Throughout the years, there have been numerous cases of cavity wall distress in the UK, which can be attributed to the corrosion of metal ties. This distress often presents itself as bulging, cracking, leaking, or in severe cases, the collapse of walls under high wind loads. It has become apparent that this issue could potentially affect almost all cavity-walled structures constructed before 1981, which amounts to around 10 million properties.
Cavity walls with metal ties have been used since the early 19th century and are particularly common in homes built in the southern and western regions of the UK during the early 20th century.
It is now evident that the longevity of bitumen and zinc coatings on these metal ties falls short of the 60-year life expectancy of the walls. This was recognised in the 1981 amendment to BS 1243, which increased the minimum zinc coating thickness on wire ties threefold.
Wall tie corrosion refers to the deterioration and eventual failure of metal ties that connect the inner and outer walls of a cavity wall, often due to the rusting of the metal. This corrosion can lead to horizontal cracking and separation of the brickwork, compromising the wall’s stability.
Here’s a more detailed explanation:
What are Wall Ties?
- Wall ties are metal fasteners that connect the inner and outer layers of a cavity wall, preventing them from separating.
- They are typically embedded in the mortar joints between the bricks.
Why do Wall Ties Corrode?
- Older wall ties, particularly those made of mild steel, were often coated with paint or a galvanized layer, which could degrade over time.
- Water ingress into the cavity wall, especially through cracks or damaged mortar, can accelerate corrosion.
- The expansion of rust can cause the wall tie to expand, potentially pushing the bricks apart.
Consequences of Wall Tie Corrosion:
- Horizontal Cracking:
As the wall ties rust and expand, they can lift the bricks above and create horizontal cracks in the mortar joints, often at regular intervals.
- Loss of Stability:
The expansion and eventual failure of wall ties can weaken the wall, making it susceptible to movement and even collapse, particularly in areas exposed to high winds.
- Bulging:
The outer leaf of the wall may bulge outwards as the ties corrode and fail, leading to a visible distortion.
- Water Ingress:
Damaged ties can allow water to penetrate the cavity, leading to dampness and potential damage to the interior of the building.
Signs of Wall Tie Corrosion:
- Horizontal Cracks:
Look for cracks that run horizontally across the wall, often aligned with the mortar joints.
- Bulging:
Notice if the outer leaf of the wall appears to be bulging outwards.
- Rust Stains:
Vertical lines of reddish-brown discoloration on the brickwork can indicate rusting ties.
- Damp Patches:
In some cases, dampness inside the building, especially if it coincides with cracking or bulging on the exterior, can be a sign of wall tie failure.
- Rattling Noises:
In high winds, a rattling sound may be heard if the outer leaf of the wall is no longer securely tied to the inner leaf.
What to do if you suspect wall tie corrosion: