Floor Screeds for London Victorian Terraces: Sand-Cement, Flowing Screed, and Anhydrite

THERE ARE TWO MAIN SCREED TYPES USED IN LONDON VICTORIAN TERRACE RENOVATIONS:

SAND-CEMENT SCREED (TRADITIONAL — THE MOST WIDELY USED): A MIX OF CEMENT (OPC — ORDINARY PORTLAND CEMENT) AND SHARP SAND, MIXED ON SITE OR SUPPLIED AS A SEMI-DRY PREMIX BAG PRODUCT: - MIX RATIO: TYPICALLY 1:3 TO 1:4.5 (CEMENT:SAND BY VOLUME) FOR A STANDARD BONDED OR UNBONDED SCREED. WATER:CEMENT RATIO (W/C): 0.4-0.55 (THE LOWER THE WATER CONTENT, THE STRONGER THE SCREED — BUT TOO DRY AND THE MIX IS UNWORKABLE). - CONSISTENCE: SEMI-DRY (DAMP BUT NOT SLUSHY — WHEN SQUEEZED INTO A BALL IN THE HAND, IT HOLDS ITS SHAPE AND DOES NOT WEEP WATER). THIS SEMI-DRY CONSISTENCY IS WHAT MAKES SAND-CEMENT SCREED SUITABLE FOR TROWELLING TO A FLAT SURFACE. - PLACEMENT: PLACED BETWEEN SCREED RAILS (TIMBER BATTENS OR STEEL SCREED PINS SET TO FINISHED LEVEL); COMPACTED WITH A TAMPING BEAM; FLOATED SMOOTH WITH A BULL FLOAT AND TROWELLED FLAT WITH A FINISHING TROWEL. A MEDIUM MACHINE FLOAT (DINGO OR SIMILAR) IS USED ON LARGE AREAS. - THICKNESS: UNBONDED SCREED (ON A POLYTHENE SEPARATOR MEMBRANE): MINIMUM 50MM. BONDED SCREED (BONDED DIRECTLY TO THE CONCRETE SLAB WITH PVA ADHESIVE OR CEMENT SLURRY): MINIMUM 25MM (BUT PRONE TO CURLING IF TOO THIN). FLOATING SCREED (OVER INSULATION OR UNDERFLOOR HEATING PIPES): MINIMUM 75MM (50MM ABOVE THE HEATING PIPES — THICKER FOR LOWER SCREED THERMAL RESISTANCE). - DRYING TIME: SAND-CEMENT SCREED DRIES SLOWLY — THE RULE OF THUMB IS 1MM PER DAY FOR THE FIRST 40MM AND 1MM PER 2 DAYS ABOVE 40MM. A 75MM SCREED TAKES APPROXIMATELY 85 DAYS TO DRY TO A RELATIVE HUMIDITY ≤ 75% (THE MOISTURE LEVEL REQUIRED BEFORE LAYING MOST FLOOR COVERINGS — WOOD, LVT, CARPET). IN LONDON IN WINTER, DRYING CAN BE EXTENDED FURTHER. - ADDITIVES: POLYPROPYLENE FIBRES (FIBROCRETE PP FIBRES — ADD AT 1KG PER M³): REDUCE PLASTIC SHRINKAGE CRACKING IN THE EARLY CURING PERIOD. NOT A SUBSTITUTE FOR STEEL REINFORCEMENT IN STRUCTURAL SCREEDS.

FLOWING (PUMPABLE) ANHYDRITE OR CALCIUM SULPHATE SCREED: FLOWING SCREEDS ARE LIQUID-APPLIED SCREED MIXES THAT ARE PUMPED ONTO THE FLOOR AND SELF-LEVEL — THEY DO NOT REQUIRE TROWELLING: - ANHYDRITE SCREED: CALCIUM SULPHATE-BASED (ANHYDRITE BINDER — CaSO4 — NOT PORTLAND CEMENT). BRANDS: HANSON CEMFLOOR ULTRA, LAFARGE GYVLON, KNAUF FLOOR. AVAILABLE AS LIQUID READYMIX (DELIVERED BY PUMP TRUCK) OR SITE-MIXED POWDER. - ADVANTAGES: VERY FLAT SURFACE (SELF-LEVELLING); FASTER PLACEMENT (A PUMP TRUCK CAN PLACE 200-300M² PER DAY — VERSUS 30-50M² FOR HAND-PLACED SAND-CEMENT); BETTER FOR UNDERFLOOR HEATING ENCAPSULATION (FLOWS AROUND PIPES; PROVIDES EXCELLENT CONTACT FOR HEAT TRANSFER); THINNER POSSIBLE OVER UFH PIPES (50MM MINIMUM ABOVE PIPES, NOT 75MM) - DISADVANTAGES: MORE EXPENSIVE THAN SAND-CEMENT PER M³; MUST BE COVERED UNTIL SET (DRAUGHT-FREE — ANHYDRITE SCREED IS SENSITIVE TO RAPID SURFACE DRYING IN THE FIRST 24-48 HOURS); ANHYDRITE SCREED MUST NOT BE USED IN WET AREAS (IT REVERTS TO GYPSUM IF PERMANENTLY WET — NOT SUITABLE FOR SHOWER FLOORS OR PERMANENTLY WET ENVIRONMENTS); MUST BE LAITANCE-REMOVED BEFORE TILING (GRIND OR SAND THE SURFACE TO REMOVE THE THIN WEAK LAITANCE LAYER BEFORE TILING — TILING DIRECTLY ON LAITANCE LEADS TO TILE DEBONDING) - POLYMER-MODIFIED CEMENT SCREED (MAPEI ULTRATOP, ARDEX K15 SLC): A HYBRID OPTION — CEMENT-BASED BUT WITH POLYMER ADDITIVES FOR SELF-LEVELLING PROPERTIES. FASTER DRYING THAN TRADITIONAL SAND-CEMENT; MORE COMPATIBLE WITH WET AREAS THAN ANHYDRITE; MORE EXPENSIVE THAN STANDARD SAND-CEMENT.

UNDERFLOOR HEATING SCREED: WHERE WATER UNDERFLOOR HEATING (HYDRONIC UFH) PIPES ARE EMBEDDED IN THE SCREED: - ANHYDRITE SCREED IS PREFERRED: IT FLOWS AROUND THE PIPES (ELIMINATES AIR VOIDS); PROVIDES BETTER THERMAL CONDUCTIVITY CONTACT THAN SAND-CEMENT (WHICH MAY LEAVE VOIDS UNDER THE PIPES) - HEATING PROTOCOL: THE UFH SYSTEM MUST NOT BE HEATED FOR AT LEAST 28 DAYS AFTER SCREED PLACEMENT (ALLOW SCREED TO CURE). THEN GRADUALLY RAISE TEMPERATURE (25°C FOR 3 DAYS; +5°C PER DAY UP TO MAX OPERATING TEMPERATURE — APPROX 45°C FLOW TEMPERATURE) BEFORE NORMAL OPERATION - FLOOR COVERING: CHECK THE FLOOR COVERING MANUFACTURER\'S REQUIREMENTS FOR SCREED MOISTURE CONTENT (MOST TIMBER FLOOR PRODUCTS REQUIRE ≤ 65% RH IN THE SCREED — MAY REQUIRE DEHUMIDIFICATION TO ACHIEVE)

SCREED THICKNESS GUIDE: - BONDED SCREED (ON CONCRETE): 25-40MM (MINIMUM) - UNBONDED SCREED (ON POLYTHENE OVER CONCRETE): 50MM MINIMUM - FLOATING SCREED (OVER INSULATION OR UFH — NO UFH PIPES): 50-65MM - FLOATING SCREED OVER UFH PIPES (SAND-CEMENT): 65-75MM TOTAL (50MM ABOVE PIPES) - FLOATING SCREED OVER UFH PIPES (ANHYDRITE): 60-70MM TOTAL (45-50MM ABOVE PIPES)

DRYING TIMES AND MOISTURE TESTING: - SAND-CEMENT SCREED: 1MM/DAY (FIRST 40MM) + 1MM/2 DAYS (ABOVE 40MM). ACCELERATED DRYING WITH DEHUMIDIFIERS AND UNDERFLOOR HEATING (RAISE GRADUALLY — SEE ABOVE): CAN HALVE DRYING TIMES. - ANHYDRITE SCREED: GENERALLY DRIES FASTER THAN SAND-CEMENT OF THE SAME THICKNESS — TYPICALLY 50MM ANHYDRITE SCREED IS READY FOR FLOOR COVERINGS IN 28-42 DAYS IN NORMAL CONDITIONS. - MOISTURE TESTING: USE A CALIBRATED HYGROMETER PROBE (TRAMEX MOIST MASTER, RS 220G RELATIVE HUMIDITY PROBE — INSERT 40% DEPTH OF SCREED) — TARGET < 75% RH FOR MOST FLOOR COVERINGS; < 65% RH FOR MOST TIMBER FLOORS.

COSTS (SUPPLY AND FIT, LONDON 2025-2026): SAND-CEMENT SCREED (50MM UNBONDED — PER M² SUPPLY AND LAY): £15-30 SAND-CEMENT SCREED (75MM FLOATING OVER UFH — PER M² SUPPLY AND LAY): £20-35 ANHYDRITE FLOWING SCREED (CEMFLOOR ULTRA — 60MM OVER UFH, PER M²): £25-45 POLYMER-MODIFIED LEVELLING COMPOUND (ARDEX K15 — 10-25MM CORRECTION LAYER, PER M²): £15-30 SCREED MOISTURE TEST (HYGROMETER PROBE — PER READING): £30-80 (PROFESSIONAL TESTING COMPANY)