The ties used to bind the two leaves of a Cavity Wall were often made of mild steel or wrought iron. Such ties were protected from corrosion by being galvanised or in earlier times by a coat of bituminous paint.
Over the course of time the galvanising or other protection can wear away. If this happens the ties will rust due to the presence of air and water in the cavity. When steel corrodes it expands in thickness, so the ends of the wall ties embedded in the outer leaf will lift the bricks above. This causes horizontal crack to appear in the brickwork. In extreme cases, the outer leaf can become separated from the inner leaf and fall off, although this very rarely leads to any further collapse of the wall.
If your surveyor suspects wall tie failure, he will recommend further investigation. This is done by locating the ties using a metal detector and then either removing a brick or the mortar or drilling a small hole and inserting an endoscope (a TV camera on a flexible probe) into the cavity to examine their condition.
If a substantial proportion of the wall ties have become corroded it will be necessary for them to be replaced; this can be an expensive operation.
Since 1981 building regulations require a higher standard of protection, and builders now use plastic or stainless steel ties, so failure of the wall ties is much less likely to occur in recently built homes.