Concrete Ground Floor Slabs for London Victorian Terraces: Specification, Insulation, and UFH

THE FIRST STAGE OF GROUND FLOOR SLAB INSTALLATION IN A LONDON VICTORIAN TERRACE IS STRIPPING OUT THE EXISTING SUSPENDED TIMBER FLOOR AND EXCAVATING DOWN TO FORMATION LEVEL (THE LEVEL AT WHICH THE NEW HARDCORE AND CONCRETE SLAB WILL BE LAID).

STRIPPING THE EXISTING FLOOR: REMOVE EXISTING FLOORBOARDS (TYPICALLY 22MM T&G SOFTWOOD — CAN BE SALVAGED IF IN GOOD CONDITION FOR REUSE ELSEWHERE). REMOVE EXISTING JOISTS (TYPICALLY 50X175MM TREATED SOFTWOOD C16 — USUALLY BEYOND ECONOMIC REPAIR AFTER 100+ YEARS IN A DAMP SUSPENDED FLOOR VOID). REMOVE WALL PLATES (TIMBER MEMBERS BUILT INTO THE EXTERNAL WALLS AT JOIST LEVEL — THESE MUST BE CAREFULLY CUT OUT AND THE MASONRY MADE GOOD — ANY REMAINING EMBEDDED TIMBER IN AN EXTERNAL WALL IS A POTENTIAL PATHWAY FOR MOISTURE AND DECAY). REMOVE EXISTING SUB-BASE (THE VOID BELOW THE SUSPENDED FLOOR IN A LONDON VICTORIAN TERRACE IS TYPICALLY FILLED TO A VARIABLE DEPTH WITH: BUILDING RUBBLE (DEMOLITION WASTE AND MORTAR FROM THE ORIGINAL CONSTRUCTION), CLINKER AND ASH (FROM THE ORIGINAL VICTORIAN COAL FIRE AND COOKING RANGE), CONTAMINATED SOIL. ALL OF THIS MATERIAL MUST BE REMOVED TO ALLOW INSPECTION OF THE SUBSOIL CONDITION AND TO ALLOW THE CORRECT DEPTH OF CLEAN HARDCORE AND CONCRETE.

EXCAVATION: THE TARGET FORMATION LEVEL IN A LONDON VICTORIAN TERRACE IS TYPICALLY 300-450MM BELOW THE EXISTING FLOORBOARD LEVEL (DEPENDING ON THE REQUIRED FINISHED FLOOR LEVEL AND THE BUILD-UP OF HARDCORE, BLINDING, DPM, PIR INSULATION, CONCRETE SLAB, AND SCREED OR TILE BED): HANSON CEMFLOOR LIQUID SCREED 65MM + TILE ADHESIVE 3-5MM + TILE 10-15MM: ADD 78-85MM. ALTERNATIVELY: POLISHED CONCRETE FINISH (FLOOR SLAB FINISHED SMOOTH AND POLISHED AS THE FINISHED FLOOR — NO SCREED REQUIRED — BUT REQUIRING HIGHER SPECIFICATION CONCRETE POUR FLATNESS AND TROWEL FINISH). FORMATION LEVEL BEARING: THE FORMATION LEVEL IN LONDON (LONDON CLAY SUBSOIL — SOUTH AND INNER LONDON) OR THAMES GRAVEL (EAST AND SOUTH LONDON ALONG THE THAMES FLOODPLAIN) MUST BE INSPECTED BY THE BUILDING CONTROL INSPECTOR AT FORMATION LEVEL TO CONFIRM ADEQUATE BEARING CAPACITY (BUILDING CONTROL HOLD POINT).

THE GROUND-BEARING CONCRETE SLAB BUILD-UP FOR A LONDON VICTORIAN TERRACE GROUND FLOOR CONVERSION IS TYPICALLY:

1. COMPACTED HARDCORE (TYPE 1 MOT SUB-BASE): TARMAC TOBASE TYPE 1 (CRUSHED LIMESTONE OR RECYCLED CONCRETE AGGREGATE — MAXIMUM 40MM PARTICLE SIZE — VIBRO-COMPACTED IN 150MM LAYERS). TOTAL DEPTH: 150MM MINIMUM (USUALLY 150-250MM DEPENDING ON EXCAVATION DEPTH AND TARGET FINISHED FLOOR LEVEL). COMPACTION: EACH 150MM LAYER VIBRO-COMPACTED WITH A PLATE COMPACTOR (WACKER PLATE) TO ACHIEVE A DENSE, NON-YIELDING SURFACE. DO NOT USE FILL MATERIAL FROM THE EXCAVATED SPOIL AS HARDCORE — IT IS CONTAMINATED AND HETEROGENEOUS.

2. SAND BLINDING (SHARP SAND — 50MM DEPTH): LAID OVER THE COMPACTED HARDCORE TO FILL VOIDS AND PROVIDE A SMOOTH, LEVEL SURFACE FOR THE DPM AND INSULATION.

3. DAMP-PROOF MEMBRANE (DPM — VISQUEEN SUPER FLEXIBLE PLUS 1200 GAUGE — 300 MICRON): POLYTHENE SHEET DPM LAPPED UP THE INTERNAL FACE OF THE EXTERNAL WALLS (TO LINK WITH THE WALL DPC — CRITICAL FOR CREATING A CONTINUOUS MOISTURE BARRIER BETWEEN GROUND AND BUILDING). LAPS BETWEEN SHEETS: MINIMUM 300MM — TAPED WITH VISQUEEN DOUBLE-SIDED SELF-ADHESIVE TAPE. VISQUEEN SUPER FLEXIBLE PLUS 1200 GAUGE (300 MICRON) IS THE INDUSTRY STANDARD DPM FOR GROUND-BEARING SLABS IN UK RESIDENTIAL CONSTRUCTION.

4. PIR RIGID INSULATION BOARD (KINGSPAN KOOLTHERM K3 FLOORBOARD — 70-100MM DEPENDING ON TARGET U-VALUE): KINGSPAN KOOLTHERM K3 (PHENOLIC FOAM CORE — LAMBDA 0.018 W/MK — THE HIGHEST PERFORMANCE RIGID FLOOR INSULATION AVAILABLE IN THE UK): 70MM K3 = U-VALUE 0.25 W/M²K (JUST MEETING APPROVED DOCUMENT L 2021 NOTIONAL DWELLING — 0.25 W/M²K); 100MM K3 = U-VALUE 0.18 W/M²K (EXCEEDING L 2021 — STRONGLY RECOMMENDED FOR NEW SLAB INSTALLATIONS WHERE THE EXCAVATION DEPTH ALLOWS). ALTERNATIVELY: CELOTEX TB4000 (PIR FOAM — LAMBDA 0.022 W/MK — 90MM FOR 0.25 W/M²K — CHEAPER THAN KOOLTHERM BUT SLIGHTLY LOWER PERFORMANCE PER MM). BOARDS LAID BUTT-JOINTED (NOT TAPED — FLOATING UNDER SLAB — THE BOARDS ARE NOT FIXED TO THE DPM OR SLAB — THEY FLOAT FREE).

5. CONCRETE SLAB (C25/P OR RC30 — 100MM MINIMUM — HAND-FLOAT OR POWER-FLOAT FINISH): CONCRETE SPECIFICATION: HANSON READYMIX C25/P (ORDINARY PORTLAND CEMENT MIX — SUITABLE FOR INTERNAL GROUND-BEARING SLAB IN A DRY CONDITION). RC30 (REINFORCED CONCRETE — REQUIRED WHERE THE SLAB CARRIES A CONCENTRATED LOAD OR WHERE SPANS EXCEED 3M WITHOUT AN INTERMEDIATE SUPPORT). REINFORCEMENT: A393 STEEL MESH (8MM BARS AT 200MM CENTRES IN BOTH DIRECTIONS — PLACED ON CONCRETE SPACERS — 50MM COVER — FOR ANY SLAB REQUIRING REINFORCEMENT). FINISH: POWER-FLOAT (HELICOPTER TROWEL FINISH) FOR A FLAT, HARD SURFACE READY FOR SCREED OR DIRECT TILE BED. MINIMUM THICKNESS: 100MM.

A NEW CONCRETE GROUND FLOOR SLAB IN A LONDON VICTORIAN TERRACE IS THE IDEAL OPPORTUNITY TO INSTALL A WET UNDERFLOOR HEATING (UFH) SYSTEM — THE PIPES ARE LAID ON TOP OF THE INSULATION BEFORE THE SCREED IS POURED, AND ARE COMPLETELY HIDDEN WITHIN THE FLOOR BUILD-UP.

UFH SYSTEM SPECIFICATION FOR A LONDON VICTORIAN TERRACE GROUND FLOOR: NU-HEAT LOOPSTRATE SYSTEM (THE MOST WIDELY SPECIFIED WET UFH SYSTEM FOR UK RESIDENTIAL RENOVATION — COMPRISING: FOIL-FACED FOAM CARRIER BOARD (30MM THICK) WITH PRE-FORMED CHANNELS FOR MLCP PIPE. UPONOR MLCP 16MM PIPE (OR NU-HEAT BRAND MLCP — MULTI-LAYER COMPOSITE PIPE — 16MM OD, 11.6MM ID — OXYGEN-BARRIER MLCP PREVENTS OXYGEN DIFFUSION INTO THE SYSTEM WHICH WOULD CAUSE CORROSION IN THE HEAT SOURCE). PIPE LAYOUT: DOUBLE-RETURN LOOP PATTERN (BIFILAR OR DOUBLE-SERPENTINE) — THE HOTTEST PIPE (FLOW) AND THE COOLEST PIPE (RETURN) RUN ADJACENT THROUGHOUT EACH ROOM, ACHIEVING A MORE EVEN FLOOR TEMPERATURE THAN A SINGLE-SERPENTINE LAYOUT.

UFH MANIFOLD (GIACOMINI R557 OR R462 — BRASS MANIFOLD — 2-8 CIRCUITS): INSTALLED IN A MANIFOLD CABINET (SURFACE-MOUNTED OR RECESSED — 300X500MM MINIMUM CABINET SPACE). FLOW TEMPERATURE: 40-50°C FLOW (LOW-TEMPERATURE UNDERFLOOR HEATING — SUITABLE FOR HEAT PUMP OUTPUT). RETURN TEMPERATURE: 30-40°C RETURN. MAXIMUM PIPE RUN PER CIRCUIT: 100M (16MM MLCP AT STANDARD FLOW RATE — LIMITS THE COVERAGE OF EACH MANIFOLD CIRCUIT — LARGER ROOMS REQUIRE MULTIPLE CIRCUITS).

SCREED OVER UFH: HANSON CEMFLOOR EASY FLOW LIQUID SCREED (CALCIUM SULPHATE ANHYDRITE LIQUID SCREED): 65MM MINIMUM DEPTH OVER PIPE (I.E. PIPE CENTRE TO TOP OF SCREED = 65MM MINIMUM). POURED LIQUID — SELF-LEVELLING — FLOWS AROUND THE PIPES WITHOUT VOIDS. HEATING COMMISSIONING: LEAVE 28 DAYS TO CURE BEFORE HEATING UP. HEAT COMMISSIONING PROTOCOL: START AT 25°C FLOW FOR 3 DAYS, INCREASE BY 5°C PER DAY TO MAXIMUM OPERATING TEMPERATURE, HOLD FOR 3 DAYS, DROP BACK TO 20°C BEFORE TILING. HEATMISER NEOSTAT V2 (OR TADO WIRELESS SMART THERMOSTAT): ZONED THERMOSTAT CONTROL FOR EACH MANIFOLD CIRCUIT — ENABLES ROOM-BY-ROOM TEMPERATURE CONTROL AND SMART HOME INTEGRATION (TADO WORKS WITH ALEXA, GOOGLE HOME, APPLE HOMEKIT).

LONDON'S DISTINCTIVE SUBSOIL CONDITIONS — PRIMARILY LONDON CLAY IN INNER AND SOUTH LONDON AND THAMES GRAVEL TERRACE IN RIVERSIDE AREAS — HAVE SPECIFIC IMPLICATIONS FOR GROUND-BEARING SLAB DESIGN.

LONDON CLAY: LONDON CLAY IS AN OVERCONSOLIDATED TERTIARY MARINE CLAY — HIGH IN PLASTICITY (IP 40-60%), SUBJECT TO SIGNIFICANT VOLUME CHANGE WITH MOISTURE CONTENT CHANGE (SHRINKAGE IN DRY SUMMERS — SWELLING IN WET WINTERS — HEAVE AND SETTLEMENT): LONDON CLAY FOUNDATION DEPTH IMPLICATIONS: THE STANDARD MINIMUM FOUNDATION DEPTH IN LONDON CLAY FOR A STRIP FOUNDATION IS 1M (OR MORE — DEEPER IN AREAS WITH ESTABLISHED TREES WHERE CLAY SHRINKAGE UNDER THE FOOTPRINT OF THE TREE CAN EXTEND 2-3M DEPTH). GROUND-BEARING SLAB RISK IN LONDON CLAY: LONDON CLAY CAN UNDERGO SWELLING WHERE IT HAS BEEN PREVIOUSLY DESICCATED (E.G. BELOW A DEMOLISHED BUILDING OR WHERE LARGE TREE ROOTS HAVE BEEN REMOVED). A GROUND-BEARING SLAB ON LONDON CLAY WHERE SWELLING IS POSSIBLE CAN BE UPLIFTED. MITIGATION: WHERE TREES HAVE BEEN REMOVED FROM THE FOOTPRINT OF THE NEW SLAB: A GROUND-BEARING SLAB IS NOT APPROPRIATE — A SUSPENDED SLAB (BEAM-AND-BLOCK OR OMNIA PRECAST SLAB) IS REQUIRED TO SPAN OVER THE POTENTIAL HEAVE ZONE. WHERE NO TREES ARE PRESENT AND THE CLAY HAS NOT BEEN PREVIOUSLY DESICCATED: A GROUND-BEARING SLAB IS ACCEPTABLE — BUILDING CONTROL MUST INSPECT FORMATION LEVEL BEFORE CASTING.

RADON: PARTS OF LONDON (PARTICULARLY SOUTH LONDON — LAMBETH, WANDSWORTH, MERTON) ARE IN LOW-TO-MEDIUM RADON ZONES (UK HEALTH SECURITY AGENCY RADON MAP). A GROUND-BEARING SLAB PROVIDES SIGNIFICANTLY BETTER RADON PROTECTION THAN AN ORIGINAL SUSPENDED TIMBER FLOOR (WHICH HAS MANY GAPS AND PENETRATIONS). WHERE THE SITE IS IN A RADON-AFFECTED ZONE: THE BUILDING REGULATIONS (APPROVED DOCUMENT C — SITE PREPARATION AND RESISTANCE TO CONTAMINANTS) REQUIRE RADON PROTECTION MEASURES: BASIC RADON PROTECTION (BELOW 10% AFFECTED AREA — MOST OF LONDON): POLYTHENE DPM AS A RADON BARRIER + WELL-SEALED AROUND ALL SERVICE PENETRATIONS. FULL RADON PROTECTION (10% OR MORE AFFECTED AREA): POSITIVE PRESSURE VENTILATION BELOW SLAB OR SUMPED MEMBRANE SYSTEM. CHECK THE UKHSA RADON MAP (RADON.CO.UK) FOR THE SPECIFIC SITE RADON LEVEL BEFORE DESIGNING THE SLAB.

CONVERTING A SUSPENDED TIMBER GROUND FLOOR TO A CONCRETE GROUND-BEARING SLAB IN A LONDON VICTORIAN TERRACE REQUIRES BUILDING REGULATIONS APPROVAL — TYPICALLY VIA A BUILDING NOTICE OR FULL PLANS APPLICATION TO THE LOCAL AUTHORITY BUILDING CONTROL (LABC) OR APPROVED INSPECTOR.

BUILDING REGULATIONS PARTS RELEVANT TO GROUND FLOOR SLAB CONVERSION: APPROVED DOCUMENT A (STRUCTURE): THE REMOVAL OF THE EXISTING TIMBER FLOOR (WHICH MAY PROVIDE LATERAL SUPPORT TO THE EXTERNAL WALLS IN THE ORIGINAL CONSTRUCTION — UNLIKELY IN A VICTORIAN TERRACE BUT SHOULD BE ASSESSED) AND THE NEW SLAB DESIGN MUST BE STRUCTURALLY ADEQUATE. BUILDING CONTROL HOLD POINT: FORMATION LEVEL INSPECTION — MANDATORY BEFORE POURING ANY HARDCORE OR CONCRETE. APPROVED DOCUMENT C (SITE PREPARATION — DAMP AND MOISTURE): THE DPM MUST BE CONTINUOUS AND LINKED TO THE WALL DPC — BUILDING CONTROL WILL CHECK THIS. APPROVED DOCUMENT L (CONSERVATION OF FUEL AND POWER — 2021 EDITION): MINIMUM U-VALUE FOR GROUND FLOOR IN A DWELLING: 0.25 W/M²K (WHERE THERMAL BRIDGING AND INSULATION CONTINUITY ARE ADEQUATELY DETAILED). WHERE A COMPLETE FLOOR REPLACEMENT IS UNDERTAKEN IN AN EXISTING BUILDING: L 2021 CONSERVATION OF FUEL AND POWER (EXISTING BUILDINGS) APPLIES — THE NEW FLOOR U-VALUE SHOULD MEET THE RELEVANT STANDARD.

PARTY WALL ACT: EXCAVATION WITHIN 3M OF AN ADJOINING OWNER'S BUILDING (AT A DEPTH THAT WOULD AFFECT THE FOUNDATIONS): IF THE EXCAVATION FOR THE NEW SLAB (TYPICALLY 300-450MM BELOW EXISTING FLOOR — WHICH IS TYPICALLY AROUND 600-900MM BELOW EXTERNAL GROUND LEVEL) FALLS WITHIN 3M OF THE PARTY WALL WITH AN ADJOINING PROPERTY: A PARTY WALL NOTICE (SECTION 6 — EXCAVATION NOTICE) MAY BE REQUIRED UNDER THE PARTY WALL ACT 1996. OBTAIN LEGAL ADVICE OR A PARTY WALL SURVEYOR ASSESSMENT BEFORE EXCAVATING CLOSE TO THE PARTY WALL.

AIR BRICKS (VENTILATION OF REMAINING SUSPENDED FLOOR VOIDS): WHERE ONLY PART OF THE GROUND FLOOR IS CONVERTED TO A SLAB (E.G. THE FRONT RECEPTION ROOM ONLY — REAR KITCHEN STILL SUSPENDED): THE AIR BRICKS SERVING THE REMAINING SUSPENDED FLOOR VOID MUST REMAIN CLEAR AND UNOBSTRUCTED — BUILDING CONTROL WILL CHECK THIS. DO NOT DPM OR CONCRETE OVER AIR BRICK OPENINGS — THE SUSPENDED FLOOR VOID MUST REMAIN VENTILATED.

GROUND FLOOR SLAB CONVERSION COSTS IN LONDON (BREAKING OUT SUSPENDED TIMBER FLOOR, EXCAVATION, HARDCORE, DPM, PIR INSULATION, CONCRETE SLAB, AND LIQUID SCREED — 2025) VARY SIGNIFICANTLY WITH THE DEPTH OF EXCAVATION REQUIRED, THE PRESENCE OF UFH, AND THE SIZE OF THE AREA.

SMALL GROUND FLOOR SLAB CONVERSION (KITCHEN ONLY — APPROXIMATELY 15M²): STRIP-OUT EXISTING TIMBER FLOOR + EXCAVATION + SPOIL REMOVAL (15M²): £600 TO £1,200. HARDCORE TYPE 1 (150MM — 2.5 TONNES — SUPPLY, SPREAD, COMPACT): £180 TO £350. SAND BLINDING (50MM — 0.75M³): £80 TO £150. DPM (VISQUEEN 1200 GAUGE — 15M²): £60 TO £120. PIR INSULATION (KINGSPAN KOOLTHERM K3 100MM — 15M²): £300 TO £550. CONCRETE SLAB (C25/P — 100MM — 1.5M³ + PUMP): £300 TO £600. HANSON CEMFLOOR LIQUID SCREED (65MM — 1.0M³ + PUMP): £400 TO £700. TOTAL MATERIALS AND LABOUR (WITHOUT UFH): £1,920 TO £3,670 (15M²).

MEDIUM GROUND FLOOR SLAB CONVERSION (FULL GROUND FLOOR — 40-50M² — INCLUDING UFH): STRIP-OUT + EXCAVATION + SPOIL REMOVAL (45M²): £1,800 TO £3,600. HARDCORE + BLINDING: £500 TO £900. DPM: £150 TO £280. PIR INSULATION (100MM KOOLTHERM K3): £800 TO £1,400. UFH SYSTEM (NU-HEAT LOOPSTRATE — 45M² — INCLUDING MLCP PIPE, MANIFOLD, STAT): £1,500 TO £2,500. CONCRETE SLAB (C25/P — 100MM — 4.5M³): £900 TO £1,600. HANSON CEMFLOOR SCREED (65MM — 2.9M³): £1,100 TO £1,800. TOTAL MATERIALS AND LABOUR (INCLUDING UFH): £6,750 TO £12,080 (45M²).

ADDITIONAL COSTS TO CONSIDER: STRUCTURAL ENGINEER (WHERE PARTY WALL NOTICE OR BUILDING CONTROL REQUIRES STRUCTURAL REPORT): £500 TO £1,500. MINI SKIP (SPOIL AND TIMBER REMOVAL): £180 TO £350 PER SKIP (TYPICALLY 2-4 SKIPS FOR A FULL GROUND FLOOR). PARTY WALL SURVEYOR: £600 TO £1,200 PER SURVEYOR WHERE REQUIRED.