Cavity Wall Tie Corrosion

Most buildings constructed since 1920 are built using cavity wall construction.

The purpose of the cavity wall design was to eliminate damp from penetrating into the inside of the building. This was achieved by building 2 separate walls (skins) with a gap between the two of typically 50mm and inserting wall ties at pre-determined spacing. This was to avoid the walls moving independently of the other.

The idea was that any water penetrating the outer skin would be unable to bridge the gap to penetrate the inner skin. Installing the wall ties on a slight incline towards the inner wall and incorporating a kink or twist in the wall tie design, any water which had penetrated the outer wall would drip harmlessly into the cavity.

This design initially worked fine and has significantly reduced the problems associated with penetrating damp since its introduction. However, many older properties were built using an insufficient amount of protective layer of galvanized coating, which in many cases has led to corrosion of the wall tie, where it is embedded in the outer skin.

Where corrosion has taken place, it is possible for the wall ties to expand up to 10 times their original thickness. Thus resulting in horizontal cracking, outward bulging of walls, lifting of roof edges and in severe cases the distinct possibility of the outer wall collapsing.

To counteract the problem, a system of wall tie replacement was developed. This involved inserting new ties by drilling through the outer wall and partly into the inner wall and tightening them to the required torque, before isolating the old ties to prevent any further structural weakening.

To determine the extent of the problem, a specialist survey is required prior to any works being carried out. This involves using metal detectors to locate existing ties and drilling a small hole alongside to view the condition of the ties, using specialist viewing equipment.