Floor Levelling in London Victorian Terraces: Screeds, Self-Levelling, and Substrate Preparation

BEFORE ANY LEVELLING WORK, THE EXISTING FLOOR CONDITION MUST BE ASSESSED. THE TYPE OF FLOOR STRUCTURE DETERMINES WHICH LEVELLING SYSTEM IS APPROPRIATE:

GROUND FLOOR — SUSPENDED TIMBER (MOST COMMON IN LONDON VICTORIAN TERRACES): THE ORIGINAL GROUND FLOOR CONSTRUCTION IS SUSPENDED PINE BOARD ON TIMBER JOISTS WITH A SUB-FLOOR VOID BELOW (VENTILATED VIA AIR BRICKS IN THE EXTERNAL WALLS). THIS FLOOR TYPE: - IS NOT SUITABLE FOR DIRECT INSTALLATION OF HEAVY CERAMIC OR STONE TILES (FLEXION IN THE TIMBER STRUCTURE CAUSES TILE AND GROUT CRACKING) - IS SUITABLE FOR ENGINEERED TIMBER, LVTILES (LVT), CARPET, AND SOFTWOOD BOARDS - CAN BE MADE SUITABLE FOR TILES BY OVERLAYING WITH 18-22MM CEMENT BOARD (HARDIE BACKER — SCREWED AT 150MM CENTRES TO THE JOISTS) + FLEXIBLE TILE ADHESIVE (BAL RAPIDFLEX, MAPEI GRANIRAPID C2FTE) - UNEVENNESS IN SUSPENDED TIMBER FLOORS IS CORRECTED BY PACKING INDIVIDUAL BOARDS THAT ARE LOW (USING HARDBOARD PACKING STRIPS) OR BY OVERLAYING WITH 12MM PLYWOOD (BS EN 636 — EXTERNAL GRADE — SCREWED AT 200MM CENTRES) BEFORE CEMENT BOARD

GROUND FLOOR — SOLID CONCRETE OR BEAM-AND-BLOCK (LATER ADDITIONS — KITCHEN EXTENSIONS, REAR ADDITIONS): CONCRETE SLABS IN EXTENSIONS AND LATER REAR ADDITIONS: - TYPICALLY HAVE SURFACE VARIATIONS OF 5-15MM ACROSS A 2M STRAIGHT-EDGE - ARE SUITABLE FOR SELF-LEVELLING COMPOUND (SLC) TO CORRECT SURFACE IRREGULARITIES (UP TO 10-15MM DEPTH IN ONE POUR — THICKER LAYERS REQUIRE MULTIPLE POURS OR SCREED) - MAY HAVE EXISTING DPM (DAMP-PROOF MEMBRANE — POLYTHENE 1200 GAUGE) BELOW OR NOT — CHECK FOR DAMP BEFORE APPLYING SLC

UPPER FLOORS — TIMBER JOIST CONSTRUCTION: FIRST FLOOR AND ABOVE IN A VICTORIAN TERRACE IS ALMOST ALWAYS SUSPENDED TIMBER JOISTS: - ORIGINAL PINE BOARDS 150-225MM WIDE WITH GAPS BETWEEN BOARDS (NATURAL SHRINKAGE OVER 100+ YEARS) - SUITABLE FOR OVERLAY WITH 12MM PLYWOOD (GLUED AND SCREWED) TO CREATE A FLAT, GAP-FREE SUBSTRATE FOR LVT, ENGINEERED TIMBER, CARPET, OR VINYL - FOR TILES ON UPPER FLOORS: 18MM HARDIE BACKER OVER THE EXISTING BOARDS + FLEXIBLE ADHESIVE — AND STRUCTURAL ASSESSMENT IF TILES ARE LARGE/HEAVY FORMAT (DEAD LOAD OF TILES + ADHESIVE ON AN UPPER FLOOR MUST BE WITHIN THE JOIST\'S STRUCTURAL CAPACITY)

ONCE THE SUBSTRATE IS ASSESSED AND PREPARED, THE APPROPRIATE LEVELLING SYSTEM IS SELECTED:

SELF-LEVELLING COMPOUND (SLC) — FOR SURFACE CORRECTION UP TO 10-15MM: SLC IS A FLOWABLE CEMENTITIOUS OR ANHYDRITE-BASED COMPOUND THAT SELF-LEVELS WHEN POURED AND SPREAD. IT PRODUCES A VERY FLAT, SMOOTH SURFACE SUITABLE FOR ALL FLOOR FINISHES.

RECOMMENDED PRODUCTS: - ARDEX K 15 DEEP BED SELF-LEVELLING COMPOUND: CAN BE APPLIED 3-20MM IN A SINGLE POUR. WALKABLE IN 3-4 HOURS. SUITABLE FOR TILES, LVT, ENGINEERED TIMBER AFTER FULL CURE (24-48 HOURS). A VERY WIDELY USED PRODUCT IN LONDON RENOVATIONS. - MAPEI ULTRAPLAN ECO: 2-20MM DEPTH. FAST-SETTING (WALKABLE 2-3 HOURS, TILING IN 3-4 HOURS). LOW-EMISSION (EC1 PLUS RATED). SUITABLE OVER UNDERFLOOR HEATING (CHECK TEMPERATURE PROTOCOL). - WEBER FLOOR 4310: 3-30MM DEPTH. GOOD FOR THICKER POURS. WALKABLE 3-5 HOURS.

SLC APPLICATION REQUIREMENTS: 1. PRIMER: ALWAYS PRIME THE SUBSTRATE BEFORE SLC. ARDEX P 51 (FOR ABSORBENT SUBSTRATES), ARDEX P 82 (FOR NON-ABSORBENT — EXISTING TILES, TERRAZZO). PRIMER PREVENTS RAPID WATER ABSORPTION FROM THE SUBSTRATE (WHICH CAUSES CRACKING). 2. SEALING GAPS: FILL ALL GAPS IN THE SUBSTRATE (EXPANSION JOINTS, PIPE PENETRATIONS) WITH FOAM BACKER ROD AND SEALANT BEFORE POURING — SLC FINDS EVERY GAP AND FLOWS OUT. 3. MIX RATIO: FOLLOW MANUFACTURER INSTRUCTION PRECISELY. OVER-WATERING REDUCES STRENGTH AND CAUSES SURFACE DUSTING AFTER CURE. 4. POUR AND SPREAD WITHIN OPEN TIME: SLC HAS AN OPEN TIME OF 10-20 MINUTES — THE FULL FLOOR AREA SHOULD BE POURED AND SPREAD WITHIN THIS WINDOW.

SAND-CEMENT SCREED (FOR DEPTHS OVER 15MM OR LARGE AREAS): A TRADITIONAL SEMI-DRY SAND-CEMENT SCREED (1:3-1:4 CEMENT:SHARP SAND MIX, TAMPED DOWN) IS USED FOR DEPTHS OF 15-75MM. SLOWER DRYING (1MM PER DAY — A 50MM SCREED REQUIRES 50 DAYS DRYING BEFORE TILING) BUT PROVIDES THERMAL MASS FOR UNDERFLOOR HEATING SYSTEMS.

FLOWING SCREED (ANHYDRITE OR CALCIUM SULPHATE SCREED — HANSON CEMFLOOR, LAFARGE THERMIA): FLOWING SCREEDS ARE PUMPED AND SELF-LEVELLING — THEY ARE THE PREFERRED SCREED FOR UNDERFLOOR HEATING INSTALLATIONS: - HANSON CEMFLOOR 65MM DEPTH OVER UFH PIPES: THE STANDARD SPECIFICATION FOR LONDON EXTENSION KITCHEN-DINERS WITH NU-HEAT OR UPONOR UNDERFLOOR HEATING - SELF-LEVELS TO ±3MM FLATNESS (VERSUS ±5-8MM FOR HAND-SCREEDED SAND-CEMENT) - WALK-ON IN 24-48 HOURS; TILING AFTER 28 DAYS - ANHYDRITE SCREED MUST BE SEALED BEFORE TILING (ARDEX P 51 DILUTED 1:3 AS PRIMER — ANHYDRITE REACTS WITH CEMENT-BASED ADHESIVE IF UNSEALED) - DRYING TIME: 1MM PER DAY FOR THE FIRST 40MM; 0.5MM PER DAY ABOVE 40MM — A 65MM SCREED DRIES IN 90 DAYS NATURALLY (OR FORCE-DRIED WITH UFH AT LOW TEMPERATURE FROM DAY 7: 25°C FOR 7 DAYS, THEN 45°C FOR 7 DAYS)

TWO IMPORTANT CONSIDERATIONS BEFORE AND AFTER FLOOR LEVELLING:

MOISTURE CHECK — CRITICAL BEFORE ANY FLOOR FINISH: RELATIVE HUMIDITY (RH) OF THE SUBSTRATE MUST BE BELOW: - 75% RH FOR MOST FLOORING TYPES (TILES, LVT, ENGINEERED TIMBER, CARPET) - 65% RH FOR SOLID TIMBER BOARDS - MEASURE WITH A CALIBRATED HYGROMETER (TRAMEX CME5, PROTIMETER SURVEYMASTER) — NOT JUST A SURFACE SCRATCH TEST

HOW LONG SHOULD SCREEDS DRY BEFORE CHECKING? - SAND-CEMENT SCREED: 1MM PER DAY (50MM = 50 DAYS MINIMUM) - ANHYDRITE FLOWING SCREED: 1MM/DAY TO 40MM, 0.5MM/DAY ABOVE 40MM — OR FORCE-DRY WITH UFH - IF MOISTURE IS STILL ELEVATED: SURFACE DPM (ARDEX MC RAPID MOISTURE CONTROL OR MAPEI PRIMER SN — TWO-COAT EPOXY-BASED MOISTURE BARRIER) CAN BE APPLIED TO ALLOW TILING BEFORE FULL DRYING (LOCKS IN RESIDUAL MOISTURE BELOW THE BARRIER)

UNDERFLOOR HEATING COMMISSIONING BEFORE TILING: FOR ANY FLOOR WITH ELECTRIC UFH OR HYDRONIC UFH BENEATH IT: - ELECTRIC UFH: CHECK ELECTRICAL RESISTANCE WITH MULTIMETER BEFORE POURING SCREED (WARMUP, NUHEAT, DCM-PRO ALL SUPPLY TEST CERTIFICATE) - HYDRONIC UFH (NU-HEAT LOOPSTRATE, UPONOR MLCP): PRESSURE-TEST PIPEWORK AT 6 BAR FOR 24 HOURS BEFORE SCREED POUR; MAINTAIN PRESSURE DURING SCREED POUR; RELEASE PRESSURE AFTER SCREED HAS CURED TO PREVENT PIPE MOVEMENT - DO NOT SWITCH ON UFH FOR THE FIRST 21 DAYS AFTER SCREED POUR (LET IT CURE NATURALLY FIRST)

COSTS FOR FLOOR LEVELLING IN LONDON (2025-2026): SELF-LEVELLING COMPOUND (ARDEX K 15 OR MAPEI ULTRAPLAN — SUPPLY AND APPLY, UP TO 10MM DEPTH, UP TO 20M²): £400-800 SAND-CEMENT SCREED (50MM, HAND-LAID, 20M²): £600-1,000 ANHYDRITE FLOWING SCREED (65MM WITH UFH, 20M², PUMPED): £800-1,400 HARDIE BACKER OVERLAY ON TIMBER FLOOR (18MM, SCREWED, INCLUDING PLYWOOD INTERMEDIATE IF REQUIRED, 20M²): £500-900 MOISTURE SURVEY AND DAMP READING REPORT: £150-300