A cavity wall tie survey is an inspection process that evaluates the condition of the metal ties connecting the inner and outer leaves of a cavity wall. The survey aims to identify any signs of corrosion, failure, or inadequacy in the wall ties, which are crucial for maintaining the structural integrity of the wall. The procedure typically involves:

• An external appraisal of the building to identify construction details, the presence of a cavity, insulation, and any visible cracks or defects.

• Using a metal detector to locate wall ties.

• Drilling small inspection holes near the ties and employing a borescope to visually assess their condition.

• Photographing the ties for further analysis and record-keeping.

This process is straightforward and usually takes less than an hour.

Cavity wall ties play a crucial role in construction by connecting the inner and outer walls of a cavity wall. Positioned horizontally within the wall cavity and secured with mortar, these ties fortify the structure by preventing moisture from traversing from the exterior to the interior wall. Their strategic design is integral to ensuring the safety and stability of the wall. In the absence of proper wall ties, or if they become corroded over time, substantial sections of brickwork—such as gables—can succumb to the forces of wind suction, leading to potential collapse.

Cavity wall ties, initially made from iron or steel, are prone to corrosion in damp conditions. To prevent this, they were galvanized with zinc. However, this protective layer can degrade over time, leading to failure. Pre-1981 wall ties often fail prematurely due to various factors such as corrosive mortars, exposure to rain, and substandard galvanizing. Fishtail wall ties expand upon corrosion, causing cracks in the outer leaf, while Wire Butterfly ties may snap without expansion, resulting in wall bulges. Replacement of older wall ties with modern stainless steel ones is recommended to prevent future failures.

Signs of cavity wall tie failure include:

• Horizontal Cracks: Found in the mortar, often at a high level, caused by the expansion of corroded wall ties on the external leaf.

• Bulging Brickwork: Indicates snapped wall ties, which can make your property unstable.

• Internal Cracking: Caused by the expansion of corroded wall ties.

• Cracks in the Render: Vertical cracks in the render.

• Roof Edges Lifting: The inner leaf carries the roof load, but as corroded wall ties expand and crack the mortar, the outer leaf takes the load, resulting in forcing the roof upwards or the gable ridge to rise.

If you observe any of these signs, it’s essential to consult a specialist for a comprehensive survey.

A cavity wall tie survey involves several steps:

• External Assessment: The surveyor examines the building’s exterior to identify construction details, the presence of a cavity, insulation, and any visible cracks or defects.

• Locating Wall Ties: A metal detector is used to find the positions of the wall ties.

• Inspection: Small inspection holes are drilled near the located ties, and a borescope (a type of endoscope) is used to visually assess their condition.

• Documentation: Photographs of the wall ties are taken for further analysis and record-keeping.

This procedure is straightforward and usually takes less than an hour.

If a survey reveals that wall ties are corroded or failing, replacement is typically recommended to maintain the structural integrity of the building. The process involves:

1. Locating Existing Ties: Identifying the positions of the old wall ties.

2. Installing New Ties: New stainless steel mechanical or resin-bonded ties are installed in an offset pattern to the existing ones.

3. Isolating Old Ties: The old ties are isolated by removing the surrounding mortar or by using grease-filled sleeves over the exposed ends to prevent further corrosion and expansion.

The work is carried out from scaffolding or temporary scaffold towers, depending on terrain and access. Stainless steel wall ties are used as they are corrosion-resistant. Replacement cavity wall ties also benefit from long-term guarantees.

The frequency of inspections depends on the age and condition of the property. For properties built before 1981, especially those in exposed or coastal areas, regular inspections are advisable due to the higher risk of wall tie corrosion. If you notice any signs of wall tie failure, such as horizontal cracking or bulging brickwork, it’s essential to arrange a survey promptly.