Plastering and Plasterwork for London Home Renovations: Skimming, Bonding, Dry Lining, and Heritage Lime Plaster Guide

BRITISH GYPSUM (PART OF THE SAINT-GOBAIN GROUP) IS THE DOMINANT PLASTER MANUFACTURER IN THE UK AND THE DEFAULT SPECIFICATION FOR GYPSUM PLASTERING IN LONDON RENOVATION. THE THISTLE RANGE COVERS ALL STANDARD APPLICATIONS.

THISTLE MULTI-FINISH (THE STANDARD LONDON SKIM COAT): THISTLE MULTI-FINISH IS THE MOST WIDELY USED FINISH PLASTER IN LONDON RENOVATION — APPLIED AS A 2MM FINISH COAT OVER A BROWNING COAT OR A PLASTERBOARD BACKING THAT HAS BEEN TAPED AND JOINTED. THISTLE MULTI-FINISH IS APPROPRIATE FOR MOST INTERNAL BACKGROUNDS INCLUDING: PLASTERBOARD (GYPROC WALLBOARD, GYPROC DUPLEX), EXISTING GYPSUM PLASTER (BACKGROUND THAT IS SOUND BUT ROUGH), BONDING COAT (THISTLE BONDING COAT), AND LIGHTWEIGHT AGGREGATE BLOCKS (AFTER SPATTERDASH DASH COAT OR PRIMER). APPLIED AT 2MM THICKNESS WET (FINISHING TROWEL TO FINAL POLISH). SETTING TIME: APPROXIMATELY 60 TO 90 MINUTES — FASTER THAN SOME MULTI-FINISH PRODUCTS, SO LONDON PLASTERERS TYPICALLY WORK IN MANAGEABLE PANELS RATHER THAN WETTING BACK AND CONTINUING TO POLISH.

THISTLE BONDING COAT (THE UNDERCOAT FOR LOW-SUCTION BACKGROUNDS): THISTLE BONDING COAT IS A GYPSUM UNDERCOAT PLASTER SUITABLE FOR LOW-SUCTION AND VARIABLE SUCTION BACKGROUNDS — INCLUDING CONCRETE, ENGINEERING BRICK, OLD OIL-PAINTED PLASTER, AND DENSE BLOCKS. APPLIED 8 TO 11MM THICK, SCRATCHED FOR KEY, AND THEN OVERCOATED WITH THISTLE MULTI-FINISH. BONDING COAT IS ESSENTIAL IN LONDON RENOVATION WHERE MIXED SUBSTRATES (PART OLD PLASTER, PART NEW BLOCK, PART CONCRETE) ARE ENCOUNTERED — IT PROVIDES A CONSISTENT BACKGROUND FOR THE FINISH COAT.

THISTLE BROWNING (UNDERCOAT FOR HIGH-SUCTION BACKGROUNDS): THISTLE BROWNING IS A GYPSUM UNDERCOAT FOR HIGH-SUCTION BACKGROUNDS SUCH AS BRICK, TERRACOTTA BLOCKS, AND LIGHTWEIGHT AERATED CONCRETE BLOCKS (THERMALITE). APPLIED 8 TO 11MM, SCRATCHED FOR KEY, AND OVERCOATED WITH MULTI-FINISH.

THISTLE RENOVATION PLASTER (FOR DAMP OR SALT-CONTAMINATED BACKGROUNDS): FOR LONDON TERRACE WALLS SUFFERING FROM RISING DAMP OR PENETRATING DAMP WHERE HYGROSCOPIC SALTS HAVE CONTAMINATED THE SUBSTRATE, ORDINARY GYPSUM PLASTERS WILL FAIL — HYGROSCOPIC SALTS ABSORB ATMOSPHERIC MOISTURE AND CAUSE BLISTERING AND BLOWING OF ORDINARY PLASTER. THISTLE RENOVATION PLASTER (A DENSE, SALT-RESISTANT RENOVATION PLASTER SYSTEM) IS SPECIFIED FOR THESE CONDITIONS. ALTERNATIVELY, KNAUF AQUAPANEL OR WEBER THANE RENOVATION PLASTER SYSTEMS ARE USED.

DRY LINING (PLASTERBOARD FIXED TO STUDS OR DIRECT-BONDED TO MASONRY WITH DOT-AND-DAB ADHESIVE) IS THE MOST COMMON INTERNAL WALL CONSTRUCTION METHOD IN LONDON RENOVATION — FASTER THAN WET PLASTER SYSTEMS, PRODUCING LOWER MOISTURE LEVELS DURING CONSTRUCTION, AND ENABLING INTEGRATION OF INSULATION AND SERVICES IN THE VOID BEHIND THE BOARD.

GYPROC WALLBOARD (STANDARD 12.5MM): THE STANDARD PLASTERBOARD FOR DRY LINING IN MOST LONDON RENOVATION APPLICATIONS. AVAILABLE IN 1200MM X 2400MM AND 1200MM X 2700MM SHEETS. FIXED TO TIMBER STUDS AT 400MM OR 600MM CENTRES (AT 400MM FOR BOARDS LAID PERPENDICULAR TO STUDS — BEST PRACTICE). FOR DIRECT BONDING TO MASONRY (DOT AND DAB), GYPROC DRYWALL ADHESIVE (BONDING AGENT) IS APPLIED IN CONTINUOUS PERIMETER BEADS AND INTERMITTENT INTERNAL DABS AT MAX 450MM CENTRES IN BOTH DIRECTIONS.

GYPROC THERMALINE BASIC AND PLUS (INSULATED PLASTERBOARD): THERMALINE BASIC (12.5MM PLASTERBOARD + 22MM EPS INSULATION) AND THERMALINE PLUS (12.5MM + 47MM EPS) ARE COMPOSITE PLASTERBOARD/INSULATION BOARDS USED FOR DRY LINING EXTERNAL WALLS IN LONDON RENOVATION — PROVIDING BOTH THE PLASTERBOARD SURFACE AND A LAYER OF INSULATION IN A SINGLE PRODUCT. FOR SOLID-WALL LONDON VICTORIAN TERRACES (WHERE THERE IS NO CAVITY), THERMALINE PLUS (47MM EPS) GIVES APPROXIMATELY 0.60-0.80 W/M'K REDUCTION IN WALL U-VALUE — IMPROVING THERMAL PERFORMANCE WITHOUT HEAVY EXTERNAL WALL INSULATION WORKS.

GYPROC MOISTURE RESISTANT (MR) BOARD (FOR BATHROOMS AND KITCHENS): GYPROC MOISTURE RESISTANT BOARD (GREEN-FACED) IS USED IN AREAS OF HIGH MOISTURE — BATHROOMS, EN SUITES, KITCHENS, AND UTILITY ROOMS. MR BOARD IS MORE RESISTANT TO SHORT-TERM WETTING THAN STANDARD WALLBOARD BUT IS NOT A WATERPROOF BOARD — FOR WET AREAS (INSIDE SHOWER ENCLOSURES), A TILE BACKER BOARD (WEDI BOARD, HARDIE BACKER, MAPEI AQUAMAT OVER MR BOARD) OR MAPEI MAPELASTIC TANKING MEMBRANE MUST BE APPLIED BEFORE TILING.

TAPE AND JOINT SYSTEMS (GYPROC EASI-FILL 60 OR KNAUF FILL+FINISH LIGHT): FOR DRY-LINED PLASTERBOARD WALLS, THE JOINTS BETWEEN BOARDS AND THE SCREW HOLES MUST BE FILLED AND FEATHERED WITH A JOINTING COMPOUND BEFORE APPLYING A MULTI-FINISH SKIM COAT. GYPROC EASI-FILL 60 (60-MINUTE SETTING COMPOUND) OR KNAUF FILL+FINISH LIGHT ARE THE STANDARD PRODUCTS USED IN LONDON RENOVATION.

ORIGINAL LIME PLASTER ON LATH (THREE-COAT LIME SYSTEM: SCRATCH COAT, FLOAT COAT, FINISH COAT — ALL LIME-BASED) IS THE AUTHENTIC INTERNAL WALL AND CEILING FINISH IN ALL LONDON PROPERTIES BUILT BEFORE APPROXIMATELY 1920. LIME PLASTER IS VAPOUR-PERMEABLE (BREATHABLE) — CRITICALLY IMPORTANT IN SOLID-WALL VICTORIAN TERRACE CONSTRUCTION WHERE MOISTURE MANAGEMENT IS THROUGH VAPOUR DIFFUSION RATHER THAN BARRIERS.

ST ASTIER NHL PRODUCTS FOR LIME PLASTER REPAIR: NATURAL HYDRAULIC LIME (NHL) IS THE MOST APPROPRIATE BINDER FOR REPAIR WORK IN LONDON VICTORIAN LIME-PLASTERED INTERIORS. ST ASTIER NHL 2.0 (WEAKEST HYDRAULIC SET — MOST FLEXIBLE, MOST VAPOUR-PERMEABLE) IS APPROPRIATE FOR FINE FINISH COATS AND WHERE THE BACKGROUND SUBSTRATE IS SOFT LIME MORTAR. ST ASTIER NHL 3.5 (MODERATE HYDRAULIC SET — STANDARD FOR FLOAT AND SCRATCH COATS IN MOST LONDON RENOVATION REPAIR WORK) IS THE MOST COMMONLY USED REPAIR LIME. ST ASTIER NHL 5.0 (STRONGER HYDRAULIC SET) IS USED WHERE SOME ADDITIONAL STRENGTH IS NEEDED (E.G., HIGH-HUMIDITY OR HIGH-TRAFFIC AREAS).

TRADITIONAL LIME PLASTER MIX RATIOS: SCRATCH COAT (BASE COAT, ONTO LATH): 1 PART NHL 3.5 : 2.5 TO 3 PARTS SHARP SAND (BY VOLUME). FLOAT COAT (DEVILLING/STRAIGHTENING COAT): 1 PART NHL 3.5 : 3 PARTS SHARP SAND. FINISH COAT: 1 PART NHL 2.0 : 1.5 TO 2 PARTS FINE SAND (SILVER SAND OR WASHED BUILDING SAND).

HAIR FIBRES IN LIME PLASTER: TRADITIONAL LIME PLASTER SCRATCH AND FLOAT COATS INCLUDED ANIMAL HAIR (CATTLE HAIR, GOAT HAIR) AS A FIBRE REINFORCEMENT TO CONTROL SHRINKAGE CRACKING. FOR AUTHENTIC HERITAGE REPAIRS IN LISTED BUILDINGS AND CONSERVATION AREA PROPERTIES IN LONDON, CATTLE OR HORSE HAIR SHOULD BE ADDED TO THE LIME MORTAR MIX (APPROXIMATELY 1.5KG HAIR PER 50KG OF NHL) FOR THE SCRATCH AND FLOAT COATS.

LONDON'S VICTORIAN TERRACE STOCK PRESENTS SPECIFIC PLASTERING CHALLENGES THAT ARE IMPORTANT TO UNDERSTAND BEFORE SPECIFYING A RENOVATION APPROACH.

EXISTING LIME PLASTER THAT IS SOUND VS FAILED: BEFORE STRIPPING ALL EXISTING PLASTER IN A RENOVATION, A CAREFUL SURVEY SHOULD ESTABLISH WHICH AREAS ARE SOUND AND WHICH ARE FAILED. SOUND LIME PLASTER (RINGS CLEANLY WHEN TAPPED, NO HOLLOW AREAS, NO CRACKING OR SALT EFFLORESCENCE) SHOULD BE RETAINED AND REPAIRED — NOT HACKED OFF AND REPLACED. STRIPPING SOUND LIME PLASTER DESTROYS A HISTORICALLY VALUABLE MATERIAL, CREATES LARGE QUANTITIES OF CONSTRUCTION WASTE, AND CAN DAMAGE ORIGINAL DECORATIVE CORNICES AND MOULDINGS.

COMPATIBILITY BETWEEN GYPSUM AND LIME PLASTER: A COMMON ERROR IN LONDON TERRACE RENOVATION IS TO APPLY MODERN GYPSUM PLASTER PRODUCTS (THISTLE MULTI-FINISH) DIRECTLY ONTO EXISTING LIME PLASTER BACKGROUNDS. GYPSUM PLASTERS ARE CHEMICALLY INCOMPATIBLE WITH LIME PLASTERS IN SOME CONDITIONS — THE DIFFERING SHRINKAGE RATES CAN CAUSE DEBONDING AND CRACKING. ALWAYS USE LIME-BASED PRODUCTS FOR REPAIRS TO EXISTING LIME PLASTER BACKGROUNDS, AND USE AN APPROPRIATE BONDING COAT (THISTLE BONDING COAT) IF APPLYING GYPSUM FINISH TO A LIME BACKGROUND THAT HAS BEEN STABILISED.

CEILING LATH FAILURE: IN MANY LONDON VICTORIAN TERRACE PROPERTIES, ORIGINAL TIMBER LATH (THIN SPLIT TIMBER STRIPS NAILED AT APPROXIMATELY 5MM GAPS TO CEILING JOISTS — THE LIME PLASTER KEYS THROUGH THE GAPS) HAS BEEN WEAKENED BY MOISTURE, TIMBER SHRINKAGE, OR MECHANICAL DAMAGE OVER 130+ YEARS. ISOLATED AREAS OF FAILED LATH (WHERE THE LIME PLASTER HAS DETACHED AND CEILING "SAUCERS" HAVE FORMED — THE CLASSIC GEORGIAN/VICTORIAN CEILING FAILURE MODE) CAN BE REPAIRED WITH NEW TIMBER LATH AND NEW LIME PLASTER. WHERE MORE THAN 30 TO 40% OF THE CEILING IS FAILED, FULL REPLACEMENT WITH NEW PLASTERBOARD (MAINTAINING THE CORNICE DETAIL WITH CAREFULLY REMOVED AND REINSTATED ORIGINAL CORNICES, OR NEW GRG REPRODUCTION CORNICES) IS TYPICALLY MORE ECONOMICAL.

THE CORRECT SEQUENCING OF PLASTERING WORKS AND ADEQUATE DRYING TIMES ARE CRITICAL FOR ACHIEVING QUALITY FINISHES AND AVOIDING PAINT FAILURE ON NEW PLASTER.

SEQUENCING: PLASTERING SHOULD NOT BEGIN UNTIL THE FOLLOWING ARE COMPLETE: BUILDING IS WATERTIGHT (ROOF REPAIR, WINDOW AND DOOR INSTALLATION). FIRST-FIX ELECTRICAL AND PLUMBING IS COMPLETE (ALL CONDUITS, BACK BOXES, PIPE CHASES, UNDERFLOOR HEATING PIPEWORK LAID AND TESTED). UNDERFLOOR HEATING SCREEDS ARE POURED AND HAVE BEEN THROUGH INITIAL DRYING (MINIMUM 7 DAYS FOR ANHYDRITE SCREED). ALL STRUCTURAL WORKS ARE COMPLETE (STEELWORK, STRUCTURAL OPENINGS, PADSTONES, LINTELS). INSULATION IS COMPLETE (WALL INSULATION, ROOF INSULATION).

DRYING TIMES: NEW PLASTER MUST DRY THOROUGHLY BEFORE PAINTING — DECORATING OVER NEW, DAMP PLASTER IS THE MOST COMMON CAUSE OF PAINT FAILURE IN LONDON RENOVATION. GUIDANCE: A 2MM SKIM COAT (MULTI-FINISH) ON PLASTERBOARD: MINIMUM 4 TO 6 WEEKS BEFORE PAINTING IN A NORMAL LONDON HEATING SEASON (HEATED BUILDING). A THREE-COAT LIME PLASTER SYSTEM: MINIMUM 6 TO 12 WEEKS (LONGER IN COOL, HUMID CONDITIONS — LIME PLASTER REQUIRES CO2 FROM THE AIR TO CARBONATE AND FULLY HARDEN). VISIBLE INDICATORS OF ADEQUATELY DRIED PLASTER: UNIFORM MID-GREY OR CREAM COLOUR (WET PLASTER IS DARK GREY OR BROWN — THE COLOUR LIGHTENS AS MOISTURE LEAVES). MISTING (DILUTED PVA SEALER — 1 PART PVA TO 5 PARTS WATER — THINNED EMULSION, OR FARROW+BALL PRIMER) SHOULD BE APPLIED BEFORE FULL COAT TO SEAL THE POROUS NEW PLASTER SURFACE.

PLASTERING COSTS IN LONDON VARY CONSIDERABLY BASED ON THE AREA, SPECIFICATION, AND EXTENT OF PREPARATION REQUIRED.

SKIM COAT ONTO PLASTERBOARD (THE MOST COMMON SPECIFICATION IN NEW LONDON EXTENSIONS AND REFURBISHMENTS): LABOUR + MATERIALS: £10 TO £18 PER M² FOR SKIM COAT ONLY ONTO ALREADY-FIXED PLASTERBOARD (TAPED AND JOINTED). PLASTERBOARD SUPPLY AND FIX (GYPROC WALLBOARD, INCLUDING TAPE AND JOINT): £15 TO £25 PER M² DEPENDING ON BOARD TYPE.

BONDING COAT + MULTI-FINISH ONTO MASONRY OR MIXED SUBSTRATES: LABOUR + MATERIALS: £18 TO £30 PER M² FOR TWO-COAT SYSTEM (BONDING COAT + MULTI-FINISH) ON MASONRY.

LIME PLASTER REPAIR (PATCH REPAIR TO EXISTING LIME PLASTER IN CONSERVATION AREA LONDON TERRACE): £35 TO £65 PER M² FOR SPECIALIST LIME PLASTERER INCLUDING MATERIALS.

FULL HERITAGE LIME PLASTER SYSTEM (THREE-COAT ON LATH — NEW WORKS OR FULL REPLACEMENT): £45 TO £80 PER M² SUPPLY AND APPLY INCLUDING HAIR FIBRES.

CEILING CORNICE REPAIR (PER LINEAR METRE OF DAMAGED HERITAGE CORNICE, LIME PLASTER IN-SITU OR GRG REPRODUCTION INSTALLATION): £60 TO £200 PER METRE DEPENDING ON PROFILE COMPLEXITY.