Floor Screed for London Home Renovations: Liquid Screed, Sand-Cement, Underfloor Heating, and Specification Guide

TRADITIONAL SAND-CEMENT SCREED (ALSO CALLED GRANO SCREED OR WET SCREED) IS MIXED ON SITE OR DELIVERED READY-MIXED AND HAS BEEN THE STANDARD SCREED TYPE FOR UK CONSTRUCTION FOR OVER A CENTURY.

MIX DESIGN: THE STANDARD SPECIFICATION FOR SAND-CEMENT SCREED IS 1:4.5 (1 PART ORDINARY PORTLAND CEMENT CEM I 42.5R : 4.5 PARTS SHARP SAND) BY VOLUME, WITH A WATER-TO-CEMENT RATIO OF APPROXIMATELY 0.45. THE MIXED SCREED SHOULD HAVE A "DAMP SAND" CONSISTENCY — IT SHOULD HOLD A BALL WHEN SQUEEZED IN THE FIST BUT SHOULD NOT RELEASE FREE WATER. TOO WET A MIX INCREASES DRYING SHRINKAGE AND CRACKING RISK; TOO DRY A MIX REDUCES WORKABILITY AND BOND.

STRENGTH: A CORRECTLY PROPORTIONED 1:4.5 SAND-CEMENT SCREED ACHIEVES APPROXIMATELY 20 TO 30 N/MM² COMPRESSIVE STRENGTH AT 28 DAYS — MORE THAN ADEQUATE FOR DOMESTIC APPLICATIONS. FOR HEAVY COMMERCIAL DUTY, A STRONGER MIX (1:3 CEMENT:SAND) OR A PROPRIETARY LEVELLING COMPOUND IS USED.

BONDED, UNBONDED, AND FLOATING SCREED: THERE ARE THREE INSTALLATION METHODS. BONDED SCREED (MINIMUM 25MM DEPTH) IS APPLIED DIRECTLY OVER A PREPARED CONCRETE SUBSTRATE THAT HAS BEEN PRIMED WITH SBR BONDING AGENT (FOSROC NITOBOND SBR OR EVERBUILD SBR BOND) — THE SCREED IS MECHANICALLY ADHERED TO THE SLAB. THIS IS THE MOST CRACK-RESISTANT OPTION WHERE SUFFICIENT ADHESION IS ACHIEVED. UNBONDED SCREED (MINIMUM 50MM DEPTH) IS APPLIED OVER A POLYTHENE SHEET (TYPICALLY VISQUEEN 1200 GAUGE 300 MICRON DAMP-PROOF MEMBRANE) WHICH ACTS AS A SLIP LAYER BETWEEN SCREED AND SLAB — THE SCREED IS FREE TO MOVE SLIGHTLY AND IS LESS LIKELY TO BE AFFECTED BY SLAB MOVEMENT. FLOATING SCREED (MINIMUM 65MM DEPTH) IS APPLIED OVER INSULATION BOARDS (CELOTEX GA4000, IKO ENERTHERM, KINGSPAN KOOLTHERM) — THE SCREED IS LAID ONTO THE INSULATION WITH NO BOND TO THE STRUCTURAL SLAB. FLOATING SCREED IS THE STANDARD FOR GROUND FLOORS IN LONDON EXTENSIONS WHERE PIR INSULATION IS REQUIRED BENEATH THE SCREED TO MEET PART L U-VALUE TARGETS.

CRACK CONTROL: POLYPROPYLENE FIBRES (SIKA VISCOCRETE FIBRES, BASF MBT FIBERMESH 150) ADDED TO THE SCREED MIX AT 900G PER CUBIC METRE SIGNIFICANTLY REDUCE PLASTIC SHRINKAGE CRACKING DURING THE EARLY CURE PERIOD. THIS IS STRONGLY RECOMMENDED FOR ALL FLOATING SCREEDS AND ALL SCREEDS OVER 60MM DEPTH.

LIQUID ANHYDRITE SCREEDS (ALSO CALLED CALCIUM SULPHATE SCREEDS OR SELF-COMPACTING SCREEDS) ARE INCREASINGLY THE DEFAULT SCREED TYPE FOR LONDON RESIDENTIAL EXTENSIONS AND RENOVATIONS INVOLVING UNDERFLOOR HEATING.

HANSON CEMFLOOR (ANHYDRITE CALCIUM SULPHATE LIQUID SCREED): CEMFLOOR IS HANSON'S PROPRIETARY LIQUID ANHYDRITE SCREED, DELIVERED IN A TRUCK AND PUMPED INTO POSITION. CEMFLOOR IS A SELF-COMPACTING LIQUID — IT FLOWS AROUND UFH PIPEWORK WITHOUT MANUAL TAMPING OR COMPACTION, ENSURING EVEN ENCAPSULATION OF THE HEATING PIPES WITHOUT VOID FORMATION (WHICH IS A COMMON FAILURE IN TRADITIONAL SCREED OVER UFH PIPES). CEMFLOOR IS AVAILABLE IN THREE STRENGTH GRADES: CT-C20-F4 (20 N/MM² COMPRESSIVE, 4 N/MM² FLEXURAL — STANDARD RESIDENTIAL), CT-C25-F5 (COMMERCIAL RESIDENTIAL), AND CT-C30-F6 (LIGHT COMMERCIAL). FOR LONDON RESIDENTIAL EXTENSIONS, CT-C20-F4 IS THE STANDARD SPECIFICATION.

MINIMUM DEPTH OVER UFH PIPES: FOR CEMFLOOR OVER UNDERFLOOR HEATING PIPEWORK, THE MINIMUM COVER IS 30MM ABOVE THE TOP OF THE PIPE (GIVING A TYPICAL INSTALLATION DEPTH OF 65MM INCLUDING A 25MM DIAMETER UFH PIPE — SO SCREED SURFACE IS 30MM ABOVE PIPE TOP). FOR BARE SLAB OR ELECTRIC HEATING MAT, MINIMUM DEPTH IS 40MM.

LAFARGE THERMIA (ALTERNATIVE CALCIUM SULPHATE LIQUID SCREED): THERMIA IS LAFARGE'S EQUIVALENT TO CEMFLOOR — SIMILAR PERFORMANCE SPECIFICATION, DELIVERED AND PUMPED SIMILARLY. BOTH HANSON CEMFLOOR AND LAFARGE THERMIA ARE WIDELY AVAILABLE ACROSS LONDON FROM READY-MIX PUMPING CONTRACTORS.

ADVANTAGES OF LIQUID SCREED OVER TRADITIONAL SCREED FOR LONDON RENOVATIONS: FASTER INSTALLATION (A 100M² FLOOR TAKES 1 TO 2 HOURS TO POUR, VERSUS 1 TO 2 DAYS FOR TRADITIONAL SCREED); BETTER FLOW AROUND UFH PIPEWORK (NO VOIDS); IMPROVED THERMAL CONDUCTIVITY (1.2 W/MK VS 0.9 W/MK FOR TRADITIONAL SCREED — BETTER HEAT OUTPUT FROM UFH); LESS LABOUR REQUIRED (NO TAMPING/COMPACTING); FLATTER FINISHED SURFACE (TYPICALLY ±3MM OVER 2M SR2 FLOOR FLATNESS TOLERANCE).

A DPM UNDER THE SCREED IS REQUIRED FOR ALL GROUND-FLOOR APPLICATIONS WHERE THE SLAB IS IN CONTACT WITH THE GROUND.

STANDARD DPM SPECIFICATION: VISQUEEN SUPER FLEXIBLE PLUS 1200 GAUGE (300 MICRON) HDPE/LLDPE SHEET DPM, LAPPED A MINIMUM OF 300MM AT ALL JOINTS (JOINTS TAPED WITH RADON-GRADE BUTYL JOINTING TAPE — VISQUEEN DELTA TAPE OR EQUIVALENT). THE DPM MUST BE LAPPED UP THE WALLS AND LINKED TO THE VERTICAL DPC (DAMP-PROOF COURSE) IN THE WALL CONSTRUCTION TO FORM A CONTINUOUS MOISTURE BARRIER. FOR LONDON CLAY GROUND CONDITIONS, WHERE GROUNDWATER AND GROUND GAS (RADON — NOT A SIGNIFICANT CONCERN IN MOST OF LONDON, BUT METHANE CAN BE A CONCERN IN BROWNFIELD LOCATIONS) ARE POTENTIAL ISSUES, GAS-RESISTANT DPM (VISQUEEN GAS-RESISTANT MEMBRANE OR GAS-TIGHT LOW-PERMEABILITY HDPE) SHOULD BE SPECIFIED WHERE A RISK ASSESSMENT INDICATES GROUND GAS RISK.

DPM FOR LIQUID SCREED: FOR LIQUID ANHYDRITE SCREEDS (CEMFLOOR/THERMIA), AN ADDITIONAL REQUIREMENT EXISTS — THE ANHYDRITE CALCIUM SULPHATE IN THE SCREED IS SENSITIVE TO MOISTURE. IF EXCESS MOISTURE IS PRESENT (FROM THE GROUND BELOW OR FROM ABOVE), THE CALCIUM SULPHATE CAN REACT AND CAUSE EXPANSION AND DISRUPTION OF THE SCREED. THE DPM MUST THEREFORE BE COMPLETELY INTACT AND WELL-LAPPED, AND THE SCREED MUST BE ALLOWED TO DRY TO BELOW 75% RELATIVE HUMIDITY (TESTED WITH A CALIBRATED HYGROMETER) BEFORE ANY MOISTURE-SENSITIVE FLOOR FINISH IS LAID OVER IT.

DPM FOR FLOATING SCREED OVER INSULATION: THE DPM IS LAID BELOW THE INSULATION. A FURTHER POLYTHENE SHEET (MINIMUM 500 GAUGE — 125 MICRON) IS PLACED BETWEEN THE INSULATION AND THE SCREED AS A SLIP LAYER AND AS PROTECTION FOR THE INSULATION SURFACE FROM THE WET SCREED.

FOR LONDON EXTENSIONS WITH WET UNDERFLOOR HEATING (WATER-BASED UFH PIPING), THE SCREED IS THE PRIMARY MEDIUM FOR HEAT DISTRIBUTION FROM THE PIPEWORK TO THE FLOOR SURFACE AND THEN TO THE ROOM ABOVE.

NU-HEAT LOOPSTRATE SCREED SYSTEM: NU-HEAT IS ONE OF THE LEADING UK UFH SYSTEM PROVIDERS AND SUPPLIES A COMPLETE TURNKEY PACKAGE INCLUDING UFH MANIFOLD, MLCP PIPEWORK (MULTILAYER COMPOSITE PIPE — TYPICALLY 15MM OR 16MM DIAMETER UPONOR MLCP OR POLYPIPE POLYPLUMB), PIPE FIXINGS, AND SCREED SPECIFICATION GUIDANCE. THE LOOPSTRATE SYSTEM USES A STRUCTURAL SCREED INSTALLATION APPROACH.

UFH COMMISSIONING BEFORE SCREEDING: BEFORE SCREED IS POURED, THE UFH PIPEWORK MUST BE PRESSURE TESTED TO 6 BAR AND HELD FOR A MINIMUM OF 30 MINUTES. THE SYSTEM SHOULD REMAIN PRESSURISED DURING SCREEDING TO MAINTAIN PIPE SHAPE UNDER THE WET SCREED WEIGHT AND TO ALLOW DETECTION OF LEAKS CAUSED BY DAMAGE DURING SCREED POURING.

SCREED DRYING AND UFH INITIAL COMMISSIONING: AFTER LAYING, LIQUID ANHYDRITE SCREED (CEMFLOOR/THERMIA) MUST BE ALLOWED TO DRY FOR A MINIMUM OF 7 DAYS BEFORE THE UFH IS INITIALLY COMMISSIONED. THE INITIAL HEAT-UP PROCESS SHOULD BE GRADUAL — THE HEATING WATER TEMPERATURE SHOULD NOT EXCEED 25°C FLOW FOR THE FIRST 3 DAYS, THEN INCREASE BY 5°C PER DAY TO THE MAXIMUM OPERATING TEMPERATURE. THIS GRADUAL HEAT-UP PROCESS (SOMETIMES CALLED "SCREED CONDITIONING") ALLOWS REMAINING MOISTURE TO ESCAPE WITHOUT CAUSING THERMAL SHOCK CRACKING. FAILING TO FOLLOW THE GRADUAL CONDITIONING PROGRAMME IS A COMMON CAUSE OF SCREED CRACKING OVER UFH IN LONDON EXTENSION PROJECTS.

SCREED DRYING TIME BEFORE FLOOR FINISH: THE SCREED MUST DRY TO BELOW 75% RELATIVE HUMIDITY (ANHYDRITE SCREED) OR BELOW 75% RH (SAND-CEMENT SCREED) BEFORE A MOISTURE-SENSITIVE FLOOR COVERING IS INSTALLED. TYPICAL DRYING TIMES: ANHYDRITE SCREED: 1 WEEK PER 10MM OF DEPTH UP TO 40MM, THEN LONGER FOR GREATER DEPTHS — FOR A 65MM SCREED: APPROXIMATELY 6 TO 8 WEEKS IN NORMAL CONDITIONS (15°C, 65% AMBIENT RH). SAND-CEMENT SCREED: APPROXIMATELY 1 DAY PER 1MM OF DEPTH — SO 65MM SCREED TAKES APPROXIMATELY 65 DAYS. FORCED DRYING WITH A DEHUMIDIFIER AND HEATER CAN SIGNIFICANTLY REDUCE THESE TIMES.

THE THICKNESS OF SCREED WITHIN A FLOOR BUILD-UP AFFECTS BOTH THERMAL PERFORMANCE (HEAT LOSS TO THE GROUND) AND THE FINISHED FLOOR LEVEL.

TYPICAL GROUND FLOOR BUILD-UP FOR LONDON REAR EXTENSION (PART L 2021, U-VALUE TARGET ≤ 0.15 W/M'K FOR GROUND FLOOR): EXISTING GROUND LEVEL: EXCAVATE TO FORMATION LEVEL. 150MM COMPACTED MOT TYPE 1 GRANULAR SUBBASE. 75MM CONCRETE BLINDING (C10/15 OR MASS CONCRETE). 300 MICRON VISQUEEN DPM (TAPED JOINTS, LAPPED TO WALLS). 100MM PIR INSULATION (CELOTEX GA4000, KINGSPAN KOOLTHERM K3, IKO ENERTHERM ALU, LAMBDA 0.022 W/MK). 125 MICRON SLIP LAYER POLYTHENE. 65MM HANSON CEMFLOOR LIQUID SCREED (INCLUDING UFH PIPEWORK AT 200MM CENTRES). FLOOR FINISH (20MM PORCELAIN TILE = 0-1MM ADD. SCREED BELOW OR DIRECT FIX TO SCREED). THIS BUILD-UP ACHIEVES A GROUND FLOOR U-VALUE OF APPROXIMATELY 0.11 TO 0.15 W/M'K DEPENDING ON FLOOR DIMENSIONS AND PERIMETER EDGE DETAIL.

EDGE INSULATION: PERIMETER EDGE INSULATION (50MM XPS EXTRUDED POLYSTYRENE UPSTAND, E.G. CELLECTA HEXATHERM XP-PE, LAPPED UNDER SCREED AND UP THE INTERNAL WALL FACE) IS REQUIRED TO REDUCE HEAT LOSS AT THE FLOOR-WALL JUNCTION (THERMAL BRIDGE). WITHOUT EDGE INSULATION, THE FLOOR PERIMETER ACTS AS A COLD BRIDGE AND THE SAP CALCULATION WILL SHOW HIGHER HEAT LOSS.

FLOOR LEVEL MANAGEMENT IN LONDON REFURBISHMENTS: IN EXISTING LONDON TERRACE GROUND-FLOOR REFURBISHMENTS, THE FLOOR BUILD-UP OFTEN NEEDS TO MATCH THE EXISTING FLOOR LEVEL IN ADJACENT ROOMS. WHERE THE EXISTING FLOOR IS TIMBER SUSPENDED (COMMON IN VICTORIAN LONDON TERRACES), THE SCREED BUILD-UP IN THE EXTENSION SHOULD BE CALCULATED TO MATCH THE EXISTING TIMBER FLOOR FINISH LEVEL, TAKING INTO ACCOUNT THE JOIST DEPTH, AIR VOID, AND FLOOR BOARD THICKNESS IN THE EXISTING CONSTRUCTION.

SCREED SUPPLY AND INSTALLATION COSTS IN LONDON VARY SIGNIFICANTLY BY SCREED TYPE AND PROJECT SCALE.

SAND-CEMENT SCREED (TRADITIONAL, LABOUR-INTENSIVE, SUITABLE FOR SMALL AREAS OR TOPPING WORKS): MATERIALS ONLY (BAGGED CEMENT + SHARP SAND): £20 TO £35 PER M² FOR 65MM DEPTH. SUPPLY AND LAY (LABOUR + MATERIALS, SMALL CONTRACTOR): £30 TO £55 PER M² FOR 65MM DEPTH. THIS IS MORE EXPENSIVE PER M² THAN LIQUID SCREED FOR AREAS OVER 40M² DUE TO HIGH LABOUR CONTENT.

LIQUID ANHYDRITE SCREED (CEMFLOOR / THERMIA, PUMP-APPLIED, SUITABLE FOR AREAS 20M² OR LARGER): SUPPLY AND PUMP DELIVERY (MATERIALS + DELIVERY + PUMP): £18 TO £30 PER M² FOR 65MM DEPTH. PREPARATION (INCLUDING DPM, INSULATION, EDGE INSULATION BEADS, UFH PIPE LAYING): £20 TO £45 PER M² ADDITIONAL DEPENDING ON SCOPE.

TYPICAL COST FOR A STANDARD 30M² LONDON REAR EXTENSION GROUND FLOOR SCREED PACKAGE (DPM, 100MM PIR INSULATION, 65MM CEMFLOOR, UFH PIPEWORK): £2,500 TO £5,500 DEPENDING ON UFH COMPLEXITY AND SPECIFICATION LEVEL.