Cavity Wall Tie Failure

Quick Answer: Cavity wall ties are the metal connectors that hold the two leaves of a cavity wall together and let them act as one structure. Older ties — particularly black "fishtail" mild steel ties common in walls built roughly from the 1920s to the 1980s — corrode over time, expanding as they rust and forcing the mortar beds apart. The classic symptom is regular horizontal cracking in the mortar joints at roughly 450mm vertical intervals (the tie spacing). Failure is both a structural problem (the leaves lose their connection and the outer leaf can bulge or buckle) and a damp problem (corroded or mortar-fouled ties bridge the cavity and carry moisture inwards). Diagnosis is confirmed by borescope inspection; the remedy is to install replacement remedial ties and isolate or remove the failed ones.

Summary

A cavity wall only works because the inner and outer leaves are tied together. The ties make two separate skins of masonry behave as a single structural element, sharing wind load and lateral restraint, while the cavity between them keeps moisture from crossing. When the ties fail, both of those functions are lost — and because the ties are buried in the mortar, the failure is invisible until the symptoms surface.

The problem is largely a problem of an era. For decades, cavity walls were built with mild steel ties, often the "fishtail" or "butterfly" pattern, with little or no corrosion protection or with coatings that did not last the life of the building. Steel rusts, and rusting steel expands — by a substantial multiple of its original volume. A tie corroding inside a horizontal mortar bed acts like a slow jack, lifting the masonry above it and opening up the bed joint. Because ties are installed in regular rows, the cracking appears in a tell-tale pattern: horizontal cracks in the mortar joints, repeating up the wall at the vertical tie spacing — roughly every 450mm.

Two things make wall tie failure matter to a tradesperson. First, it is structural: as the ties go, the outer leaf loses its restraint and can bow, bulge or, in severe cases, become unstable — a real safety issue, especially on tall or exposed elevations. Second, it is a damp issue: a corroded tie, or a tie fouled with mortar droppings, forms a bridge across the cavity, giving moisture a direct path from the wet outer leaf to the inner leaf — penetrating damp that no amount of internal treatment will cure until the bridge is removed. Anyone diagnosing damp on a cavity wall, or pricing structural repairs, needs to be able to recognise tie failure.

Key Facts

• Function of wall ties — connect the inner and outer leaves of a cavity wall so they act structurally as one, providing lateral restraint and load sharing; they must not bridge the cavity for moisture.
• At-risk era — walls built roughly from the 1920s to the 1980s commonly used mild steel ties; corrosion-related failure is widespread in this stock.
• The corrosion mechanism — rusting steel expands significantly, forcing apart the mortar bed it sits in ("oxide jacking").
• Signature crack pattern — regular horizontal cracking in the mortar bed joints, repeating vertically at roughly 450mm centres (the standard tie spacing), often most visible on exposed elevations.
• Other symptoms — bulging or bowing of the outer leaf, lifting or stepped distortion around openings, cracking concentrated at corners and verges, and in severe cases visible movement.
• Tie spacing (modern standard) — generally a maximum of 900mm horizontally and 450mm vertically, with additional ties around openings and at movement joints/verges — older walls may have fewer ties than this.
• Damp consequence — corroded ties and ties fouled with mortar droppings bridge the cavity and conduct moisture to the inner leaf, causing penetrating damp.
• Diagnosis — visual survey of the crack pattern plus borescope (endoscope) inspection through small drilled holes to see the actual tie condition; a metal detector helps locate ties.
• Remedy — install remedial replacement ties (mechanical or resin-fixed) at the correct density, and isolate or remove the failed ties so they cannot continue to jack the masonry.
• Standards — modern ties and remedial ties are covered by BS EN 845-1; remedial tie installation is covered by BS 8208 and industry (e.g. CSIC/PCA) guidance.

Quick Reference Table

Detailed Guidance

How and why ties fail

A cavity wall tie spends its life embedded in a damp horizontal mortar joint, in the cavity, exposed to whatever moisture crosses or condenses there. Mild steel with inadequate protection corrodes in that environment. The critical point is that corroding steel expands — rust occupies several times the volume of the original metal — so a tie rusting inside a 10mm mortar bed slowly forces the brick or block above it upward. Multiply that across a whole row of ties and the bed joint opens up; multiply it across the rows and you get the repeating horizontal crack pattern. As the ties also lose section, they weaken, and the structural connection between the leaves degrades — so you get expansion damage and loss of restraint at the same time.

Exposed, weather-facing elevations fail first and worst, because they are the wettest. Coastal and high-exposure locations accelerate everything.

Recognising the symptoms

The diagnosis usually starts from the crack pattern, because it is so distinctive:

• Regular horizontal cracking in the mortar bed joints, repeating up the wall at roughly 450mm vertical centres. Nothing else produces that signature — it directly mirrors the rows of ties.
• The cracking is often most pronounced on the most exposed elevations and can be concentrated at corners, verges and around openings.
• In more advanced cases, the outer leaf bows or bulges outward — visible by sighting along the wall — because it has lost its connection to the inner leaf and its lateral restraint.
• There may be lifting or distortion of masonry over windows and doors.
• And there may be penetrating damp on the inner face with no other explanation, because the failing ties are bridging the cavity.

Distinguish this from other movement: subsidence cracks are usually diagonal and tapering; thermal/shrinkage cracks are irregular. The regular, horizontal, evenly-spaced pattern is the wall tie fingerprint.

Confirming the diagnosis

Visual evidence points to it; borescope inspection confirms it. The process: locate the ties (a metal detector or rangefinder helps), drill small inspection holes into the cavity, and use a borescope to look at the actual ties — their type, their degree of corrosion, and whether the cavity is also fouled with mortar droppings sitting on the ties (which both bridges damp and was often a sign of poor original workmanship). Several ties at different heights and elevations should be checked, because failure is rarely uniform. Where bulging or instability is present, a structural engineer should assess it — tie failure can move from a maintenance issue to a safety issue, particularly on tall, exposed or gable walls.

The remedy: remedial ties and isolating the failures

The repair has two parts, and both matter:

1. Install remedial replacement ties. These are proprietary ties — mechanically expanding, or resin-bonded — drilled and fixed through the outer leaf into the inner leaf to re-establish the structural connection. They are installed to a calculated density and pattern (typically referencing the modern spacing and adding ties at openings, corners and verges, with the actual specification driven by the wall's exposure and condition).
2. Isolate or remove the failed ties. This is the step that is sometimes skipped and should not be — a corroding original tie left in place keeps expanding and keeps jacking the masonry. Failed ties are either physically removed or "isolated" so they can no longer transmit expansion forces or bridge moisture across the cavity.

Where the cavity is fouled with mortar droppings, that debris is dealt with too, because it is an independent damp bridge. After the structural and damp bridges are addressed, any cracked masonry and pointing is made good.

The damp angle — don't treat the symptom

If you are called to penetrating damp on a cavity wall built in the at-risk era and you cannot find an obvious external defect — pointing, render, gutters, sills all sound — suspect the ties. A corroded or mortar-fouled tie is a textbook cavity bridge, and internal replastering or "damp-proofing" the inner face will simply be covered up again within a year or two while the bridge keeps conducting water. The cure is to remove the bridge, which means dealing with the ties. This ties directly into the wider damp diagnosis process in rising damp vs penetrating damp and penetrating damp.

Frequently Asked Questions

How do I know if a wall has cavity wall tie failure?

The giveaway is the crack pattern: regular horizontal cracking in the mortar bed joints, repeating up the wall at roughly 450mm intervals — that even, horizontal spacing directly mirrors the rows of ties and nothing else produces it. Supporting signs are bulging of the outer leaf, cracking concentrated at corners and verges, distortion around openings, and unexplained penetrating damp. It is most common and most visible on exposed, weather-facing elevations of walls built roughly between the 1920s and 1980s. Confirmation comes from a borescope inspection of the actual ties through small drilled holes.

Is cavity wall tie failure dangerous?

It can be. Wall ties are structural — they tie the two leaves together and give the outer leaf its lateral restraint. As ties corrode and lose section, the outer leaf progressively loses its connection to the inner leaf and can bow, bulge and ultimately become unstable, particularly on tall, exposed or gable walls. At the same time the rust expansion is actively cracking the masonry. Early-stage failure is a maintenance issue; advanced failure with visible bulging is a genuine safety concern and should be assessed by a structural engineer before anyone works on or near the wall.

Can corroded wall ties cause damp?

Yes — and it is often missed. A cavity wall stays dry partly because the cavity itself stops moisture crossing from the wet outer leaf to the inner leaf. A corroded tie, or a tie fouled with mortar droppings, forms a physical bridge across the cavity that conducts water straight to the inner leaf. The result is penetrating damp on the inside face with no obvious external defect to explain it. Internal damp-proofing will not fix it; the bridge has to be removed, which means dealing with the ties and clearing any cavity debris.

What does it cost to fix and is it disruptive?

The work — drilling, borescope survey, installing remedial ties and isolating the failed ones — is done almost entirely from outside the building, through the outer leaf, so it is far less disruptive to the occupants than most structural repairs; there is usually no need to strip internal finishes. The cost depends on the wall area, the number of ties required (driven by exposure and condition), access (scaffold for upper storeys and gables), and how much making-good and re-pointing is needed afterwards. Properly specified, it is a long-term fix — remedial ties are designed to outlast the original mild steel by a wide margin.

Regulations & Standards

• Building Regulations 2010, Part A — structural requirements, including lateral restraint and the integrity of cavity walls.

• BS EN 845-1 — specification for ancillary components for masonry, including wall ties (new and remedial).

• BS 8208 — guide to assessment of suitability of external cavity walls (relevant to assessing ties and cavity condition).

• BS 5628 / BS EN 1996 (Eurocode 6) — design of masonry structures, including tie provision and spacing.

• PCA (Property Care Association) / CSIC remedial wall tie guidance — industry guidance on diagnosis and remedial tie installation.

• BRE guidance on wall tie corrosion — Building Research Establishment guidance on identification and repair.

• Property Care Association — Structural waterproofing and wall ties — industry guidance on remedial wall tie work

• BRE — Wall tie corrosion and replacement — Building Research Establishment guidance

• GOV.UK — Approved Document A (Structure) — structural requirements for masonry walls

• Historic England — Cavity walls and wall ties — assessment in older buildings

• rising damp vs penetrating damp — diagnosing damp on cavity walls and ruling causes in or out

• penetrating damp — penetrating damp from external defects, including cavity bridging

• structural calculations — when a structural engineer is needed to assess wall movement

• surveying damp — survey methods including borescope inspection of cavities