Ceiling Repair and Restoration for London Victorian Terraces: Lath and Plaster, Plasterboard, and Cornice

BEFORE DECIDING ON A REPAIR OR REINSTATEMENT APPROACH FOR A DETERIORATED LATH AND PLASTER CEILING IN A LONDON VICTORIAN TERRACE, THE EXTENT AND CAUSE OF THE FAILURE MUST BE DIAGNOSED.

TYPES OF LATH AND PLASTER CEILING FAILURE: KEYING FAILURE (THE PLASTER KEYS BEHIND THE LATHS HAVE SEPARATED — MOST COMMON FAILURE MODE): THE LIME PLASTER WAS APPLIED IN A SEMI-LIQUID STATE THAT SQUEEZED BETWEEN THE TIMBER LATHS AND HARDENED AS "KEYS" (NIBS) BEHIND THE LATHS. AS THE TIMBER LATHS EXPAND AND CONTRACT WITH MOISTURE CHANGES, THE KEYS GRADUALLY SHEAR — THE PLASTER FACE STAYS IN PLACE INITIALLY, HELD BY FRICTION AND COHESION, BUT EVENTUALLY DROPS IN LARGE SHEETS. DIAGNOSIS: TAP THE CEILING WITH A KNUCKLE — A HOLLOW, DEAD SOUND (VS THE SOLID SOUND OF WELL-KEYED PLASTER) INDICATES KEY FAILURE. SAFE AREAS SOUND SOLID; FAILED AREAS SOUND HOLLOW.

LATH DECAY (TIMBER LATHS HAVE ROTTED — COMMON AFTER ROOF LEAKS OR PERSISTENT CONDENSATION): IF THE TIMBER LATHS HAVE DETERIORATED (ROTTEN, FRIABLE), THE SUBSTRATE FOR REPAIRS IS COMPROMISED — PATCHING OVER DECAYED LATHS IS A TEMPORARY FIX AT BEST. LATH CONDITION CAN BE ASSESSED BY PROBING WITH A BRADAWL THROUGH A FAILED AREA.

PLASTER SHRINKAGE CRACKS (HAIRLINE CRACKS IN THE SKIM COAT — COSMETIC ONLY): FINE CRACKS IN THE FINISH PLASTER ARE COMMON IN LONDON VICTORIAN TERRACES AND ARE USUALLY COSMETIC — THE RESULT OF SEASONAL MOVEMENT AND THERMAL CYCLING. THESE DO NOT INDICATE KEY FAILURE AND CAN BE ADDRESSED WITH A FLEXIBLE FILLER (TOUPRET FIBERTAPE OR POLYCELL FINE SURFACE FILLER) OR A FULL SKIM COAT.

DIAGNOSIS PRINCIPLE: BEFORE ANY WORK, TAP THE ENTIRE CEILING TO MAP HOLLOW AREAS. HOLLOW AREAS = KEY FAILURE. SOLID AREAS = INTACT. DECIDE ON REPAIR (PATCH HOLLOW AREAS ONLY) OR FULL REPLACEMENT (WHERE MORE THAN 50% OF THE CEILING IS HOLLOW OR COMPROMISED).

WHERE THE LATH AND PLASTER CEILING IS PARTIALLY FAILED (HOLLOW AREAS UP TO 50% OF THE TOTAL CEILING), REPAIR IS FEASIBLE — FULL REPLACEMENT IS NOT NECESSARY IF THE LATHS ARE SOUND.

APPROACH 1: LIME PLASTER REPAIR (THE MOST SYMPATHETIC — PRESERVES THE ORIGINAL MATERIALS AND MOVEMENT CHARACTERISTICS): THE FAILED PLASTER IS REMOVED BACK TO SOUND KEYED AREAS. EXISTING LATHS ARE CHECKED FOR SOUNDNESS AND ANY DECAYED LATHS REPLACED. LIME SCRATCH COAT (HYDRAULIC LIME PLASTER — HANSON PLASMOR OR ST ASTIER NHL 3.5 + SHARP SAND 1:2.5 MIX) IS APPLIED IN A 10-12MM COAT OVER THE LATHS — CROSSED AND SCRATCHED BEFORE SETTING. AFTER 24 TO 48 HOURS: FLOAT COAT (NHL 3.5 + SILVER SAND 1:3 — 6-8MM) IS APPLIED. AFTER FLOAT COAT SETS (7 TO 14 DAYS FOR HYDRAULIC LIME): FINISH COAT (FINE LIME PUTTY PLASTER — CHARCOAL LIME FINE COAT OR LIMELITE RENOVATING PLASTER) IS APPLIED AT 2-3MM AND WORKED SMOOTH. TOTAL DEPTH: 18 TO 22MM — MATCHING THE ORIGINAL THREE-COAT SYSTEM.

APPROACH 2: BONDING COAT + MULTI-FINISH (MODERN PLASTER REPAIR ON SOUND EXISTING LATH AND PLASTER BACKGROUND): WHERE A FAST, COST-EFFECTIVE PATCH IS REQUIRED AND LIME AUTHENTICITY IS NOT A PRIORITY: EXISTING FAILED PLASTER REMOVED TO SOUND EDGES. BONDING COAT (BRITISH GYPSUM THISTLE BONDING — OR KNAUF BONDING COAT) APPLIED 8MM THICK TO THE EXISTING PLASTER BACKGROUND (AROUND PATCH EDGES) AND OVER THE EXPOSED LATH AREAS. HESSIAN SCRIM TAPE IS APPLIED OVER THE JOINT BETWEEN NEW AND EXISTING PLASTER (PREVENTING CRACK PROPAGATION AT THE REPAIR EDGE). MULTI-FINISH SKIM (BRITISH GYPSUM THISTLE MULTI-FINISH — 2MM) APPLIED OVER THE BONDING COAT AFTER 24H.

CRITICAL DETAIL AT REPAIR EDGES: THE JUNCTION BETWEEN THE EXISTING LIME PLASTER CEILING AND ANY NEW REPAIR IS A CRACK-RISK LOCATION. PVA BOND (UNIBOND PVA DILUTED 1:3) APPLIED TO THE EXISTING PLASTER EDGE IMPROVES ADHESION OF THE NEW BONDING COAT — BUT LIME PLASTER SHOULD NOT BE BONDED WITH PVA ALONE (PVA FILMS OVER AS IT DRIES, REDUCING SUCTION).

WHERE MORE THAN 50% OF THE CEILING HAS FAILED KEYING, OR WHERE THE LATHS ARE DECAYED, FULL CEILING REPLACEMENT IS THE MOST RELIABLE LONG-TERM SOLUTION. IN A LONDON VICTORIAN TERRACE, THE CHOICE IS BETWEEN: REPLACING LATH AND PLASTER LIKE-FOR-LIKE (INSTALLING NEW TIMBER LATHS AND LIME PLASTERING — USED IN LISTED BUILDINGS AND SOME CONSERVATION AREA RESTORATIONS) OR REPLACING WITH MODERN PLASTERBOARD (THE STANDARD APPROACH IN NON-LISTED LONDON VICTORIAN TERRACE RENOVATION).

PLASTERBOARD REPLACEMENT PROCESS: STEP 1: STRIP ALL FAILED LATH AND PLASTER (SIGNIFICANT DUST AND DEBRIS — POLYTHENE SHEETING PROTECTION OF THE ROOM BELOW AND SKIP FOR DISPOSAL ESSENTIAL). STEP 2: CHECK STRUCTURAL CONDITION OF CEILING JOISTS — PROBE FOR ROT, CHECK MOISTURE CONTENT (MOISTURE METER — LESS THAN 18% BEFORE PLASTERBOARD FIX). STEP 3: FIX 12.5MM BRITISH GYPSUM GYPROC WALLBOARD (STANDARD PLASTERBOARD — 12.5MM FOR CEILING TO MEET MINIMUM STABILITY REQUIREMENTS FOR CEILING-FIXED PLASTERBOARD; SOME SPECIFIERS USE 15MM FOR INCREASED RIGIDITY ON CEILINGS). FIX WITH 3.5MM X 38MM DRYWALL SCREWS AT 300MM CENTRES INTO CEILING JOISTS. STEP 4: TAPE AND JOINT (BRITISH GYPSUM GYPROC JOINT CEMENT + JOINTING TAPE) AT ALL BOARD EDGES AND JOINTS. STEP 5: SKIM COAT (BRITISH GYPSUM THISTLE MULTI-FINISH — 2MM) OVER THE ENTIRE CEILING.

ACOUSTIC CONSIDERATION: IN LONDON CONVERTED TERRACE FLATS, REPLACING A LATH AND PLASTER CEILING (WHICH HAS INHERENT ACOUSTIC MASS — A 3-COAT LIME PLASTER CEILING IS APPROXIMATELY 25-30 KG/M²) WITH LIGHTWEIGHT PLASTERBOARD (12.5MM GYPROC = 10.2 KG/M²) CAN REDUCE THE ACOUSTIC PERFORMANCE OF THE CEILING. IF ACOUSTIC PERFORMANCE IS A CONCERN, CONSIDER 15MM GYPROC (12.3 KG/M²) PLUS INDEPENDENT CEILING SYSTEM WITH ROCKWOOL ACOUSTIC QUILT ABOVE.

THE CORNICE RUNNING AROUND THE CEILING PERIMETER OF A LONDON VICTORIAN TERRACE LIVING ROOM OR BEDROOM IS ONE OF THE MOST DISTINCTIVE PERIOD FEATURES — AND ONE OF THE MOST COMMONLY DAMAGED OR MISSING ELEMENTS IN LONDON RENOVATION PROJECTS. REPAIR AND REINSTATEMENT OF THE VICTORIAN CORNICE IS THEREFORE BOTH A TECHNICAL AND AN AESTHETIC PRIORITY IN PERIOD PROPERTY RENOVATION.

GRC (GLASS FIBRE REINFORCED CEMENT) AND GRG (GLASS REINFORCED GYPSUM) CORNICE (THE STANDARD APPROACH FOR CORNICE REINSTATEMENT IN LONDON VICTORIAN TERRACES): GRG (GLASS REINFORCED GYPSUM) CORNICE — MANUFACTURED BY STEVENSONS OF NORWICH (THE PRINCIPAL UK MANUFACTURER OF ARCHITECTURAL PERIOD CORNICE PROFILES) — IS THE MOST COMMONLY SPECIFIED PRODUCT FOR CORNICE REINSTATEMENT IN LONDON VICTORIAN TERRACE PROPERTIES. GRG CORNICE IS CAST FROM A MOULD TAKEN FROM THE ORIGINAL OR FROM A STANDARD PATTERN BOOK PROFILE — THE RESULTING SECTION IS PRECISELY MATCHED TO THE EXISTING CORNICE PROFILE. INSTALLATION: GRG CORNICE IS FIXED TO THE CEILING AND WALL JUNCTION WITH COVING ADHESIVE (BRITISH GYPSUM THISTLE COVING ADHESIVE OR GYPROC COVING FIXATIVE) AND SUPPLEMENTARY CUT MASONRY NAILS OR SCREWS INTO TIMBER NOGGINGS ABOVE THE PLASTERBOARD CEILING EDGE.

IN-SITU RUN LIME CORNICE (FOR CONSERVATION AREAS OR LISTED BUILDING RESTORATION): THE ORIGINAL METHOD OF CREATING A VICTORIAN CORNICE WAS BY "RUNNING" THE CORNICE IN-SITU WITH A ZINC TEMPLATE (SCREED) DRAWN ALONG A RUNNING RULE AND HORSED RAIL FIXED TO THE CEILING AND WALL — APPLYING SUCCESSIVE COATS OF LIME PLASTER UNTIL THE FULL PROFILE IS BUILT UP. THIS METHOD IS NOW ONLY USED BY SPECIALIST LIME PLASTERERS FOR HIGH-VALUE LISTED BUILDING RESTORATION IN LONDON (E.G. GEORGIAN TERRACES IN BELGRAVIA, KENSINGTON, AND MARYLEBONE).

CEILING REPAIR COSTS IN LONDON VARY SIGNIFICANTLY BASED ON THE EXTENT OF DAMAGE, THE REPAIR APPROACH, AND THE SIZE OF THE CEILING.

LATH AND PLASTER PATCH REPAIR (LIME PLASTER — PER M² OF REPAIRED AREA): MATERIAL (NHL 3.5 LIME, SAND, SCRIM) + LABOUR FOR SMALL PATCH (UP TO 2 M²): £120 TO £250 PER M². BONDING COAT + MULTI-FINISH PATCH (MODERN GYPSUM — PER M² OF REPAIR): £60 TO £120 PER M².

FULL CEILING REPLACEMENT (LATH AND PLASTER TO PLASTERBOARD + SKIM — PER M²): STRIP AND DISPOSE OF LATH AND PLASTER: £15 TO £25 PER M². 12.5MM GYPROC BOARD + FIX + TAPE + SKIM COAT: £25 TO £40 PER M². TOTAL REPLACEMENT: £40 TO £65 PER M².

CORNICE (GRG PROFILE — SUPPLY AND FIT PER LINEAR METRE): STANDARD GRG VICTORIAN CORNICE 100-150MM GIRTH (STEVENSONS OF NORWICH OR SIMILAR — SUPPLIED IN 2M OR 3M LENGTHS): SUPPLY: £15 TO £35 PER LINEAR METRE. INSTALLATION (FIX, MITRE CORNERS, FILL AND SAND): £10 TO £25 PER LINEAR METRE. TOTAL: £25 TO £60 PER LINEAR METRE.

IN-SITU RUN LIME CORNICE (SPECIALIST LIME PLASTERER — FOR LISTED BUILDINGS): £150 TO £350 PER LINEAR METRE (SIGNIFICANTLY MORE EXPENSIVE — ONLY FOR LISTED BUILDINGS OR VERY HIGH SPECIFICATION CONSERVATION RESTORATIONS).

TYPICAL LONDON VICTORIAN TERRACE FRONT RECEPTION ROOM CEILING REPAIR PROJECT (FULL CEILING REPLACEMENT 20 M² + GRG CORNICE REINSTATEMENT 16 LINEAR METRES): CEILING REPLACEMENT: £800 TO £1,300. GRG CORNICE: £400 TO £960. TOTAL: £1,200 TO £2,260.