Cavity Wall Tie Failure: Causes, Symptoms, and Remediation

A cavity wall is built with two separate 'leaves' of masonry — typically 102.5mm of facing brickwork on the outside and 100mm of blockwork (or brickwork) on the inside — separated by a 50–75mm cavity. The cavity prevents rainwater from tracking across to the inner leaf and provides a space for insulation.

Wall ties are metal components embedded in the mortar beds of both leaves, spanning the cavity. They perform two critical structural functions: 1. They resist wind-induced lateral forces trying to push the outer leaf outward or inward 2. They transfer loads and help maintain the structural stability of the two-leaf assembly

**Why ties fail — corrosion**: Virtually all cavity wall tie failures in the UK are caused by corrosion (rust) of mild-steel ties. Mild steel was used for ties in the vast majority of UK housing built between approximately 1930 and 1981. The Building Regulations change in 1981 introduced requirements for stainless steel or galvanised ties — but galvanised mild steel ties were still used through the 1980s and early 1990s, and galvanising can fail over time.

• The corrosion process:
• •Moisture in the cavity attacks the mild steel tie over decades
• •Rust forms and expands — the expansion factor of rust compared to steel is approximately 4–6 times the original volume
• •The expanding corrosion product pushes outward in the mortar bed, causing horizontal cracking along the bed joints at the tie levels
• •As corrosion progresses, the tie cross-section reduces until it can no longer transfer load
• •In severe cases, the tie is effectively broken — the outer leaf loses its connection to the inner leaf

• **When corrosion is accelerated**:
• •Salt air (coastal locations) — not typically a problem in London
• •Acidic or aggressive mortar mixes
• •Cavity wall insulation that retains moisture against the tie
• •Cavity wall insulation blown in around ties that were already partially corroded

**Why poorly specified or missing ties cause failure**: In some properties — particularly pre-1930 cavity walls and early cavity wall construction — ties were either not installed or were installed at inadequate frequency. BS 5628 (replaced by Eurocode 6) sets out tie spacing requirements: typically 2.5 per m² of wall area, at no more than 450mm horizontal spacing and 450mm vertical spacing. Pre-1981 cavity walls may have ties at inadequate spacing or of inadequate embedment depth.

Cavity wall tie failure has characteristic visual signs that a competent surveyor or contractor can identify:

**Horizontal cracking in the external brickwork**: The most characteristic sign. Corrosion expansion pushes the mortar joint outward, creating a horizontal crack or step crack along the mortar bed line. This cracking pattern typically appears at repeating heights — corresponding to the tie positions, usually every 2–4 brick courses.

**Key distinction**: Horizontal cracking from tie corrosion appears at consistent intervals (corresponding to tie spacing), is relatively level, and often runs across a large section of the wall. It is distinguishable from diagonal cracking (which suggests structural settlement or thermal movement) and from isolated cracks at openings (which suggest lintel movement).

**Bowing of the outer leaf**: In more advanced cases, the outer leaf may be visibly bowing outward. The tie is no longer restraining the outer leaf, which begins to act as an unbraced wall between structural support points (foundations, roof, and other restraints). The level at which bowing begins can indicate the tie level that has failed first.

• **How to confirm the diagnosis**:
• •**Borescope inspection**: A small hole (typically 10mm diameter) is drilled through the mortar joint in the outer leaf at the suspected tie position. A flexible borescope camera is inserted to visually inspect the tie condition. This is the fastest, most cost-effective diagnostic approach.
• •**Metal detector sweep**: A specialist tie-detection device (e.g., Cavi-Scan) can be swept over the wall surface to map tie positions without drilling. It identifies the presence and depth of metal at tie positions — but cannot confirm tie condition (corroded ties may still be detectable as metal).
• •**Structural engineer's assessment**: For any property showing visible bowing of the outer leaf, a structural engineer should inspect and assess the structural risk before any other works proceed.

• **Properties most at risk**:
• •Housing built 1930–1975: mild steel butterfly ties with no protective coating — typically the most corroded
• •Housing built 1975–1985: butterfly or double-triangle ties with limited galvanising
• •Housing built post-1985: generally lower risk, but galvanised ties can also fail in aggressive environments

Once tie failure is confirmed by borescope inspection or structural engineer assessment, remediation involves:

**1. Install new ties**: New wall ties are driven through the outer leaf into the inner leaf using a specialist drill — typically a helical stainless steel tie driven through a 12–14mm hole drilled at the appropriate position. The tie threads into the outer and inner leaf and, once installed, provides a structural connection equivalent to (or better than) the original ties.

• A specialist wall tie replacement contractor will:
• •Map the existing tie pattern
• •Establish the remediation grid (typically 900mm centres horizontally × 450mm vertically, or to structural engineer specification)
• •Drill holes, drive and torque new ties
• •Repoint the drilled holes in lime or cement mortar

**2. What happens to the old corroded ties**: The old ties are not removed — this would require demolishing and rebuilding the outer leaf. The new ties are installed at positions offset from the failed ties. The failed ties remain in place but are no longer relied upon structurally.

• **3. If the outer leaf is bowing**:
• Where the outer leaf has bowed outward, the remediation is more complex:
• •Helical tie remediation alone may be insufficient if the outer leaf has moved significantly
• •A specialist contractor will assess whether the outer leaf can be pulled back to vertical (using temporary props and restraints)
• •In severe cases, rebuilding the affected section of outer leaf masonry may be necessary — the most expensive scenario

**4. If cavity wall insulation is present**: Cavity wall insulation blown into the cavity complicates both the diagnosis (making borescope inspection harder) and the tie installation (the new tie must penetrate the insulation). Specialist contractors have the appropriate drill bits and tie types for insulated cavities.

• **Costs (London 2025)**:
• •Borescope survey (up to 10 locations drilled and inspected): £300–£600
• •Helical tie installation — standard two-storey semi-detached: £1,200–£2,800
• •Helical tie installation — larger detached property or with additional scaffolding: £2,500–£5,500
• •Rebuilding of bowed outer leaf section (per m²): £200–£450/m² including scaffold
• •Party Wall Agreement will be needed if any part of the affected wall is on or near the boundary — this adds time and cost for surveyor fees

**Guarantee**: Specialist wall tie contractors typically provide a 20–30 year guarantee on helical tie installations. Warranties are often provided through the BBA (British Board of Agrément) for the tie system used.