Cavity wall ties hold the two leaves of a cavity wall together. They’re the small metal connectors hidden inside the wall cavity, transferring wind and lateral load from the outer leaf to the inner leaf so the building behaves as one structural unit. When they fail, the outer leaf is essentially a freestanding skin, and skins fall down.
On most Sydney homes built before about 1980, the original ties are twisted galvanised iron. Service life was always going to be limited by the galvanising, and many of those ties are now well past it. The good news is that failure leaves a clear visual signature, and the repair is non-destructive.
What wall ties do
A cavity wall is two separate leaves of brickwork, an outer weather-exposed leaf and an inner structural leaf, with an air gap between them. The cavity provides insulation and a drainage path for any water that gets through the outer leaf.
Without ties, those two leaves are two unrelated walls. Wind pressure acting on the outer leaf has nothing to push against; the leaf can flex, bow, and eventually fail. Ties at roughly 450 by 900mm centres convert the two leaves into a single structural cavity wall that behaves predictably under load.
Why ties fail
The same expansion mechanism that destroys lintels destroys wall ties. As the galvanising on the original tie weathers away, the steel underneath rusts. Rust takes up to seven times the volume of the original metal. Inside the joint, that expansion forces the courses apart, lifting the surrounding bricks and creating regular horizontal cracking.
Eventually the tie corrodes through completely. At that point the outer leaf has lost the connection to the inner leaf at that location. Over a whole elevation, with most of the ties failing within a similar window, the cumulative effect is a wall that’s no longer structurally tied at all.
Four warning signs
1. Horizontal cracks at regular intervals
The most distinctive signature. A series of horizontal cracks running through mortar joints, evenly spaced 450–900mm apart, often the full length of a wall. The spacing is the giveaway, it matches the original tie pattern.
2. Bulging or bowing of the outer leaf
Sight along an external wall from a corner. If the wall isn’t plumb, if you can see a curve or a lean, the outer leaf is moving away from the structure behind it. A few millimetres is significant; anything more than 10mm is structural and urgent.
3. Rust staining at joints
Brown weep marks emerging from horizontal joints, often coinciding with the cracking pattern above. The corroded tie is shedding rust and water is carrying it through the joint to the surface.
4. New cracking after wind events
If you spot fresh cracking, or existing cracks widening, after a major storm, you’re seeing the wall responding to wind load it can no longer resist. This is the latest warning sign before serious structural failure.
What happens if you wait
Wall tie failure is progressive. Once a few ties go, load is redistributed to the survivors, which then fail faster. Eventually the cumulative loss of connection means the outer leaf is unsupported, and at that point a strong wind can push significant areas of brickwork out of plumb or, in extreme cases, off the wall entirely.
The repair is much simpler when caught early. Remedial wall ties are stainless steel and designed to be installed through the existing wall, small holes drilled, ties installed, holes pointed up to match. From outside, the work is invisible. The longer you wait, the more cracking has to be repaired in addition to the tie installation.
If you’ve seen any of the four signs above, especially horizontal cracking at regular intervals, it’s worth getting a diagnosis. We can usually confirm wall tie failure from photos before any site visit, the pattern is that distinctive.