Floor Screed for London Homes: Liquid Anhydrite, Sand-Cement, and UFH Compatibility Guide

TRADITIONAL SAND-CEMENT SCREED (SOMETIMES CALLED SHARP SAND SCREED OR MORTAR SCREED) IS MIXED ON SITE FROM SHARP SAND AND ORDINARY PORTLAND CEMENT IN A RATIO OF APPROXIMATELY 4:1 OR 4.5:1 (SAND:CEMENT BY WEIGHT), WITH WATER ADDED TO PRODUCE A SEMI-DRY (STIFF CRUMBLE) CONSISTENCY FOR BONDED AND UNBONDED APPLICATIONS OR A WETTER SEMI-FLUID CONSISTENCY FOR FLOWING SCREED APPLICATIONS. MINIMUM THICKNESS DEPENDS ON INSTALLATION TYPE: BONDED (ADHERED DIRECTLY TO CONCRETE SLAB) — MINIMUM 25MM; UNBONDED (OVER DPM OR SLIP MEMBRANE) — MINIMUM 50MM; FLOATING (OVER INSULATION) — MINIMUM 65MM TO 75MM.

SAND-CEMENT SCREED HAS A RELATIVELY HIGH DRYING RATE REQUIREMENT: GENERALLY 1MM OF DEPTH PER WEEK FOR THE FIRST 40MM AND 0.5MM PER WEEK THEREAFTER, GIVING APPROXIMATELY 50 DAYS FOR A 65MM FLOATING SCREED TO DRY SUFFICIENTLY FOR TILING (RESIDUAL MOISTURE CONTENT <75% RH IN THE SCREED). THIS DRYING TIME IS FREQUENTLY THE RATE-LIMITING STEP IN A LONDON REFURBISHMENT PROJECT — TILES CANNOT BE LAID, ENGINEERED TIMBER CANNOT BE INSTALLED, AND MOISTURE-SENSITIVE FINISHES CANNOT BE APPLIED UNTIL THE SCREED IS DRY.

LIQUID CALCIUM SULPHATE (ANHYDRITE) SCREED — MARKETED AS HANSON CEMFLOOR, LAFARGE AGILIA FLOOR, AND FLOWCRETE FLOWSCREED — IS PUMP-DELIVERED TO SITE IN A READY-MIXED SELF-LEVELLING FORMULATION. IT FLOWS OVER INSULATION OR UNDERFLOOR HEATING PIPES UNDER ITS OWN WEIGHT, SELF-LEVELS TO A NEAR-PERFECT FLAT SURFACE, AND REQUIRES MINIMUM MECHANICAL WORKING AFTER POURING. MINIMUM THICKNESS OVER UFH PIPE: 65MM FROM TOP OF PIPE. MINIMUM THICKNESS IN FLOATING APPLICATION WITHOUT UFH: 40MM.

CRITICAL ADVANTAGE: LIQUID ANHYDRITE SCREED ACHIEVES WALK-ON STRENGTH IN 24 TO 48 HOURS AND REACHES SUFFICIENT DRYNESS FOR TILING IN APPROXIMATELY 1MM PER DAY (VS. 1MM PER WEEK FOR TRADITIONAL SCREED). A 65MM CEMFLOOR SCREED IS DRY ENOUGH FOR TILING IN APPROXIMATELY 65 DAYS WITHOUT HEATING BUT ONLY 28 DAYS IF THE BUILDING IS WARMED TO 15-20'C WITH VENTILATION — DRAMATICALLY FASTER THAN SAND-CEMENT.

HANSON CEMFLOOR IS THE MOST WIDELY USED LIQUID ANHYDRITE SCREED IN UK RESIDENTIAL CONSTRUCTION. IT IS A CALCIUM SULPHATE (ANHYDRITE) BINDER-BASED FLOWING SCREED DELIVERED READY-MIXED BY PUMP TRUCK. TYPICAL COMPRESSIVE STRENGTH AT 28 DAYS: 20 TO 30 N/MM' DEPENDING ON GRADE. CEMFLOOR PRODUCTS INCLUDE CEMFLOOR ULTRA (STANDARD RESIDENTIAL AND COMMERCIAL FLOWING SCREED), CEMFLOOR RAPID (ACCELERATED-DRYING FORMULATION FOR TIME-CRITICAL PROJECTS), AND CEMFLOOR UFH (OPTIMISED FOR UNDERFLOOR HEATING APPLICATIONS WITH ENHANCED THERMAL CONDUCTIVITY).

THERMAL CONDUCTIVITY OF CEMFLOOR ANHYDRITE SCREED: 1.80 TO 2.00 W/MK (COMPARED TO 0.98 W/MK FOR TRADITIONAL SAND-CEMENT SCREED). THIS SIGNIFICANTLY HIGHER THERMAL CONDUCTIVITY MEANS THAT FOR A GIVEN UFH OUTPUT, CEMFLOOR SCREED TRANSFERS HEAT TO THE FLOOR SURFACE MORE EFFICIENTLY THAN SAND-CEMENT SCREED — REDUCING REQUIRED FLOW TEMPERATURE BY APPROXIMATELY 2 TO 4'C AND IMPROVING HEAT PUMP EFFICIENCY WHERE HEAT PUMPS SUPPLY THE SYSTEM.

IMPORTANT CONSTRAINTS OF ANHYDRITE SCREED: ANHYDRITE SCREED MUST NOT BE USED IN PERMANENTLY DAMP CONDITIONS (SUCH AS UNVENTILATED SCREED IN CONTACT WITH A RISING-DAMP FLOOR WITHOUT A DPM) BECAUSE CALCIUM SULPHATE IS SOLUBLE IN WATER AND WILL DEGRADE OVER TIME. ANHYDRITE SCREED MUST BE PRIMED WITH A SUITABLE SURFACE HARDENER AND SEALER (HANSON CEMFLOOR SURFACE HARDENER, ARDEX P 51, OR WEBER.FLOOR PRIME) BEFORE TILING OR RESIN INSTALLATION — THE PRIMER CLOSES THE SURFACE PORES AND PREVENTS BOND FAILURE. ENGINEERED TIMBER FLOORS INSTALLED OVER ANHYDRITE SCREED MUST USE AN UNCOUPLING MAT OR ACOUSTIC MEMBRANE BECAUSE DIRECT ADHESIVE-BOND OF TIMBER TO ANHYDRITE WITHOUT A PRIMER CARRIES RISK OF BOND FAILURE DUE TO LAITANCE ON THE SCREED SURFACE.

THE INSTALLATION METHOD FOR SCREED — BONDED, UNBONDED, OR FLOATING — DETERMINES THE MINIMUM REQUIRED THICKNESS AND THE STRUCTURAL PERFORMANCE OF THE FINISHED FLOOR.

BONDED SCREED IS MECHANICALLY KEYED AND ADHESIVELY BONDED TO THE STRUCTURAL CONCRETE SLAB BELOW. THE SLAB IS SCABBLED (MECHANICAL SURFACE PREPARATION) AND A SBR (STYRENE-BUTADIENE RUBBER) PRIMER AND BONDING AGENT IS APPLIED BEFORE THE SCREED IS POURED. BONDED SCREED WORKS COMPOSITELY WITH THE SLAB, SO MINIMUM THICKNESS CAN BE AS LOW AS 25MM FOR TRADITIONAL SCREED AND 35MM FOR LIQUID SCREED. BONDED SCREED IS THE THINNEST POSSIBLE OPTION AND BEST WHERE FLOOR-TO-CEILING HEIGHT IS CONSTRAINED.

UNBONDED SCREED IS SEPARATED FROM THE SLAB BY A DPM OR POLYTHENE SLIP MEMBRANE — THE SCREED IS FREE TO MOVE RELATIVE TO THE SLAB. MINIMUM 50MM FOR TRADITIONAL SCREED. UNBONDED SCREED IS USED WHERE A DPM IS REQUIRED FOR MOISTURE PROTECTION OR WHERE THE SLAB SURFACE IS TOO POOR OR CONTAMINATED FOR BONDING.

FLOATING SCREED IS APPLIED OVER A COMPRESSIBLE INSULATION LAYER (UNDERFLOOR INSULATION, ACOUSTIC INSULATION, OR UFH PIPE PANELS). BECAUSE THE SCREED HAS NO RIGID SUPPORT BENEATH IT, MINIMUM THICKNESS MUST BE GREATER TO ACHIEVE ADEQUATE STRUCTURAL PERFORMANCE. FOR TRADITIONAL SAND-CEMENT SCREED: MINIMUM 65MM (DOMESTIC USE) TO 75MM (COMMERCIAL USE). FOR LIQUID ANHYDRITE SCREED: MINIMUM 65MM ABOVE THE TOP OF UFH PIPE WHERE UFH IS PRESENT; 40MM MINIMUM WITHOUT UFH.

IN A TYPICAL LONDON REAR EXTENSION OR GROUND FLOOR RENOVATION COMBINING INSULATION AND UFH: DPM OVER CONCRETE SLAB + 100MM PIR INSULATION (KINGSPAN THERMAFLOOR TF70) + NU-HEAT LOOPSTRATE PANELS (30MM) + HANSON CEMFLOOR ULTRA AT 65MM = TOTAL BUILD-UP OF 195MM ABOVE SLAB LEVEL.

WHERE THE PROJECT PROGRAMME REQUIRES MINIMAL DRYING TIME, POLYMER-MODIFIED AND RAPID-SETTING SCREEDS ALLOW EARLIER APPLICATION OF FLOOR FINISHES.

ARDEX A 45 FEATHEREDGE SMOOTHING COMPOUND (ARDEX RAPID-CURING SMOOTHING UNDERLAYMENT) CAN BE APPLIED AS A SELF-LEVELLING TOPPING OVER AN EXISTING SOUND SCREED, CONCRETE, OR TIMBER FLOOR AT 2MM TO 10MM THICKNESS, REACHING FOOT TRAFFIC STRENGTH IN 30 MINUTES AND BEING TILE-READY IN 3 HOURS. IT IS NOT A PRIMARY SCREED REPLACEMENT BUT IS USED TO LEVEL IMPERFECT EXISTING FLOORS BEFORE TILING OR ENGINEERED TIMBER INSTALLATION.

WEBER.FLOOR 4310 RAPID-SETTING SELF-LEVELLING COMPOUND (SAINT-GOBAIN) IS SIMILAR — A PRE-MIXED SELF-LEVELLING TOPPING AT 2MM TO 30MM DEPTH, WALKABLE IN 2 HOURS AND TILE-READY IN 24 HOURS.

FOR A FULL-DEPTH RAPID FLOATING SCREED, MAPEI TOPCEM PRONTO IS A RAPID-HARDENING SCREED (CALCIUM SULFOALUMINATE BINDER) THAT CAN BE APPLIED AT 30MM TO 80MM, ACHIEVES WALK-ON STRENGTH IN 12 HOURS, AND CAN BE TILED AFTER 24 HOURS. COMPRESSIVE STRENGTH AT 28 DAYS: 25 N/MM'. THIS IS APPROXIMATELY 60 TIMES FASTER THAN TRADITIONAL SAND-CEMENT SCREED FOR TILE-READY STATUS.

THE RELATIONSHIP BETWEEN SCREED AND UNDERFLOOR HEATING IS CRITICAL TO BOTH THERMAL PERFORMANCE AND LONG-TERM SCREED INTEGRITY. INCORRECT UFH COMMISSIONING SEQUENCE IS A COMMON CAUSE OF CRACKED SCREED IN LONDON RESIDENTIAL PROJECTS.

FOR LIQUID ANHYDRITE (CEMFLOOR) SCREED OVER UFH PIPES: THE SCREED MUST NOT BE SUBJECTED TO HEATING UNTIL IT HAS REACHED SUFFICIENT MATURITY — MINIMUM 7 DAYS FOR HANSON CEMFLOOR ULTRA, 5 DAYS FOR CEMFLOOR RAPID. THE COMMISSIONING SEQUENCE IS: DAY 0-7: AMBIENT CURE (DO NOT HEAT). DAY 7: BEGIN HEATING AT 25'C FLOW TEMPERATURE FOR 3 DAYS. DAY 10: INCREASE TO 40'C MAXIMUM FOR 4 DAYS. DAY 14 ONWARD: NORMAL OPERATING TEMPERATURES. THIS GRADUATED HEATING PREVENTS THERMAL SHOCK CRACKING — A VERY COMMON DEFECT WHERE SCREED IS HEATED TOO QUICKLY AFTER POURING.

FOR SAND-CEMENT SCREED OVER UFH: THE MINIMUM PERIOD BEFORE HEATING IS 21 DAYS. THE COMMISSIONING SEQUENCE IS SIMILAR BUT EXTENDED.

AFTER HEATING COMMISSIONING, SCREED MUST BE DRIED DOWN TO RESIDUAL MOISTURE CONTENT BELOW THE FLOOR COVERING MANUFACTURER'S MAXIMUM BEFORE INSTALLATION. TYPICAL LIMITS: TILES (MAPEI ULTRABOND ECO TILE ADHESIVE): <75% RH OR 3.5% CM MOISTURE. ENGINEERED TIMBER: <65% RH OR 3.0% CM. CARPET/LVT: <75% RH. MOISTURE MEASUREMENT IS PERFORMED WITH A CALIBRATED HYGROMETER (PROTIMETER SURVEYMASTER SM) OR CALCIUM CARBIDE BOMB TEST (CM TEST) — SURFACE MOISTURE METERS ARE NOT RELIABLE FOR SCREED MOISTURE ASSESSMENT.

SCREED COSTS FOR LONDON RESIDENTIAL PROJECTS DEPEND ON TYPE, THICKNESS, AND FLOOR AREA. THE FOLLOWING ARE INDICATIVE FOR SUPPLY AND INSTALLATION.

TRADITIONAL SAND-CEMENT FLOATING SCREED (65MM, OVER INSULATION, INCLUDING LABOUR AND MATERIALS): £20 TO £35 PER M'. MINIMUM AREA FOR COST-EFFECTIVE SITE MIXING VERSUS PUMP DELIVERY IS APPROXIMATELY 50M'.

HANSON CEMFLOOR LIQUID ANHYDRITE FLOATING SCREED (65MM, INCLUDING PUMP DELIVERY AND INSTALLATION): £22 TO £38 PER M'. PUMP DELIVERY IS INCLUDED IN THE CEMFLOOR CONTRACT PRICE AND THE MINIMUM DELIVERY VOLUME IS TYPICALLY 20M' (1 TRUCK LOAD). FOR SMALL AREAS BELOW 20M', A SITE-BATCHED LIQUID SCREED OR PREMIUM PER-M' RATE MAY APPLY.

MAPEI TOPCEM PRONTO RAPID SCREED (30-65MM, SMALL AREA OR TIME-CRITICAL APPLICATION): £40 TO £65 PER M'.

ARDEX A 45 SELF-LEVELLING COMPOUND TOPPING (2-10MM LEVELLING LAYER OVER EXISTING FLOOR): £12 TO £22 PER M' INCLUDING PRIMER.

FOR A TYPICAL LONDON REAR EXTENSION OF 25M' FLOOR AREA, A COMPLETE SCREED SYSTEM (CEMFLOOR 65MM FLOATING OVER 100MM INSULATION) COSTS APPROXIMATELY £750 TO £1,100 SUPPLY AND INSTALL, PLUS INSULATION BOARDS AT APPROXIMATELY £800 TO £1,200 DEPENDING ON PIR SPECIFICATION.