Wall Tie Failure in London Victorian Terraces: Survey, Replacement, and Retrofit

WALL TIE FAILURE IN LONDON BRICK-CAVITY CONSTRUCTION IS CAUSED PRIMARILY BY CORROSION OF THE ORIGINAL STEEL TIE — A PROCESS DRIVEN BY MOISTURE IN THE CAVITY, WHICH IN LONDON CAN BE SIGNIFICANT DUE TO THE WET TEMPERATE CLIMATE AND THE PRESENCE OF MORTAR DROPPINGS ON THE ORIGINAL CAVITY TIES (A VERY COMMON CONSTRUCTION DEFECT IN CAVITY WALLS OF ALL PERIODS — MORTAR DROPPINGS FALLING INTO THE CAVITY DURING BRICKLAYING AND BRIDGING THE DPC OR ACCUMULATING ON TIES).

TIE TYPES AND TYPICAL FAILURE TIMESCALES: TWISTED WIRE MILD STEEL TIE (ORIGINAL INTER-WAR CONSTRUCTION — 1920S-1940S): 3MM MILD STEEL WIRE, TWISTED IN A BUTTERFLY OR FIGURE-8 PATTERN. LIFESPAN IN MODERATE EXPOSURE: 40-60 YEARS. CURRENT STATUS: MOST TWISTED WIRE TIES FROM THE 1920S-1940S ARE NOW AT END OF SERVICEABLE LIFE (80-100 YEARS OLD) — MANY HAVE FAILED OR ARE NEAR FAILURE. BUTTERFLY AND FISH-TAIL TIES (POST-WAR — 1950S-1970S): STAMPED MILD STEEL, BUTTERFLY OR FISHTAIL SHAPE. SIMILAR CORROSION VULNERABILITY TO TWISTED WIRE TIES. CURRENT STATUS: 1950S TIES (70+ YEARS OLD) — MANY NEAR END OF LIFE; 1970S TIES (50+ YEARS) — EARLY STAGE OF CORROSION IN WET EXPOSURES.

CORROSION EXPANSION MECHANISM: WHEN MILD STEEL CORRODES (RUSTS), THE RUST PRODUCTS (IRON OXIDES) HAVE A SIGNIFICANTLY LARGER VOLUME THAN THE ORIGINAL STEEL — UP TO 6-8X THE ORIGINAL VOLUME FOR ADVANCED CORROSION. IN A WALL TIE, THIS EXPANSION OCCURS AT THE POINT WHERE THE TIE IS EMBEDDED IN THE MORTAR JOINT OF THE OUTER LEAF BRICKWORK — THE EXPANDING RUST EXERTS A SPLITTING FORCE ON THE HORIZONTAL MORTAR JOINT ABOVE AND BELOW THE TIE. THE RESULT: HORIZONTAL CRACKING OF THE OUTER LEAF BRICKWORK AT THE HEIGHT OF EVERY TIE COURSE (TYPICALLY AT 450MM VERTICAL INTERVALS — THE STANDARD TIE SPACING IN INTER-WAR AND POST-WAR CAVITY WALL CONSTRUCTION). THIS HORIZONTAL CRACKING IS THE MOST DISTINCTIVE AND DIAGNOSTIC SYMPTOM OF WALL TIE FAILURE IN LONDON PROPERTIES.

IDENTIFYING WALL TIE FAILURE ON A LONDON TERRACE PROPERTY REQUIRES RECOGNISING THE CHARACTERISTIC SYMPTOMS AND DISTINGUISHING TIE FAILURE FROM OTHER CAUSES OF HORIZONTAL CRACKING.

SYMPTOMS OF WALL TIE FAILURE: HORIZONTAL CRACKING IN THE OUTER LEAF BRICKWORK: THE MOST DIAGNOSTIC SYMPTOM — FINE HORIZONTAL CRACKS RUNNING ALONG THE MORTAR JOINTS AT REGULAR INTERVALS (TYPICALLY EVERY 450MM HEIGHT — THE STANDARD TIE VERTICAL SPACING). BOWING OR BULGING OF THE OUTER BRICK LEAF: IN ADVANCED CASES, THE OUTER LEAF HAS LOST SUFFICIENT TIE CONNECTION THAT IT BEGINS TO BOW OUTWARD UNDER WIND PRESSURE — VISIBLE AS A CONVEX OUTWARD PROFILE WHEN VIEWED OBLIQUELY. STAINING AND WATER PENETRATION: FAILED TIE JOINTS (CRACKED HORIZONTALLY) ADMIT WATER DIRECTLY INTO THE CAVITY — INCREASING THE RISK OF DAMPNESS PENETRATING TO THE INNER LEAF.

DIFFERENTIATING TIE FAILURE FROM OTHER CAUSES: DIAGONAL STAIRCASE CRACKING (USUALLY DIFFERENTIAL SETTLEMENT — NOT TIE FAILURE). VERTICAL CRACKING AT WINDOW REVEALS (LINTEL OR FRAME DEFLECTION — NOT TIE FAILURE). HORIZONTAL CRACKING AT DPC LEVEL (MOISTURE EXPANSION OF BRICKWORK BELOW DPC — NOT TIE FAILURE — UNLESS ALSO APPEARING AT TIE INTERVALS ABOVE DPC).

FORMAL SURVEY METHODS FOR WALL TIE FAILURE IN LONDON PROPERTIES: METAL DETECTOR SURVEY (FIRST STEP — CHEAPEST): A CALIBRATED METAL DETECTOR (PROTOVALE REBARSCANNER RB4 OR ELCOMETER 324) SWEPT ACROSS THE OUTER FACE OF THE BRICKWORK CAN DETECT THE POSITION OF METAL TIES BELOW THE SURFACE — AND MAY REVEAL IRREGULAR SPACING OR MISSING TIES WHERE CORROSION HAS CAUSED COMPLETE LOSS. ENDOSCOPIC INSPECTION (BORESCOPE CAMERA — THROUGH A 10MM HOLE DRILLED THROUGH A MORTAR JOINT INTO THE CAVITY): THE MOST RELIABLE NON-DESTRUCTIVE METHOD — A FLEXIBLE BORESCOPE CAMERA (TESLONG TS99 OR MILWAUKEE M12 INSPECTION CAMERA) INSERTED INTO THE CAVITY AT TIE LEVEL CAN DIRECTLY VIEW THE TIE CONDITION (CORROSION PRODUCTS, SECTION LOSS, MORTAR DROPPING BRIDGES). REPORT AND SPECIFICATION: THE FORMAL OUTPUT OF A WALL TIE SURVEY IS A REPORT IDENTIFYING: NUMBER OF FAILING TIES, TIE TYPE, EXTENT OF OUTER LEAF DISPLACEMENT, RECOMMENDED REPLACEMENT TIE TYPE AND DENSITY, AND COST ESTIMATE.

WALL TIE REPLACEMENT ON A LONDON TERRACE INVOLVES INSTALLING NEW STAINLESS STEEL REPLACEMENT TIES THROUGH THE OUTER LEAF INTO THE INNER LEAF — A PROCESS CARRIED OUT BY A SPECIALIST WALL TIE REPLACEMENT CONTRACTOR USING A HAMMER DRILL AND APPROPRIATE ANCHOR FIXINGS.

HELIFIX WT2 STAINLESS STEEL WALL TIE (THE MOST WIDELY SPECIFIED REPLACEMENT WALL TIE IN THE UK FOR BRICK CAVITY WALLS — INCLUDING LONDON VICTORIAN AND INTER-WAR TERRACE PROPERTIES): THE HELIFIX WT2 IS A GRADE 304 STAINLESS STEEL HELICAL-ANCHOR TIE (6MM DIAMETER SQUARE-SECTION STAINLESS STEEL, HELICALLY WOUND) THAT IS DRILLED AND DRIVEN THROUGH THE OUTER LEAF MORTAR JOINT INTO THE INNER LEAF USING A ROTARY DRILL. INSTALLATION: DRILL 10MM HOLE THROUGH THE OUTER LEAF MORTAR JOINT TO THE INNER LEAF (OR THROUGH BOTH LEAVES SIMULTANEOUSLY — 200MM TOTAL DEPTH IN A 100MM OUTER LEAF + 75MM CAVITY + 100MM INNER LEAF CAVITY WALL). INJECT POLYURETHANE RESIN (HELIFIX COBA — SINGLE-PART INJECTED INTO THE HOLE PRIOR TO TIE INSERTION — FOR SECURE EMBEDMENT IN THE INNER LEAF). DRIVE HELIFIX WT2 TIE INTO THE HOLE (ROTARY DRILL — THE HELICAL PROFILE CUTS INTO THE INNER LEAF AND RESIN FOR A SECURE GRIP). CAPACITY: HELIFIX WT2 (6MM SQUARE SECTION) — TESTED PULL-OUT RESISTANCE: >4KN (SINGLE TIE IN A 100MM BRICK INNER LEAF — HELIFIX TECHNICAL DATA). MATERIAL: GRADE 304 STAINLESS STEEL — LIFESPAN WELL IN EXCESS OF 100 YEARS.

RAWLPLUG TYPHOON WALL TIE (ALTERNATIVE TO HELIFIX WT2 — ALSO WIDELY USED IN LONDON FOR WALL TIE REPLACEMENT): SIMILAR CONCEPT — 6MM STAINLESS STEEL TIE WITH RESIN INJECTION — INSTALLED WITH A RAWLPLUG DRILL-DRIVER ACCESSORY. TESTED PULL-OUT: >3KN (RAWLPLUG TECHNICAL DATA). REMCON SYSTEM (RESIN-INJECTED STAINLESS STEEL TIE — LARGER 8MM SECTION FOR HIGH-LOAD APPLICATIONS OR WIDE CAVITIES): SUITABLE WHERE THE CAVITY IS WIDER THAN STANDARD (100MM CAVITY — WHERE HELIFIX WT2 REACH IS BORDERLINE FOR ENGAGEMENT IN THE INNER LEAF).

THE CORRECT SPECIFICATION FOR WALL TIE REPLACEMENT IN A LONDON TERRACE PROPERTY MUST ADDRESS: THE NUMBER OF NEW TIES PER UNIT AREA (TIE DENSITY), THE LOCATION (AT EXISTING TIE POSITIONS OR REDISTRIBUTED), AND THE TREATMENT OF THE FAILED ORIGINAL TIES.

WALL TIE REPLACEMENT DENSITY (BS 5628-1 AND PCA GUIDE TO WALL TIE REPLACEMENT): BS 5628-1 (CODE OF PRACTICE FOR USE OF MASONRY) SPECIFIES THE MINIMUM TIE DENSITY FOR CAVITY WALLS IN TERMS OF TIES PER M²: STANDARD ZONE (INTERNAL AREA AWAY FROM EDGES AND OPENINGS): 2.5 TIES/M² MINIMUM. EDGE AND OPENING ZONES (WITHIN 150MM OF ALL OPENINGS, EDGES, AND MOVEMENT JOINTS): 5 TIES/M² MINIMUM. FOR WALL TIE REPLACEMENT IN A LONDON TERRACE PROPERTY (TYPICALLY A 9M HIGH X 6M WIDE FRONT ELEVATION = 54M² — MINUS WINDOW OPENINGS): STANDARD ZONE TIES: 2.5 X (54M² - OPENINGS) = APPROXIMATELY 100-130 REPLACEMENT TIES. ADDITIONAL EDGE/OPENING ZONE TIES: TYPICALLY 20-40 ADDITIONAL TIES AROUND WINDOW OPENINGS.

TREATMENT OF FAILED ORIGINAL TIES: CUTTING BACK: ORIGINAL FAILED TIES CANNOT USUALLY BE REMOVED (THEY ARE EMBEDDED IN THE MORTAR JOINT OF BOTH LEAVES — REMOVAL WOULD INVOLVE CUTTING OUT BRICKS). THE FAILED TIES ARE INSTEAD CUT BACK AT THE OUTER FACE OF THE OUTER LEAF (USING AN ANGLE GRINDER AND TUNGSTEN CARB DISC) SO THAT THE TIP IS BELOW THE MORTAR JOINT SURFACE — ELIMINATING THE EXPANDING RUST PRESSURE ON THE OUTER LEAF JOINT AND PREVENTING FURTHER CRACKING FROM THOSE TIES. INJECTION GROUTING OF CRACKED JOINTS: WHERE HORIZONTAL CRACKING HAS OCCURRED AT TIE LEVEL, THE CRACKS SHOULD BE RE-POINTED WITH NHL 3.5 LIME MORTAR (AFTER CUTTING BACK THE OLD TIE) AND THE JOINT SURFACE REBUILT. MAJOR BULGING (>25MM): WHERE THE OUTER LEAF HAS BULGED SIGNIFICANTLY, WALL TIE REPLACEMENT ALONE MAY NOT BE SUFFICIENT — THE OUTER LEAF MAY NEED TO BE TAKEN DOWN AND REBUILT (USING MATCHING BRICK — RECLAIMED LONDON STOCK OR IBSTOCK EQUIVALENT).

WALL TIE REPLACEMENT IN A LONDON TERRACE PROPERTY HAS SPECIFIC BUILDING REGULATIONS AND PARTY WALL ACT IMPLICATIONS THAT SHOULD BE UNDERSTOOD BEFORE WORK STARTS.

BUILDING REGULATIONS: WALL TIE REPLACEMENT IS A STRUCTURAL REPAIR THAT IS NOTIFIABLE UNDER BUILDING REGULATIONS (APPROVED DOCUMENT A — STRUCTURE). A FULL PLANS BUILDING REGULATIONS APPLICATION (OR A BUILDING NOTICE) SHOULD BE SUBMITTED TO THE LOCAL AUTHORITY BUILDING CONTROL (LABC) OR AN APPROVED INSPECTOR (SUCH AS NHBC OR BRE). THE BUILDING CONTROL INSPECTOR WILL REQUIRE: A SCHEDULE OF WORKS (NUMBER AND TYPE OF REPLACEMENT TIES, TREATMENT OF FAILED TIES, CRACKING REPAIR). EVIDENCE OF COMPLIANCE WITH THE TIE MANUFACTURER'S PULL-OUT TESTS (HELIFIX WT2 OR RAWLPLUG TYPHOON — ON-SITE PULL-OUT TEST REQUIRED BY SOME INSPECTORS TO CONFIRM ACHIEVED INSTALLATION CAPACITY). A STRUCTURAL ENGINEER'S APPROVAL OF THE SPECIFICATION WHERE SIGNIFICANT BULGING OR STRUCTURAL MOVEMENT HAS OCCURRED.

PARTY WALL ACT: A WALL TIE REPLACEMENT PROGRAMME AFFECTING A WALL SHARED WITH AN ADJACENT PROPERTY (A PARTY WALL OR PARTY FENCE WALL) REQUIRES A PARTY WALL NOTICE (SECTION 2 OF THE PARTY WALL ACT 1996 — WORKS TO A PARTY STRUCTURE). WALL TIE REPLACEMENT IN THE FRONT OR REAR ELEVATION WALLS OF A LONDON TERRACE THAT ARE ENTIRELY WITHIN THE OWNER'S OWN PROPERTY (I.E. NOT SHARED WITH A NEIGHBOUR) DOES NOT REQUIRE A PARTY WALL NOTICE. HOWEVER: IN A LONDON VICTORIAN SEMI-DETACHED TERRACE, THE SIDE GABLE WALL MAY BE A PARTY WALL — CHECK OWNERSHIP AND LEGAL STATUS BEFORE ASSUMING NO PARTY WALL OBLIGATION.

HEALTH AND SAFETY (CDM 2015): WALL TIE REPLACEMENT WORKS THAT INVOLVE SCAFFOLD (ALMOST ALWAYS REQUIRED FOR ELEVATIONS ABOVE 3M) ARE NOTIFIABLE UNDER CDM 2015 (CONSTRUCTION DESIGN AND MANAGEMENT REGULATIONS 2015) IF THE WORKS LAST MORE THAN 30 WORKING DAYS WITH MORE THAN 20 WORKERS SIMULTANEOUSLY, OR IF THE OVERALL DURATION IS MORE THAN 500 PERSON-DAYS.

WALL TIE REPLACEMENT COSTS IN LONDON DEPEND ON THE NUMBER OF ELEVATIONS AFFECTED, THE HEIGHT OF THE PROPERTY (SCAFFOLD REQUIREMENTS), THE DEGREE OF BULGING AND CRACKING THAT REQUIRES ADDITIONAL MASONRY REPAIR, AND THE TIE REPLACEMENT SYSTEM SPECIFIED.

SURVEY COST (PRELIMINARY — METAL DETECTOR + BORESCOPE INSPECTION + WRITTEN REPORT): £300 TO £800 FOR A STANDARD LONDON TERRACE (TWO-STOREY — FRONT ELEVATION SURVEY ONLY). £600 TO £1,500 FOR A FULL SURVEY OF ALL ELEVATIONS (FRONT, REAR, AND SIDE GABLE — INCLUDING BORESCOPE INSPECTION AT MULTIPLE POINTS).

WALL TIE REPLACEMENT (HELIFIX WT2 OR RAWLPLUG TYPHOON — STAINLESS STEEL — RESIN-INJECTED): COST PER TIE (INSTALLED — LABOUR AND MATERIALS — HELIFIX WT2 SUPPLY + INSTALLATION THROUGH BOTH LEAVES): £8 TO £15 PER TIE (LABOUR AND MATERIALS). TYPICAL FRONT ELEVATION OF A TWO-STOREY LONDON VICTORIAN TERRACE (6M WIDE X 6M HIGH — 36M² — MINUS WINDOWS — APPROXIMATELY 70-100 REPLACEMENT TIES): MATERIALS AND INSTALLATION: £560 TO £1,500. SCAFFOLD (TUBE AND FIT — FRONT ELEVATION — 2 STOREY — 1-2 WEEKS): £800 TO £2,000. TOTAL FRONT ELEVATION (TWO-STOREY — INCLUDING SCAFFOLD): £1,360 TO £3,500.

FULL PROPERTY WALL TIE REPLACEMENT (ALL ELEVATIONS — FRONT, REAR, GABLE — 3-STOREY LONDON TERRACE — INCLUDING SCAFFOLD, ALL ELEVATIONS): MATERIALS AND INSTALLATION (200-350 TIES): £1,600 TO £5,250. SCAFFOLD (ALL ELEVATIONS — 3-STOREY): £2,500 TO £6,000. TOTAL ALL ELEVATIONS (3-STOREY): £4,100 TO £11,250.

ADDITIONAL MASONRY REPAIR (WHERE HORIZONTAL CRACKING REQUIRES REPOINTING AND CUT-BACK OF FAILED TIES): £2 TO £5 PER CRACK (CUT-BACK OLD TIE WITH ANGLE GRINDER + TUCK-POINT WITH NHL 3.5). OUTER LEAF REBUILD (MAJOR BULGING — TAKE DOWN AND REBUILD OUTER LEAF IN MATCHING BRICK): £150 TO £350 PER M² (DEPENDENT ON BRICK SOURCING AND MATCHING REQUIREMENT).