Concrete Slab Floors for London Victorian Terrace Extensions and Refurbishments

THE GROUND PREPARATION FOR A CONCRETE SLAB FLOOR IN A LONDON VICTORIAN TERRACE EXTENSION OR GROUND FLOOR REFURBISHMENT IS THE FOUNDATION ON WHICH THE ENTIRE FLOOR BUILD-UP DEPENDS — INADEQUATE PREPARATION LEADS TO FUTURE SLAB HEAVE, SUBSIDENCE, DAMP, OR DIFFERENTIAL SETTLEMENT BETWEEN THE SLAB AND THE SURROUNDING STRUCTURE.

EXCAVATION DEPTH: THE EXCAVATION DEPTH FOR A NEW GROUND-BEARING SLAB IN A REAR EXTENSION ON A LONDON VICTORIAN TERRACE IS DETERMINED BY THE REQUIRED SLAB FORMATION LEVEL — WORKING BACKWARDS FROM THE REQUIRED FINISHED FLOOR LEVEL (FFL) WHICH IS TYPICALLY MATCHED TO THE INTERNAL FLOOR LEVEL OF THE EXISTING PROPERTY: FROM THE REQUIRED FFL DEDUCT: THICKNESS OF FLOOR FINISH (TYPICALLY 10-20MM FOR CERAMIC/PORCELAIN TILE, 14-22MM FOR ENGINEERED TIMBER, 2-4MM FOR LVT); THICKNESS OF SCREED (TYPICALLY 65-75MM LIQUID SCREED SUCH AS HANSON CEMFLOOR OR LAFARGE THERMIA IF UFH, OR 65-75MM SAND AND CEMENT SCREED IF NO UFH); THICKNESS OF PIR INSULATION (TYPICALLY 100MM KINGSPAN KOOLTHERM K3 FLOORBOARD OR CELOTEX GA4000 PIR — SEE BELOW); THICKNESS OF DPM (1200 GAUGE VISQUEEN SUPER FLEXIBLE PLUS DPM — APPROXIMATELY 0.3MM — NEGLIGIBLE BUT INCLUDE IN FORMATION LEVEL CALCULATION); THICKNESS OF CONCRETE SLAB (TYPICALLY 100MM MINIMUM FOR A GROUND-BEARING SLAB IN A RESIDENTIAL EXTENSION — BUILDING CONTROL WILL TYPICALLY REQUIRE MINIMUM 100MM C25/P CONCRETE — SEE BELOW); THICKNESS OF COMPACTED HARDCORE BASE (TARMAC TOPBASE TYPE 1 LIMESTONE AGGREGATE OR EQUIVALENT CLEAN ANGULAR HARDCORE — MINIMUM 150MM COMPACTED DEPTH — 200-250MM LOOSE DEPTH BEFORE COMPACTION USING WACKER PLATE); LONDON-SPECIFIC CONSIDERATIONS: (1) LONDON CLAY — LONDON CLAY HAS HIGH VOLUME-CHANGE POTENTIAL (HEAVE AND SHRINKAGE WITH MOISTURE CONTENT CHANGES — PARTICULARLY RELEVANT WHERE TREES ARE PRESENT) — WHERE TREES ARE WITHIN APPROXIMATELY 1.5X MATURE TREE HEIGHT OF THE PROPOSED SLAB, A STRUCTURAL ENGINEER SHOULD BE CONSULTED ON WHETHER PRECAUTIONS ARE NEEDED (ANTI-HEAVE BOARDS, COMPRESSIBLE LAYER, OR VOID-FORM SYSTEM) TO PROTECT THE SLAB AGAINST CLAY HEAVE; (2) MADE GROUND AND FILL — LONDON HAS EXTENSIVE AREAS OF MADE GROUND AND HISTORIC FILL (RUBBLE, ASH, CINDERS) PARTICULARLY IN THE BACK GARDEN AREAS OF VICTORIAN TERRACES — MADE GROUND IS NOT A SUITABLE DIRECT BEARING STRATUM FOR A GROUND-BEARING SLAB WITHOUT CONSOLIDATION — BUILDING CONTROL WILL TYPICALLY REQUIRE CONFIRMATION OF FOUNDING CONDITIONS; (3) CONTAMINATION — IN SOME PARTS OF LONDON (PARTICULARLY AREAS WITH INDUSTRIAL HISTORY) GROUND CONTAMINATION MAY AFFECT THE FILL MATERIAL — REPORT TO BUILDING CONTROL IF UNUSUAL MATERIALS OR ODOURS ARE ENCOUNTERED DURING EXCAVATION.

THE SLAB BUILD-UP ABOVE THE COMPACTED HARDCORE (FROM BOTTOM UP) IS: BLINDING LAYER + DPM + PIR INSULATION + STRUCTURAL MESH + CONCRETE.

BLINDING LAYER (SHARP SAND BLINDING — TYPICALLY 50MM COMPACTED DEPTH): A THIN LAYER OF SHARP SAND IS LAID AND COMPACTED OVER THE COMPACTED HARDCORE TO PROVIDE A SMOOTH, LEVEL SURFACE FOR THE DPM — PREVENTS THE HARDCORE AGGREGATE FROM PUNCTURING THE DPM; DPM (DAMP-PROOF MEMBRANE — PROTECTS THE SLAB AND FLOOR CONSTRUCTION FROM GROUND MOISTURE AND SOIL GAS): SPECIFICATION: VISQUEEN SUPER FLEXIBLE PLUS 1200 GAUGE POLYTHENE SHEET DPM (1200 GAUGE = 300 MICRON THICKNESS — THE BUILDING REGULATIONS MINIMUM FOR A GROUND FLOOR DPM IS 1000 GAUGE — 1200 GAUGE IS THE STANDARD SPECIFICATION FOR NEW-BUILD AND EXTENSION SLABS IN LONDON); LAP AND TAPE: MINIMUM 150MM OVERLAP AT ALL JOINTS — TAPED WITH VISQUEEN POLYTHENE TAPE OR FULLY WELDED — CRITICAL THAT LAPS ARE PROPERLY SEALED TO PREVENT MOISTURE OR RADON INGRESS AT JOINTS; DPM MUST BE LAPPED UP THE INTERNAL FACE OF ALL SURROUNDING WALLS TO AT LEAST DPC HEIGHT — CONNECTED TO THE WALL DPC — TO COMPLETE THE CONTINUOUS HORIZONTAL MOISTURE BARRIER BENEATH THE SLAB; WHERE RADON PROTECTION IS REQUIRED (PART C AND THE RADON MAP — LONDON IS GENERALLY IN THE LOWER-RISK ZONES FOR RADON BUT SOME AREAS REQUIRE RADON PROTECTION — THE BUILDING CONTROL OFFICER WILL ADVISE): THE DPM ALSO ACTS AS A RADON BARRIER — ALL JOINTS MUST BE FULLY SEALED; PIR INSULATION (RIGID CLOSED-CELL PIR (POLYISOCYANURATE) INSULATION BOARD — INSTALLED ABOVE THE DPM AND BELOW THE CONCRETE SLAB OR (ALTERNATIVELY IN SOME SPECIFICATIONS) ABOVE THE SLAB AND BELOW THE SCREED): SPECIFICATION: KINGSPAN KOOLTHERM K3 FLOORBOARD (RATED COMPRESSIVE STRENGTH APPROXIMATELY 150 KPA — SUITABLE FOR RESIDENTIAL FLOOR LOADING) OR CELOTEX GA4000 (100 KPA — ALSO SUITABLE FOR RESIDENTIAL) — AT 100MM DEPTH: U-VALUE OF COMBINED SLAB/INSULATION ASSEMBLY APPROXIMATELY 0.18-0.22 W/M²K DEPENDING ON FLOOR PLAN GEOMETRY (EDGE EFFECTS INCREASE U-VALUE — LARGER FLOOR AREA = BETTER EFFECTIVE U-VALUE); PART L 2021 TARGET: 0.15 W/M²K FOR NEW GROUND FLOORS (MAXIMUM — LOWER IS BETTER) — 100MM KINGSPAN KOOLTHERM K3 TYPICALLY ACHIEVES 0.15-0.20 W/M²K DEPENDING ON AREA AND PLAN GEOMETRY — FOR BETTER PERFORMANCE, INCREASE INSULATION DEPTH TO 120MM OR 150MM; STRUCTURAL MESH (REINFORCING MESH FABRIC — BINDS THE CONCRETE SLAB AND PROVIDES CRACK CONTROL): SPECIFICATION: A193 MESH (AREA OF STEEL 193 MM²/M — 7MM BAR AT 200MM CENTRES EACH WAY) — THE MINIMUM STANDARD MESH FOR A GROUND-BEARING RESIDENTIAL SLAB; A252 MESH (8MM BAR AT 200MM CENTRES — SLIGHTLY HEAVIER — USED WHERE HEAVIER LOCALISED LOADS ARE EXPECTED OR WHERE THE ENGINEER SPECIFIES); SUPPORT: MESH MUST BE SUPPORTED AT MID-DEPTH OF THE CONCRETE SLAB USING PLASTIC SPACER CHAIRS (BAR CHAIRS — MINIMUM 25MM COVER TO ALL STEEL FROM THE BASE) — NOT RESTING ON THE INSULATION BOARD.

THE CONCRETE GRADE AND CASTING METHOD FOR A RESIDENTIAL EXTENSION GROUND-BEARING SLAB IN LONDON ARE CRITICAL TO THE LONG-TERM PERFORMANCE OF THE FLOOR — WEAK OR POORLY CAST CONCRETE LEADS TO SURFACE DUSTING, CRACKING, INSUFFICIENT STRENGTH, AND POTENTIAL BUILDING CONTROL NON-COMPLIANCE.

CONCRETE GRADE (BS 8500-1 / BS EN 206 — DESIGNATED CONCRETE — DESIGNED MIX): C25/P (GEN 3 — THE MINIMUM BUILDING CONTROL STANDARD FOR A GROUND-BEARING RESIDENTIAL SLAB IN MANY LONDON BOROUGHS — THE DESIGNATION C25/P: C25 = MINIMUM 28-DAY CHARACTERISTIC COMPRESSIVE CUBE STRENGTH 25 N/MM², P = PRESCRIBED MIX WITH DEFINED WORKABILITY — SLUMP OR FLOW CLASS) — SPECIFY AS GEN 3 FROM THE CONCRETE SUPPLIER (HANSON READYMIX, CEMEX, AGGREGATE INDUSTRIES READYMIX, BREEDON CONCRETE — ALL HAVE READY-MIX PLANTS ACROSS LONDON); RC30 OR C30/P (GEN 4 — THE MORE COMMON CURRENT SPECIFICATION USED BY GOOD CONTRACTORS FOR LONDON RESIDENTIAL SLABS — BETTER DURABILITY, LOWER W/C RATIO, SLIGHTLY HIGHER STRENGTH — RECOMMENDED WHERE SULPHATE EXPOSURE CLASS DS-1 OR DS-2 IS APPLICABLE — LONDON CLAY GROUNDWATER IS TYPICALLY IN ACEC CLASS AC-1 OR AC-2 — THE BUILDING CONTROL OFFICER OR STRUCTURAL ENGINEER WILL ADVISE ON SULPHATE CLASS); WORKABILITY: SLUMP CLASS S3 (100-150MM SLUMP) IS APPROPRIATE FOR A GROUND-BEARING SLAB POURED AROUND MESH REINFORCEMENT — TOO STIFF (S1 OR S2) MAKES COMPACTION AND LEVELLING DIFFICULT; VOLUME: CALCULATE VOLUME (LENGTH X WIDTH X 0.1M) + 5-10% WASTE ALLOWANCE — ORDER FROM READY-MIX SUPPLIER — DO NOT MIX SITE-MIXED CONCRETE FOR A STRUCTURAL SLAB (INCONSISTENT QUALITY); CASTING PROCEDURE: (1) PLACE READY-MIX CONCRETE IN SECTIONS — DO NOT OVERFILL OR DUMP FULL LOAD IN ONE PLACE (MOVES MESH AND INSULATION); (2) COMPACT CONCRETE WITH A POKER VIBRATOR (TYPICALLY 38MM OR 50MM DIAMETER — USED IN OVERLAPPING INSERTIONS NOT MORE THAN 600MM APART) — CRITICAL FOR ELIMINATING VOIDS AND ACHIEVING FULL COMPACTION AROUND MESH; (3) SCREED LEVEL WITH STRAIGHT EDGE OR SCREED RAIL (DATUM PEGS OR LASER LEVEL) TO ACHIEVE FLAT LEVEL FORMATION FOR INSULATION AND SCREED; (4) FLOAT SURFACE FLAT WITH A BULL FLOAT OR HAND FLOAT — DO NOT OVER-TROWEL WHILE WET (SURFACE LAITANCE RISK); (5) PROTECT FROM FROST AND DRYING (COVER WITH POLYTHENE SHEET FOR MINIMUM 3 DAYS IN COLD WEATHER — MINIMUM CURING PERIOD 7 DAYS BEFORE LOADING OR SCREED APPLICATION).

THE SCREED LAYER OVER THE CONCRETE SLAB (AND WHERE APPLICABLE OVER THE INSULATION ABOVE THE SLAB) PROVIDES THE LEVEL, SMOOTH, HIGH-TOLERANCE SURFACE REQUIRED FOR THE FLOOR FINISH — AND (WHERE UFH IS INSTALLED) ENCAPSULATES THE UFH PIPEWORK AND PROVIDES THERMAL MASS FOR HEAT EMISSION.

LIQUID SCREED (CALCIUM SULPHATE ANHYDRITE SCREED — ALSO CALLED FLOW SCREED OR POURED SCREED — THE DOMINANT CHOICE FOR UFH FLOOR SYSTEMS AND LARGE-AREA INSTALLATIONS IN LONDON): MAIN PRODUCTS: HANSON CEMFLOOR (HANSON UK — A READY-MIX CALCIUM SULPHATE SCREED DELIVERED IN A MIXER TRUCK OR PUMP TRUCK — THE MOST WIDELY USED LIQUID SCREED IN THE UK RESIDENTIAL MARKET); LAFARGE THERMIA FLOOR (SAINT-GOBAIN — CALCIUM SULPHATE LIQUID SCREED WITH ENHANCED THERMAL CONDUCTIVITY — 1.6-2.0 W/MK — PARTICULARLY SUITED TO UFH APPLICATIONS); ADVANTAGES OF LIQUID SCREED: FASTER INSTALLATION (A TEAM OF TWO CAN INSTALL 200-300M² PER DAY VS 30-50M²/DAY FOR HAND-APPLIED SAND AND CEMENT SCREED — SIGNIFICANT SAVING IN LABOUR TIME ON LARGE FLOOR AREAS); SELF-LEVELLING (FLOWS AND LEVELS ITSELF TO A FLAT SURFACE — FLATNESS TOLERANCE SR2 IS ACHIEVABLE — SR1 WITH SURFACE GRINDING); ENCAPSULATES UFH PIPES WITHOUT VOIDS (FLOWS AROUND ALL PIPEWORK); DEPTH: MINIMUM 50MM ABOVE TOP OF UFH PIPE FOR LIQUID SCREED OVER UFH (TYPICALLY UFH PIPE IS 16MM OD — MINIMUM TOTAL SCREED DEPTH 65-70MM); CURING TIME: LIQUID SCREED IS TYPICALLY WALKABLE AT 24-48 HOURS — FULL CURE FOR HARD FLOOR FINISHES 28-42 DAYS — CALCIUM SULPHATE SCREED IS SENSITIVE TO MOISTURE DURING CURING (MUST BE COVERED AND PROTECTED FROM RAIN); SURFACE TREATMENT: CALCIUM SULPHATE SCREED MUST BE SANDED (ABRADED) AFTER INITIAL CURE TO REMOVE THE LAITANCE LAYER (A WEAK CALCIUM SULPHATE SURFACE SKIN) BEFORE APPLICATION OF ADHESIVE FLOOR TILES OR TIMBER — IF NOT SANDED, ADHESION OF FLOOR FINISH MAY FAIL; SAND AND CEMENT SCREED (TRADITIONAL — SEMI-DRY CONSISTENCY — TROWELLED AND TAMPED BY HAND — LOWER MATERIAL COST BUT HIGHER LABOUR COST PER M² FOR LARGE AREAS): MIX: 1 PART OPC : 3.5 PARTS SHARP SAND (SEMI-DRY MIX — CONSISTENCY OF DAMP SAND — SHOULD HOLD ITS SHAPE WHEN SQUEEZED BUT NOT SQUEEZE OUT WATER) — MINIMUM DEPTH 65MM (UNBONDED OR FLOATING — NOT BONDED TO CONCRETE SLAB); OVER UFH: MINIMUM 65MM ABOVE TOP OF PIPE; CURING: 28 DAYS MINIMUM FOR HARD FLOOR FINISH APPLICATION; SHRINKAGE CRACKING: SAND AND CEMENT SCREED IS MORE PRONE TO SHRINKAGE CRACKING THAN LIQUID SCREED — BAY SIZES SHOULD BE CONTROLLED (MAX 40M² OR MAX DIMENSION 8M UNLESS MOVEMENT JOINTS ARE FORMED — SIKA JOINT SEALANT AT ALL MOVEMENT JOINTS).

CONCRETE GROUND-BEARING SLAB AND SCREED COSTS FOR LONDON VICTORIAN TERRACE EXTENSIONS AND GROUND FLOOR REFURBISHMENTS (LONDON 2025 — APPROXIMATE RANGE — SUPPLY AND FIT — GROUNDWORKS/FLOORING CONTRACTOR — LABOUR AND MATERIALS).

GROUND PREPARATION AND EXCAVATION (DIG OUT TO FORMATION LEVEL — REMOVE EXISTING MATERIAL — HAND EXCAVATION WHERE ACCESS IS RESTRICTED — PER M² OF FLOOR AREA): APPROXIMATELY £15-35/M² (MECHANICALLY ASSISTED — MINI EXCAVATOR ACCESS THROUGH PROPERTY OR SIDE GATE — LONDON RATES); APPROXIMATELY £30-60/M² (FULL HAND EXCAVATION WHERE NO MECHANICAL ACCESS — TYPICAL FOR INNER-LONDON TERRACE WITH NO SIDE GATE ACCESS — ALL MATERIAL CARRIED THROUGH PROPERTY — LONDON PREMIUM). HARDCORE COMPACTION (TYPE 1 COMPACTED — 150MM MINIMUM DEPTH — SUPPLY AND COMPACT — WACKER PLATE COMPACTION — PER M²): APPROXIMATELY £12-22/M² (SUPPLY AND COMPACT). DPM (1200 GAUGE VISQUEEN SUPER FLEXIBLE PLUS — SUPPLY AND LAY — PER M²): APPROXIMATELY £3-6/M² (SUPPLY AND LAY FLAT). PIR INSULATION (100MM KINGSPAN KOOLTHERM K3 OR CELOTEX GA4000 — SUPPLY AND LAY — PER M²): APPROXIMATELY £22-38/M² (100MM DEPTH — SUPPLY AND LAY). CONCRETE SLAB (100MM C25/P OR C30/P — READY-MIX — SUPPLY, PUMP/PLACE, COMPACT, AND FLOAT — PER M²): APPROXIMATELY £20-38/M² (SUPPLY AND PLACE — LONDON RATES — CONCRETE PUMP TYPICALLY REQUIRED FOR RESTRICTED-ACCESS REAR EXTENSIONS — PUMP HIRE ADDS APPROXIMATELY £400-700 PER POUR). LIQUID SCREED OVER UFH (HANSON CEMFLOOR OR LAFARGE THERMIA — MINIMUM 65MM ABOVE PIPE — SUPPLY AND POUR — PER M²): APPROXIMATELY £18-30/M² (SUPPLY AND POUR — PUMP-APPLIED). TOTAL CONCRETE SLAB FLOOR PACKAGE (GROUND PREPARATION + HARDCORE + DPM + 100MM PIR INSULATION + 100MM CONCRETE SLAB + LIQUID SCREED — EXCLUDING UFH PIPEWORK AND SCREED SURFACE PREPARATION — PER M²): APPROXIMATELY £90-190/M² (MECHANICALLY ASSISTED EXCAVATION — TYPICAL LONDON EXTENSION WITH SOME MECHANICAL ACCESS); APPROXIMATELY £120-240/M² (FULL HAND EXCAVATION — RESTRICTED ACCESS — INNER-LONDON TERRACE — LONDON PREMIUM FOR HAND WORK). NOTE: THESE RATES ARE FOR GROUNDWORK AND SLAB CONSTRUCTION ONLY — THEY EXCLUDE FLOOR FINISH, UFH MANIFOLD AND PIPEWORK, PLUMBING AND HEATING CONNECTIONS, AND ANY STRUCTURAL ELEMENT OF THE EXTENSION WALLS, ROOF, OR FOUNDATIONS.