Concrete Floors for London Victorian Terrace Extensions and Renovations

A NEW SINGLE-STOREY REAR EXTENSION TO A LONDON VICTORIAN TERRACE REQUIRES A GROUND-BEARING CONCRETE SLAB — THE FLOOR SLAB THAT SITS ON THE COMPACTED HARDCORE BED AND PROVIDES THE STRUCTURAL FLOOR OF THE EXTENSION. MODERN THERMAL REGULATIONS REQUIRE THAT THE SLAB IS INSULATED TO MEET PART L 2021 REQUIREMENTS (MAXIMUM U-VALUE OF 0.13 W/M²K FOR A NEW EXTENSION GROUND FLOOR).

GROUND-BEARING SLAB BUILD-UP (BOTTOM TO TOP — STANDARD SPECIFICATION FOR LONDON REAR EXTENSION): 1. SUBGRADE: LONDON CLAY OR MADE GROUND (REMOVE TOPSOIL AND ANY FILL MATERIAL TO UNDISTURBED GROUND — MINIMUM 150MM BELOW FINISHED FORMATION LEVEL). 2. TYPE 1 AGGREGATE: 150MM COMPACTED TARMAC TOPBASE TYPE 1 (LEAN MIX CONCRETE 100MM IF SOFT GROUND — BUILD CONTROL MAY REQUIRE TRIAL PIT INSPECTION TO CONFIRM GROUND CONDITION). 3. BLINDING CONCRETE: 50MM C10 BLINDING CONCRETE (SMOOTH SURFACE FOR DPM INSTALLATION — PREVENTS HARDCORE PUNCTURING THE DPM). 4. DPM: 1200 GAUGE POLYTHENE DPM (VISQUEEN SUPER FLEXIBLE PLUS — 300 MICRON) — LAPPED 300MM AT JOINTS, SEALED WITH DUCT TAPE, TURNED UP AT EDGES TO LINK WITH WALL DPC. 5. INSULATION: KINGSPAN THERMAFLOOR TF70 PIR INSULATION (100MM THICKNESS — ACHIEVES 0.12 W/M²K U-VALUE FOR GROUND FLOOR WITH 150MM CONCRETE SLAB ABOVE). ALTERNATIVELY: CELOTEX GA4000 100MM PIR. LAID IN TWO LAYERS OF 50MM WITH STAGGERED JOINTS TO MINIMISE COLD BRIDGING AT BOARD JOINTS. 6. STRUCTURAL CONCRETE SLAB: 150MM C25/30 CONCRETE (HANSON READYMIX OR CEMEX) WITH A393 STEEL MESH (8MM BAR AT 200MM CENTRES IN BOTH DIRECTIONS) IN THE MIDDLE OF THE SLAB. LAID MONOLITHICALLY (IN ONE POUR), VIBRATED, AND LEVELLED TO ± 5MM OVER 3M SPAN (FLOOR FLATNESS TOLERANCE BS 8204).

EDGE INSULATION: THE THERMAL BRIDGE AT THE PERIMETER OF THE GROUND-BEARING SLAB (WHERE THE SLAB EDGE MEETS THE WALL) IS A MAJOR SOURCE OF HEAT LOSS. EDGE INSULATION (PERINSUL OR WÄRMEDÄMMELEMENT THERMOSTOP — LOAD-BEARING THERMAL BREAK BLOCK UNDER THE INNER LEAF WALL ON TOP OF THE SLAB EDGE) REDUCES THIS THERMAL BRIDGE — RECOMMENDED ON ALL LONDON EXTENSION GROUND SLABS.

LIQUID SCREED (ALSO CALLED FLOWING SCREED, ANHYDRITE SCREED, OR SELF-COMPACTING SCREED) IS THE STANDARD SPECIFICATION FOR UNDERFLOOR HEATING INSTALLATIONS IN LONDON REAR EXTENSIONS AND WHOLE-HOUSE RENOVATIONS — IT PROVIDES EXCELLENT THERMAL CONDUCTIVITY, EMBEDS UFH PIPES COMPLETELY, AND SELF-LEVELS TO A SMOOTH, FLAT FINISH.

HANSON CEMFLOOR (THE MOST WIDELY SPECIFIED LIQUID SCREED IN LONDON — CALCIUM SULPHATE / ANHYDRITE BINDER): HANSON CEMFLOOR IS A CALCIUM SULPHATE (ANHYDRITE) BINDER LIQUID SCREED SUPPLIED BY TRUCK IN READY-MIX FORM — PUMPED INTO THE ROOM AND SPREAD ACROSS THE UFH PIPE LAYOUT USING A SPREADER BAR. SELF-LEVELLING (FLOWS TO FILL AROUND UFH PIPES AND BETWEEN JOIST BAYS). TYPICAL INSTALLED DEPTH: 65MM ABOVE THE INSULATION SURFACE (MINIMUM 30MM COVER OVER UFH PIPES — PIPE OUTSIDE DIAMETER TYPICALLY 16MM, SO MINIMUM 16MM + 14MM COVER = 30MM ABOVE PIPE). FLOW RATE: ONE PUMP TRUCK CAN LAY APPROXIMATELY 1,000 M² PER DAY — MAKING CEMFLOOR PARTICULARLY EFFICIENT FOR LONDON RENOVATION PROJECTS WITH MULTIPLE ROOMS. CURING: CEMFLOOR REQUIRES 24 TO 48 HOURS BEFORE WALK-ON AND 7 TO 14 DAYS MINIMUM BEFORE LIGHT LOADING. DRYING: CEMFLOOR TAKES APPROXIMATELY 1MM OF SCREED DEPTH PER DAY TO DRY (A 65MM SCREED TAKES APPROXIMATELY 65 DAYS UNDER NATURAL DRYING CONDITIONS — OR CAN BE ACCELERATED WITH UFH COMMISSIONING CYCLING FROM DAY 7). SCREED MUST DRY TO BELOW 75% RH (BS EN ISO 29767 HYGROMETER TEST) BEFORE LVT, TILE ADHESIVE, OR CARPET IS LAID.

ARDEX FEATHER FINISH (SELF-LEVELLING COMPOUND FOR EXISTING SOLID FLOORS THAT ARE UNEVEN OR HAVE SURFACE DEFECTS): FOR EXISTING SOLID FLOORS IN LONDON VICTORIAN TERRACES (TYPICALLY 100MM CONCRETE LAID IN THE 1950S-70S WITHOUT INSULATION) THAT ARE UNEVEN OR HAVE DAMAGED SURFACES, A SELF-LEVELLING COMPOUND OVER THE EXISTING SLAB PROVIDES A FLAT SUBSTRATE FOR THE FINISHED FLOOR COVERING WITHOUT FULL SCREED REPLACEMENT: ARDEX A45 MIDI SELF-LEVELLING COMPOUND (3MM TO 30MM DEPTH — 24-HOUR WALK-ON STRENGTH; AVAILABLE IN BAGS FROM TRAVIS PERKINS).

POLISHED CONCRETE IS AN INCREASINGLY POPULAR FINISH FLOORING CHOICE IN LONDON HIGH-SPECIFICATION KITCHEN-DINERS AND OPEN-PLAN LIVING SPACES — PARTICULARLY IN CONTEMPORARY REAR EXTENSION PROJECTS WHERE A RAW, INDUSTRIAL AESTHETIC IS DESIRED.

POLISHED CONCRETE CAN BE ACHIEVED BY TWO METHODS: METHOD 1: POLISH-IN-PLACE (THE STRUCTURAL CONCRETE SLAB ITSELF IS POLISHED — USING DIAMOND-IMPREGNATED GRINDING PADS ON A FLOOR GRINDER): THE EXISTING OR NEW STRUCTURAL SLAB IS GROUND PROGRESSIVELY WITH DIAMOND PADS FROM COARSE (16 GRIT) TO FINE (1500 GRIT) — REVEALING THE AGGREGATE WITHIN THE CONCRETE AND PRODUCING A MIRROR-FINISH OR SATIN-FINISH SURFACE. CONCRETE AGGREGATE MIX SPECIFICATION: FOR A POLISHED CONCRETE FLOOR IN A LONDON EXTENSION, THE CONCRETE MIX SHOULD SPECIFY THE AGGREGATE (TYPICALLY 20MM ROUNDED GRAVEL OR CONTRASTING STONE — RIVER PEBBLE, FLINT, OR GRANITE — WHICH WILL BE EXPOSED BY POLISHING). EXPOSED AGGREGATE DESIGN: THE HOMEOWNER CHOOSES THE AGGREGATE COLOUR AND FINISH DEPTH — HEAVIER GRINDING EXPOSES MORE AGGREGATE (SALT-AND-PEPPER EFFECT — EVENLY DISTRIBUTED AGGREGATE VISIBLE); LIGHTER GRINDING PRODUCES A CREAM FINISH (LESS AGGREGATE VISIBLE). METHOD 2: MICRO-CEMENT OR MICROTOPPING OVERLAY (A THIN CEMENT-BASED COATING APPLIED OVER AN EXISTING SLAB TO PROVIDE A POLISHED CONCRETE APPEARANCE WITHOUT FULL SLAB REPLACEMENT): ARDEX ARDIFLEX (2MM TO 4MM MICROTOPPING OVER PRIMED CONCRETE, TILES, OR PLYWOOD — POLISHED WITH A ROTARY BUFFER). DEGUSSA UCRETE (RESIN-MODIFIED FLOOR COATING IN CONCRETE AESTHETIC — EXTREMELY DURABLE, CHEMICAL RESISTANT). MICRO-CEMENT IS POPULAR IN LONDON BECAUSE IT CAN BE APPLIED OVER EXISTING FLOORS WITHOUT FULL STRUCTURAL DEMOLITION.

THE MOST COMMON USE OF A LIQUID SCREED IN A LONDON RENOVATION IS TO EMBED UNDERFLOOR HEATING PIPES — THE LIQUID SCREED ENCAPSULATES THE PIPE, PROVIDING INTIMATE THERMAL CONTACT AND AN EVEN HEAT DISTRIBUTION ACROSS THE FLOOR.

UFH PIPE SPECIFICATION (UNDER LIQUID SCREED IN A LONDON EXTENSION): NU-HEAT LOOPSTRATE PRO (16MM OD MLCP — MULTI-LAYER COMPOSITE PIPE, ALUMINIUM BARRIER LAYER — OXYGEN BARRIER ESSENTIAL FOR WET UFH SYSTEMS). UPONOR COMFORT PIPE PLUS (16MM OD MLCP — SIMILAR SPECIFICATION). POLYPIPE UNDERFLOOR HEATING PIPE (15MM OR 16MM OD POLYBUTYLENE). PIPE LAYOUT: SNAKE OR SNAIL PATTERN — MAXIMUM LOOP LENGTH 120M FOR 16MM PIPE (PRESSURE DROP CONSTRAINT). PIPE SPACING: 100MM TO 200MM CENTRES — CLOSER SPACING FOR HIGHER-DEMAND AREAS (KITCHEN-DINER, BATHROOMS), WIDER FOR BEDROOMS AND LOW-LOAD AREAS. MANIFOLD: GIACOMINI R57 MANIFOLD (ACTUATOR-CONTROLLED ROOM-BY-ROOM THERMOSTAT ZONES — EACH ROOM HAS A SEPARATE LOOP FROM THE MANIFOLD ALLOWING INDEPENDENT CONTROL). SCREED COVERAGE ABOVE PIPE: MINIMUM 30MM ABOVE PIPE OUTER DIAMETER (SO 30MM + 16MM PIPE OD = 65MM MINIMUM SCREED DEPTH — THE STANDARD HANSON CEMFLOOR UFH INSTALLATION DEPTH). COMMISSIONING: AFTER CEMFLOOR CURING (7 DAYS MINIMUM), THE UFH IS COMMISSIONED BY GRADUALLY RAISING THE FLOW TEMPERATURE FROM 20°C TO 45°C OVER 7 DAYS — THIS DRIVES MOISTURE OUT OF THE SCREED AND PREVENTS THERMAL SHOCK CRACKING.

CONCRETE FLOOR COSTS IN LONDON DEPEND ON WHETHER THE WORK INVOLVES A NEW STRUCTURAL SLAB, A LIQUID SCREED, OR A POLISHED CONCRETE FINISH.

NEW INSULATED GROUND-BEARING SLAB (LONDON REAR EXTENSION — COMPLETE BUILD-UP FROM FORMATION TO CONCRETE SURFACE): HARDCORE TYPE 1 (150MM): £8 TO £15 PER M² SUPPLY AND COMPACT. BLINDING CONCRETE (50MM C10): £10 TO £18 PER M². DPM (VISQUEEN 1200G): £3 TO £6 PER M². PIR INSULATION (KINGSPAN TF70 100MM — 2X50MM): £25 TO £40 PER M². STRUCTURAL SLAB (150MM C25/30 + A393 MESH — READY-MIX CONCRETE AND LABOUR): £45 TO £70 PER M². TOTAL INSULATED SLAB (PER M²): £91 TO £149 PER M² SUPPLY AND INSTALL (EXCLUDING FORMWORK AND SHUTTERING).

LIQUID SCREED (HANSON CEMFLOOR 65MM OVER UFH — PER M²): £15 TO £25 PER M² SUPPLY AND PUMP (MINIMUM ORDER TYPICALLY 30 M² FOR PUMPED DELIVERY). UFH PIPE LAYOUT AND MANIFOLD INSTALLATION: £25 TO £40 PER M² ADDITIONAL. TOTAL UFH SCREED SYSTEM: £40 TO £65 PER M².

SELF-LEVELLING COMPOUND (ARDEX A45 3-10MM OVER EXISTING SLAB): £15 TO £30 PER M² SUPPLY AND INSTALL.

POLISHED CONCRETE (GRIND AND POLISH EXISTING SLAB — PER M²): £60 TO £120 PER M² (DEPENDING ON SPECIFICATION, GRIT SEQUENCE, AND SEALER CHOICE).

MICRO-CEMENT OVERLAY (ARDIFLEX 2-4MM OVER EXISTING FLOOR — SUPPLY AND INSTALL): £70 TO £130 PER M² SUPPLY AND INSTALL.