Concrete Screed for London Victorian Terraces: Sand-Cement, Liquid Screed, and UFH Specification

TRADITIONAL SAND-CEMENT SCREED (ALSO KNOWN AS A SEMI-DRY SCREED OR SAND-CEMENT FLOOR SCREED) IS THE MOST WIDELY USED SCREED TYPE IN LONDON VICTORIAN TERRACE RENOVATION PROJECTS FOR GROUND FLOOR EXTENSIONS AND CONVERSIONS. IT IS MIXED ON SITE FROM SHARP WASHED SAND AND ORDINARY PORTLAND CEMENT AND APPLIED BY HAND.

MIX SPECIFICATION (BS 8204-1:2003 — SCREEDS, BASES AND IN SITU FLOORINGS — PART 1: CONCRETE BASES AND CEMENT SAND LEVELLING SCREEDS): STANDARD MIX: 1 PART CEMENT : 4 PARTS CLEAN SHARP WASHED SAND (BY VOLUME) — THIS PRODUCES A SCREED WITH A COMPRESSIVE STRENGTH OF APPROXIMATELY 25 N/MM² AT 28 DAYS — SUITABLE FOR THE GROUND FLOOR OF A DOMESTIC EXTENSION. FOR A STRONGER SCREED (WHERE THE SCREED MUST SUPPORT HEAVY POINT LOADS — E.G., UNDER HEAVY KITCHEN UNITS OR A FREESTANDING BATH): 1:3 MIX (CEMENT : SHARP SAND) — APPROXIMATELY 35-40 N/MM² AT 28 DAYS. WATER: THE CORRECT WATER CONTENT FOR A SAND-CEMENT SCREED IS CRITICAL — THE MIX SHOULD BE SEMI-DRY (THE BALL TEST: TAKE A HANDFUL OF MIXED SCREED AND SQUEEZE IT — THE BALL SHOULD HOLD ITS SHAPE WITHOUT CRUMBLING, BUT SHOULD NOT RELEASE FREE WATER WHEN SQUEEZED). TOO WET = WEAK, SHRINKAGE CRACKS, LONG DRYING TIME. TOO DRY = POOR WORKABILITY, DIFFICULT TO COMPACT, POOR BOND TO SUBSTRATE.

SCREED DEPTH (BS 8204-1:2003): BONDED SCREED (SCREED BONDED DIRECTLY TO THE CONCRETE SLAB SUBSTRATE — NO DPM BETWEEN SCREED AND SLAB): MINIMUM 25MM DEPTH. MOST COMMONLY SPECIFIED AT 40-50MM IN A RESIDENTIAL EXTENSION. UNBONDED SCREED (SCREED APPLIED OVER A POLYTHENE SLIP LAYER — VISQUEEN 1200 GAUGE — WHICH ALLOWS THE SCREED TO MOVE INDEPENDENTLY OF THE SLAB — REQUIRED WHERE THE SLAB SURFACE IS CONTAMINATED OR WHERE A DPM IS REQUIRED BETWEEN SLAB AND SCREED): MINIMUM 50MM DEPTH. FLOATING SCREED (SCREED APPLIED OVER A COMPRESSIBLE INSULATION LAYER — TYPICALLY 100MM KINGSPAN KOOLTHERM K3 PIR OR CELOTEX GA4000 PIR — THE SCREED IS NOT BONDED TO ANY RIGID SUBSTRATE BUT RESTS ON THE COMPRESSIBLE INSULATION): MINIMUM 65MM DEPTH (AND TYPICALLY REINFORCED WITH A150/A142 STEEL MESH — A142 MESH AT MID-DEPTH — TO CONTROL CRACKING OVER THE COMPRESSIBLE INSULATION). NOTE: IN A LONDON VICTORIAN TERRACE REAR EXTENSION WITH UFH, THE STANDARD FLOOR BUILD-UP IS: STRUCTURAL CONCRETE SLAB + 100MM PIR INSULATION + 65MM FLOATING SCREED (WITH UFH PIPES EMBEDDED WITHIN) — THE 65MM DEPTH COVERS THE 16MM UFH PIPE (16MM UPONOR MLCP PIPE) WITH A MINIMUM 50MM COVER ABOVE THE PIPE CENTRELINE (BS 8204-1 REQUIREMENT FOR UFH PIPE COVER).

FIBRE REINFORCEMENT (AN ALTERNATIVE TO STEEL MESH FOR CRACK CONTROL): POLYPROPYLENE FIBRES (PROPEX FIBERMESH 150 — 19MM POLYPROPYLENE FIBRES — ADDED TO THE SCREED MIX AT A RATE OF 0.9 KG/M³) DISTRIBUTED THROUGHOUT THE SCREED BODY — REDUCES PLASTIC SHRINKAGE CRACKING DURING THE EARLY DRYING PERIOD WITHOUT REQUIRING THE LABOUR OF PLACING STEEL MESH.

LIQUID ANHYDRITE SCREED (ALSO KNOWN AS CALCIUM SULPHATE SCREED, SELF-LEVELLING SCREED, OR FLOWING SCREED) IS INCREASINGLY SPECIFIED FOR LONDON VICTORIAN TERRACE REAR EXTENSION GROUND FLOOR SLABS WITH UNDERFLOOR HEATING — BECAUSE OF ITS SUPERIOR SELF-LEVELLING CHARACTERISTICS, FASTER PLACEMENT, AND BETTER THERMAL CONTACT WITH UFH PIPES COMPARED WITH TRADITIONAL SAND-CEMENT SCREED.

HANSON CEMFLOOR (THE MOST WIDELY SPECIFIED LIQUID ANHYDRITE SCREED IN THE UK RESIDENTIAL MARKET — SUPPLIED BY HANSON CEMENT — PART OF HEIDELBERGCEMENT GROUP): A CALCIUM SULPHATE BINDER (ANHYDRITE — CaSO4 — RATHER THAN PORTLAND CEMENT CaO) MIXED WITH SAND AND WATER TO FORM A VERY FLUID, SELF-LEVELLING SLURRY THAT IS PUMPED FROM A TRUCK AND POURED ONTO THE SUBSTRATE — LEVELLING ITSELF TO A FLAT SURFACE BY GRAVITY. KEY ADVANTAGES OVER SAND-CEMENT SCREED: FLATNESS: HANSON CEMFLOOR ACHIEVES SR1 SURFACE REGULARITY (MAXIMUM 3MM DEVIATION UNDER 2M STRAIGHTEDGE — BS 8204-1:2003 SURFACE REGULARITY CLASS SR1 — THE HIGHEST FLATNESS STANDARD — ACHIEVED WITHOUT MANUAL LEVELLING) — COMPARED TO SAND-CEMENT WHICH TYPICALLY ACHIEVES SR2-SR3 WITHOUT SKILLED FINISHING. THERMAL PERFORMANCE: THE LIQUID CEMFLOOR COMPLETELY SURROUNDS THE UFH PIPES (NO AIR VOIDS AROUND THE PIPE — WHICH WOULD INSULATE THE PIPE FROM THE SCREED) — SIGNIFICANTLY BETTER THERMAL CONTACT THAN SAND-CEMENT SCREED WHERE AIR POCKETS FORM AROUND THE PIPES IF COMPACTION IS POOR. REDUCED DEPTH: HANSON CEMFLOOR CAN BE APPLIED AT 40MM OVER UFH PIPE (WITH 16MM PIPE, 40MM TOTAL DEPTH GIVES 24MM COVER ABOVE THE PIPE CENTRELINE — LESS THAN THE 50MM MINIMUM FOR SAND-CEMENT — BUT HANSON CEMFLOOR IS STRONGER AND MORE CRACK-RESISTANT THAN SAND-CEMENT AT THE SAME DEPTH, JUSTIFYING THE REDUCED COVER). HOWEVER: THE MOST COMMONLY SPECIFIED CEMFLOOR DEPTH IS STILL 50-65MM (TO PROVIDE ADEQUATE COVER AND IMPROVE THERMAL MASS FOR THE UFH SYSTEM). PLACEMENT SPEED: A TRUCK OF CEMFLOOR CAN PLACE APPROXIMATELY 300-500M² OF SCREED IN A DAY (VERSUS 80-120M² PER DAY FOR HAND-LAID SAND-CEMENT) — SIGNIFICANTLY REDUCING PROGRAMME DURATION FOR LARGE AREAS.

HANSON CEMFLOOR DRYING: CEMFLOOR DRIES FASTER THAN SAND-CEMENT: APPROXIMATELY 1MM PER DAY FOR THE FIRST 40MM, THEN SIMILAR TO SAND-CEMENT BEYOND. A 50MM CEMFLOOR SCREED IS TYPICALLY DRY ENOUGH FOR FLOOR FINISHES (75% RH) IN 4-6 WEEKS (VERSUS 8-12 WEEKS FOR A 65MM SAND-CEMENT SCREED).

WHERE UNDERFLOOR HEATING (UFH) PIPES ARE EMBEDDED WITHIN THE FLOOR SCREED (THE MOST COMMON UFH CONFIGURATION IN A LONDON VICTORIAN TERRACE REAR EXTENSION), THE PIPE SPECIFICATION AND INSTALLATION SEQUENCE CRITICALLY AFFECTS THE PERFORMANCE OF BOTH THE UFH SYSTEM AND THE SCREED.

UFH PIPE SPECIFICATION: UPONOR MLCP (MULTILAYER COMPOSITE PIPE — ALUMINIUM-SANDWICHED POLYETHYLENE — THE MOST WIDELY SPECIFIED UFH PIPE IN UK RESIDENTIAL APPLICATIONS): 16MM OD (16MM OUTSIDE DIAMETER — THE STANDARD RESIDENTIAL UFH PIPE SIZE — 2MM WALL — 12MM INTERNAL BORE — FLOW RATE OF APPROXIMATELY 1.3 L/MIN AT 0.1 M/S VELOCITY — ADEQUATE FOR RESIDENTIAL UFH CIRCUITS). ALTERNATIVE: HEP2O (HIGH-DENSITY POLYETHYLENE — SINGLE LAYER — SIMPLER AND CHEAPER — APPROXIMATELY £0.40-0.60 PER METRE VS £0.80-1.20 FOR UPONOR MLCP — THE ALUMINIUM LAYER IN MLCP PROVIDES OXYGEN BARRIER AND BETTER RESISTANCE TO KINKING DURING INSTALLATION). UFH PIPE SPACING: MOST COMMONLY 150-200MM CENTRES (150MM FOR HIGHER-OUTPUT AREAS — E.G., UNDER LARGE STONE TILE OR WHERE THE INSULATION LEVEL IS LOWER; 200MM FOR STANDARD KITCHEN EXTENSION WITH GOOD INSULATION). MAXIMUM CIRCUIT LENGTH: 80-100M PER CIRCUIT (TO MAINTAIN ADEQUATE FLOW AND AVOID EXCESSIVE PRESSURE DROP ACROSS THE CIRCUIT). PIPE FIXING: UFH PIPES ARE CLIPPED TO THE INSULATION SURFACE (POLYPIPE UFH CLIPS — PUSH-FIT INTO POLYSTYRENE INSULATION — OR UPONOR STAPLE RAIL) BEFORE THE SCREED IS POURED. THE PIPES MUST BE PRESSURE TESTED (MINIMUM 4 BAR HYDROSTATIC PRESSURE — HOLD FOR 24 HOURS — CONFIRM NO PRESSURE DROP) BEFORE THE SCREED IS POURED OVER THEM.

NU-HEAT LOOPSTRATE (THE ALTERNATIVE — THE MOST WIDELY SPECIFIED PRE-GROOVED INSULATION BOARD SYSTEM FOR UFH WITH LIQUID SCREED IN UK RESIDENTIAL RENOVATIONS — NU-HEAT UK, AXMINSTER, DEVON): A FACTORY-MOULDED PIR INSULATION BOARD (100MM THICK) WITH PRE-FORMED GROOVES AT 200MM CENTRES TO ACCEPT THE UFH PIPE — THE PIPE SNAPS INTO THE GROOVE WITHOUT SEPARATE CLIPS — HALVES THE INSTALLATION TIME FOR UFH PIPE LAYING — USED WITH HANSON CEMFLOOR LIQUID SCREED (50-65MM OVER THE PIPE — WHICH SITS IN THE GROOVE AT THE TOP SURFACE OF THE INSULATION BOARD).

SCREED DRYING IS ONE OF THE MOST IMPORTANT PROGRAMME CONSIDERATIONS IN A LONDON VICTORIAN TERRACE EXTENSION PROJECT — AND ONE OF THE MOST FREQUENTLY OVERLOOKED BY CLIENTS AND CONTRACTORS ALIKE, LEADING TO FLOOR FINISH FAILURES WHEN TILES CRACK, ADHESIVE DEBONDS, OR ENGINEERED TIMBER CUPS BECAUSE THE SCREED BENEATH WAS NOT DRY ENOUGH.

SAND-CEMENT SCREED DRYING RATE (RULE OF THUMB): FIRST 40MM DEPTH: APPROXIMATELY 1MM PER DAY (40 DAYS TO DRY THE FIRST 40MM TO 75% RH). BEYOND 40MM DEPTH: APPROXIMATELY 0.5MM PER DAY (EACH ADDITIONAL MM ABOVE 40MM TAKES 2 DAYS TO DRY). EXAMPLE: A 65MM FLOATING SCREED (STANDARD FOR UFH) DRYING TIME: 40 DAYS (FOR THE FIRST 40MM) + (65-40) x 2 DAYS = 40 + 50 = 90 DAYS. APPROXIMATELY 3 MONTHS. NOTE: THESE ARE APPROXIMATE RATES — ACTUAL DRYING DEPENDS ON AMBIENT TEMPERATURE AND HUMIDITY, VENTILATION, AND WHETHER THE SCREED WAS MIXED AT THE CORRECT WATER CONTENT (OVER-WATERED SCREED DRIES MUCH MORE SLOWLY). IN WINTER IN LONDON (UNHEATED EXTENSION SHELL — LOW TEMPERATURE — HIGH HUMIDITY): DRYING IS SIGNIFICANTLY SLOWER THAN THE RULE-OF-THUMB — A 65MM SCREED LAID IN DECEMBER MAY NOT BE DRY UNTIL APRIL.

HANSON CEMFLOOR DRYING RATE: FASTER THAN SAND-CEMENT: APPROXIMATELY 1MM PER DAY FOR THE FIRST 40MM, THEN MARGINALLY FASTER THAN SAND-CEMENT BEYOND 40MM. A 50MM CEMFLOOR TYPICALLY ACHIEVES LESS THAN 75% RH IN 4-6 WEEKS UNDER NORMAL AMBIENT CONDITIONS. UFH-ASSISTED DRYING: RUNNING THE UFH AT A CONTROLLED PROGRAMME (STARTING AT 15°C AND INCREASING BY 5°C PER DAY TO MAXIMUM 35°C — HOLD FOR 3-4 WEEKS — THEN REDUCE BACK TO MINIMUM) ACCELERATES DRYING OF BOTH CEMFLOOR AND SAND-CEMENT SCREED AND IS RECOMMENDED BY MOST LIQUID SCREED MANUFACTURERS.

MEASURING MOISTURE CONTENT: THE CORRECT METHOD FOR MEASURING SCREED MOISTURE BEFORE LAYING FLOOR FINISH IS THE HYGROMETER PROBE (FLOOR-MOUNTED HYGROMETER — PLACED IN A SEALED HOOD ON THE SCREED SURFACE FOR MINIMUM 72 HOURS — MEASURES THE RELATIVE HUMIDITY OF THE AIR IN CONTACT WITH THE SCREED SURFACE — THE MOST ACCURATE METHOD): TARGET: LESS THAN 75% RH FOR MOST ADHESIVE SYSTEMS AND FLOOR FINISHES. NOT THE SURFACE MOISTURE METER (CARBIDE BOMB TEST OR SURFACE CAPACITANCE METER — THESE ARE INACCURATE FOR SCREED — THEY MEASURE THE SURFACE MOISTURE, NOT THE MOISTURE CONTENT OF THE WHOLE DEPTH OF THE SCREED — A SCREED CAN APPEAR DRY ON THE SURFACE BUT STILL BE WET AT DEPTH — ONLY THE HYGROMETER PROBE TEST IS RELIABLE).

THE FLATNESS OF THE SCREED SURFACE DETERMINES WHETHER THE FLOOR FINISH CAN BE LAID SUCCESSFULLY WITHOUT LIPPAGE (WHERE ADJACENT TILES OR BOARDS SIT AT DIFFERENT HEIGHTS — CAUSED BY THE SCREED SURFACE BEING UNEVEN) OR HOLLOW SPOTS (WHERE THE FLOOR FINISH IS NOT SUPPORTED BY THE SCREED — TYPICALLY BECAUSE THE SCREED HAS SHRUNK OR SUNK AWAY FROM THE FINISHED SURFACE).

BS 8204-7:2003 AND BS EN 13454 — SURFACE REGULARITY CLASSES (APPLICABLE TO SCREEDS BEFORE LAYING FLOOR FINISHES): SR1 (THE HIGHEST FLATNESS STANDARD — BEST QUALITY — REQUIRED FOR THE MOST DEMANDING FLOOR FINISHES SUCH AS LARGE FORMAT STONE TILES 600X600MM AND ABOVE — ENGINEERED TIMBER OVER 150MM WIDE GLUED DOWN): MAXIMUM 3MM DEVIATION UNDER A 2M STRAIGHTEDGE. SR2 (NORMAL QUALITY — REQUIRED FOR STANDARD CERAMIC OR PORCELAIN TILES UP TO 600MM — ENGINEERED TIMBER FLOATING INSTALLATION): MAXIMUM 5MM DEVIATION UNDER A 2M STRAIGHTEDGE. SR3 (UTILITY QUALITY — FOR CARPETS, THICK VINYL, AND OTHER FLEXIBLE FINISHES THAT ACCOMMODATE SURFACE IRREGULARITY): MAXIMUM 10MM DEVIATION UNDER A 2M STRAIGHTEDGE.

HOW TO ACHIEVE SR1 WITH SAND-CEMENT SCREED: REQUIRES SKILLED SCREED LAYING (EXPERIENCED SCREEDER USING DEPTH PINS AND A LONG STRAIGHT EDGE — NOT A GENERAL LABOURER). SCREED GAUGE PINS SET AT CORRECT DEPTH ACROSS THE FLOOR AREA. STRAIGHT EDGE (FEATHER EDGE) WORKED FROM THE PINS. FINISH WITH A POWER FLOAT (RIDGID 36-INCH POWER FLOAT — ONLY WHERE THE SCREED IS ACCESSIBLE — NOT PRACTICAL IN SMALL ROOMS). COMMON RESULT WITH SAND-CEMENT (SKILLED CONTRACTOR): SR2 — 3-5MM DEVIATION UNDER 2M. SR1 WITH SAND-CEMENT IS ACHIEVABLE BUT REQUIRES SIGNIFICANT SKILL AND CARE.

HOW TO ACHIEVE SR1 WITH LIQUID SCREED (HANSON CEMFLOOR): CEMFLOOR SELF-LEVELS TO SR1 BY GRAVITY (NO MANUAL LEVELLING REQUIRED BEYOND INITIAL POUR AND MINIMAL RAKING TO SPREAD) — THE SELF-LEVELLING PROPERTY OF THE FLUID CEMFLOOR IS THE FUNDAMENTAL ADVANTAGE OVER SAND-CEMENT FOR FLATNESS. NOTCHING: AFTER POURING AND BEFORE THE CEMFLOOR STARTS TO SET, THE SURFACE SHOULD BE 'NOTCHED' (BROKEN WITH A SCREED NOTCHING RAKE — A ROLLER WITH PINS — THAT BREAKS THE SURFACE TENSION AND RELEASES TRAPPED AIR BUBBLES) — CRITICAL FOR ACHIEVING A CONSISTENTLY FLAT, VOID-FREE SURFACE.

SCREED COSTS IN LONDON (2025) FOR A STANDARD REAR EXTENSION GROUND FLOOR (40M² — THE TYPICAL AREA FOR A SINGLE-STOREY REAR EXTENSION ON A LONDON VICTORIAN TERRACE).

SAND-CEMENT FLOATING SCREED (65MM — OVER 100MM PIR INSULATION — WITH A142 STEEL MESH — UFH PIPES EMBEDDED): MATERIALS (SHARP SAND + CEMENT + A142 MESH + MIXING TOOL HIRE): APPROXIMATELY £300 TO £600 FOR 40M². LABOUR (SCREED GANG — 2 OPERATIVES — TYPICALLY 1-2 DAYS FOR 40M²): £600 TO £1,200. TOTAL SAND-CEMENT FLOATING SCREED (40M²): £900 TO £1,800.

HANSON CEMFLOOR LIQUID SCREED (50-65MM — OVER NU-HEAT LOOPSTRATE UFH INSULATION BOARD WITH UFH PIPES — PUMPED): CEMFLOOR MATERIAL (TRUCK SUPPLY — MINIMUM ORDER TYPICALLY 3M³ — 3M³ COVERS APPROXIMATELY 60M² AT 50MM): APPROXIMATELY £500 TO £700 FOR 40M² WORTH (PRO-RATED FROM MINIMUM ORDER — SO MINIMUM ORDER COST APPLIES — MINIMUM ORDER £600-900 FOR TRUCK SUPPLY IN LONDON). PUMP COST (SELF-LEVELLING SCREED PUMP — HIRED OR PROVIDED BY CEMFLOOR CONTRACTOR): £150 TO £300 FOR HALF DAY. LABOUR (1 OPERATIVE — TYPICALLY 2-4 HOURS FOR 40M² CEMFLOOR): £100 TO £200. TOTAL HANSON CEMFLOOR LIQUID SCREED (40M²): £850 TO £1,400 (BUT MINIMUM ORDER COSTS MEAN THE COST PER M² IS HIGHER FOR SMALL AREAS — LIQUID SCREED IS MOST COST-EFFECTIVE FOR AREAS OF 80M² AND ABOVE).

SELF-LEVELLING COMPOUND (ARDEX K15, MAPEI ULTRATOP 1C — FOR PATCHING/LEVELLING AN EXISTING SCREED OR CONCRETE SLAB BEFORE LAYING FLOOR FINISH — NOT A FULL-DEPTH SCREED): ARDEX K15: £25-40 PER 25KG BAG (COVERS APPROXIMATELY 3-4M² AT 6MM DEPTH). FOR 40M² AT 6MM AVERAGE LEVELLING: 10-14 BAGS = £250-560 (MATERIALS ONLY — LABOUR ADDITIONAL: £200-400). TOTAL LEVELLING COMPOUND (40M² — MODEST LEVELLING): £450 TO £960.

RECOMMENDED APPROACH FOR LONDON VICTORIAN TERRACE EXTENSION: HANSON CEMFLOOR IS PREFERRED FOR AREAS OVER 60M² (BETTER FLATNESS, FASTER PROGRAMME, BETTER UFH CONTACT). SAND-CEMENT IS OFTEN MORE COST-EFFECTIVE AND PRACTICAL FOR SMALLER AREAS (UNDER 40M²) WHERE THE MINIMUM ORDER COST OF CEMFLOOR IS NOT JUSTIFIED.