Concrete Floor Repair and Replacement in London Victorian Homes: A Complete Guide

BEFORE DECIDING WHETHER TO REPAIR OR REPLACE THE EXISTING CONCRETE SLAB, A PROPER ASSESSMENT IS NEEDED. THE FOLLOWING INDICATORS GUIDE THE DECISION:

SYMPTOMS SUGGESTING REPLACEMENT IS NEEDED: - RISING DAMP TIDE MARK AT SKIRTING LEVEL (WHITE SALT EFFLORESENCE — CALCIUM SULPHATE OR HYGROSCOPIC NITRATE SALTS) — INDICATES DPM IS ABSENT OR FAILED - FLOOR FEELS COLD TO THE TOUCH EVEN WITH CARPET — INDICATES NO THERMAL INSULATION - HOLLOW SOUND WHEN TAPPED — INDICATES DELAMINATION OF SCREED FROM SLAB - SIGNIFICANT CRACKING — INDICATES POSSIBLE SETTLEMENT, SHRINKAGE, OR HEAVE - FLOOR LEVELS INCONSISTENT ACROSS ROOM — SETTLEMENT OF POORLY COMPACTED HARDCORE BENEATH - MOISTURE METER READING ABOVE 75% RELATIVE HUMIDITY AT SLAB SURFACE (USING HYGROMETER WITH 72-HOUR EQUILIBRATION) — INDICATES ACTIVE DAMP

CONDITION ASSESSMENT APPROACH: 1. CORE SAMPLE: DRILL A 100MM CORE TO INSPECT THE BUILD-UP — ASSESS DEPTH OF SLAB, PRESENCE/ABSENCE OF DPM, HARDCORE QUALITY 2. GROUND INVESTIGATION: INSPECT BENEATH SLAB IF ACCESS ALLOWS — CHECK FOR VOIDS, RUBBLE HARDCORE, MADE GROUND 3. DAMP MAPPING: CALCIUM CARBIDE (SPEEDY) MOISTURE TEST ON SCREED SAMPLES — MORE ACCURATE THAN ELECTRICAL RESISTANCE METERS 4. LEVEL SURVEY: CHECK EXISTING FLOOR LEVELS WITH LASER LEVEL — DETERMINE TOLERANCE FOR RAISING FINISHED FLOOR LEVEL (AFFECTS WINDOW SILL HEIGHTS, DOOR THRESHOLDS, STEP INTO EXTENSION)

REPAIR VS REPLACEMENT DECISION: REPAIR (RESCREED ONLY) IS SUITABLE WHERE: DPM IS CONFIRMED PRESENT AND INTACT; SLAB IS STRUCTURALLY SOUND; DELAMINATION IS LIMITED TO SCREED SURFACE ONLY; LEVEL TOLERANCES ALLOW A THIN OVERLAY. IN MOST LONDON VICTORIAN TERRACES TACKLED IN THE MID-20TH CENTURY, THE DPM IS ABSENT — MAKING FULL REPLACEMENT THE MORE RELIABLE LONG-TERM SOLUTION.

A COMPLIANT MODERN GROUND FLOOR SLAB IN A LONDON VICTORIAN TERRACE (POST-RENOVATION) FOLLOWS A SPECIFIC BUILD-UP. THE TOTAL THICKNESS ADDED VERSUS THE EXISTING SLAB TOP LEVEL IS APPROXIMATELY 100-150MM (DEPENDING ON INSULATION SPECIFICATION) — WHICH MUST BE RECONCILED WITH DOOR THRESHOLDS AND ADJACENT FLOOR LEVELS.

RECOMMENDED BUILD-UP FROM BOTTOM TO TOP:

1. EXISTING SUBSOIL / HARDCORE: COMPACT EXISTING GROUND. IF EXISTING HARDCORE IS RUBBLE (COMMON IN LONDON), ASSESS AND COMPACT. IF GROUND HAS BEEN REDUCED (SLAB BROKEN OUT), LAY NEW 150MM TYPE 1 GRANULAR FILL (TARMAC TOPBASE TYPE 1) COMPACTED IN LAYERS.

2. CONCRETE SLAB: MINIMUM 100MM REINFORCED C25/30 CONCRETE (HANSON READYMIX C25P OR EQUIVALENT). A393 STEEL MESH (8MM BARS AT 200MM CENTRES EACH WAY) IN THE SLAB. ALTERNATIVELY 150MM UNREINFORCED C25/30 FOR LIGHTLY LOADED FLOORS. PERIMETER EDGE INSULATION REQUIRED TO PREVENT THERMAL BRIDGING AT THE WALL/FLOOR JUNCTION.

3. DAMP PROOF MEMBRANE (DPM): 1200-GAUGE (300 MICRON) POLYETHYLENE DPM (VISQUEEN SUPER FLEXIBLE PLUS OR EQUIVALENT) LAID OVER THE SLAB — WITH ALL LAPS TAPED AND EDGES TURNED UP TO DPC LEVEL OR ABOVE. THE DPM MUST LINK TO THE WALL DPC. WHERE ACTIVE DAMP IS PRESENT, CONSIDER Newton SYSTEM 500 STUDDED MEMBRANE OR SIKA WATERTIGHT BASEMENT SYSTEM UNDER THE SLAB FOR BELT-AND-BRACES.

4. RIGID INSULATION: MINIMUM 75MM PIR BOARD (CELOTEX GA4000, IKO ENERTHERM, OR XTRATHERM THIN-R) — ACHIEVING PART L (2021) FLOOR U-VALUE REQUIREMENT OF 0.13 W/M²K WITH 100MM PIR UNDER 65MM SCREED. FOR UNDERFLOOR HEATING, THE INSULATION LAYER IS CRITICAL — MINIMUM 100MM PIR (LAMBDA 0.022-0.022 W/MK).

5. SCREED: TRADITIONAL SAND:CEMENT SCREED (1:3.5 MIX, CT-C25-F5 PER BS EN 13813): MINIMUM 65MM OVER INSULATION (75MM OVER UNDERFLOOR HEATING PIPES). ALLOW 4-6 WEEKS DRYING AT 25MM/WEEK BEFORE TILES, OR USE LIQUID CALCIUM SULPHATE SCREED (HANSON CEMFLOOR, LAFARGE AGILIA): 40-50MM, FASTER DRYING (TYPICALLY 10-14 DAYS WITH FORCED VENTILATION), SELF-LEVELLING. NOTE: CALCIUM SULPHATE SCREED REQUIRES PRIMING BEFORE TILING (MAPEI PRIMER L OR ARDEX P51) — THIS IS FREQUENTLY FORGOTTEN AND CAUSES TILE BOND FAILURE.

6. FLOOR FINISH: ACCORDING TO SPECIFICATION — PORCELAIN TILES, WOOD FLOORING, CARPET (NOT RECOMMENDED OVER UFH), VINYL/LVT.

DAMP MANAGEMENT IS THE MOST CRITICAL ELEMENT OF A LONDON GROUND FLOOR SLAB — GETTING THIS WRONG MEANS THE FLOOR WILL REQUIRE REPLACEMENT AGAIN WITHIN 5-15 YEARS.

STANDARD 1200-GAUGE POLYETHYLENE DPM: THE WORKHORSE SOLUTION — VISQUEEN SUPER FLEXIBLE PLUS 1200G (300 MICRON) OR EQUIVALENT. KEY INSTALLATION RULES: - ALL LAPS MINIMUM 300MM AND TAPED WITH VISQUEEN SELF-ADHESIVE TAPE - EDGES TURNED UP MINIMUM 150MM AT WALL JUNCTIONS - DPM MUST LINK CONTINUOUSLY TO THE WALL DPC - NO PUNCTURES FROM SCREWS, PIPE PENETRATIONS, OR SHARP HARDCORE - IF LAID ABOVE INSULATION (WARM FLOOR), ENSURE NO COLD BRIDGING AT PERIMETER

BONDED EPOXY DPM ON TOP OF EXISTING SLAB (REPAIR SCENARIO): WHERE REPLACING THE ENTIRE SLAB IS NOT FEASIBLE, AN EPOXY DPM APPLIED TO THE SURFACE OF THE EXISTING SLAB CAN PROVIDE A BARRIER — TYPICALLY: SIKA IGASOL DM 2K, MAPEI MAPEPROOF, OR REMMERS BETOSURF-IMP. TWO-COAT SYSTEM. THIS IS A LESS RELIABLE SOLUTION THAN A PHYSICAL DPM — IT RELIES ON THE EPOXY BOND TO A POTENTIALLY DAMP OR CONTAMINATED SUBSTRATE AND CANNOT DEAL WITH POSITIVE WATER PRESSURE.

STUDDED MEMBRANE SYSTEM (CAVITY DRAINAGE — MANAGES RATHER THAN EXCLUDES DAMP): NEWTON SYSTEM 500 HDPE STUDDED MEMBRANE (8MM STUD HEIGHT), WITH NEWTON BASEDRAIN PERIMETER CHANNEL AND TWIN-PUMP SUMP (NEWTON TITAN 210 OR SIMER 2305B). DIVERTS WATER AROUND AND BENEATH THE FLOOR RATHER THAN EXCLUDING IT. MORE EXPENSIVE BUT APPROPRIATE WHERE ACTIVE WATER INGRESS IS PRESENT AND CANNOT BE EXCLUDED. USED IN BASEMENT CONVERSIONS AND CELLARS.

LINKING TO WALL DPC: THE MOST COMMON FAILURE POINT IN LONDON RENOVATION PROJECTS IS FAILURE TO LINK THE FLOOR DPM TO THE WALL DPC. IF THERE IS NO DPC (PRE-1875 PROPERTY — MANY LONDON TERRACES), A RETROSPECTIVE DPC INJECTION (WYKAMOL ULTRAGRADE DPC CREAM, SAFEGUARD DRYZONE) SHOULD BE CARRIED OUT BEFORE THE NEW SLAB IS LAID, AND THE DPM LAPPED AND SEALED TO THE TREATED BED JOINT.

CONCRETE FLOOR WORK IS DISRUPTIVE — THE HOME IS TYPICALLY UNINHABITABLE DURING GROUNDWORK — AND REQUIRES CAREFUL SEQUENCING RELATIVE TO OTHER TRADES.

TYPICAL GROUND FLOOR SLAB REPLACEMENT COSTS (LONDON): - BREAK OUT EXISTING SLAB AND DISPOSE (30M² ROOM): £1,500-3,000 (INCLUDING SKIP HIRE) - SUPPLY AND LAY NEW 100MM REINFORCED SLAB (MATERIALS + LABOUR): £2,000-4,000 - DPM (1200G PE, LAPS AND EDGES): £300-600 - 100MM PIR INSULATION BOARDS (CELOTEX GA4000): £800-1,500 - 65MM SAND:CEMENT SCREED (SUPPLIED AND LAID, BAYS, SCREEDED): £1,500-3,000 TOTAL (GROUNDWORK ONLY, EXCLUDING FLOOR FINISH): £6,000-12,000 FOR A TYPICAL 30M² GROUND FLOOR (TWO ROOMS)

LIQUID SCREED PREMIUM: ADD £300-600 FOR LIQUID CALCIUM SULPHATE SCREED VS SAND:CEMENT IN A 30M² AREA.

UNDERFLOOR HEATING ADDITION: ADD £1,500-3,000 FOR UFH PIPE AND MANIFOLD SUPPLY AND FIT.

SEQUENCING: 1. BREAK OUT EXISTING SLAB — TYPICALLY 1-2 DAYS 2. GROUND INVESTIGATION / REDUCED LEVEL DIG IF NEEDED — 1 DAY 3. HARDCORE REINSTATEMENT AND COMPACT — 1 DAY 4. SLAB POUR AND CURE (MINIMUM 7 DAYS BEFORE LOADING) — 7-10 DAYS 5. DPM LAY — HALF DAY 6. INSULATION LAY — HALF DAY 7. SCREED (SAND:CEMENT) — 1 DAY; THEN 4-6 WEEKS DRYING 8. FLOOR FINISH — AFTER DRYING PERIOD TOTAL PROGRAMME FROM BREAK-OUT TO SCREED READY FOR FINISH: 3-4 WEEKS (EXCLUDING DRYING)

NOTE: BUILDING CONTROL INSPECTION IS REQUIRED AT FOUNDATION/SLAB STAGE FOR EXTENSIONS — FOR MAIN HOUSE FLOOR REPLACEMENT, INSPECTION IS NOT TYPICALLY REQUIRED BUT CONFIRM WITH LOCAL AUTHORITY.