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Cavity wall tie corrosion is a common problem which only affects cavity constructed walls usually built prior to the 1980's.
Cavity walls consist of two 'skins' of masonry separated by a hollow space (cavity). The skins are commonly masonry, such as brick or concrete block. The inner skin is structural and load bearing taking the weight of the floors and roof structure. The cavity and outer skin provide an independent envelope around the building to protect against damp, a common problem associated with solid walls. Over recent years it has become common to fill the cavity void with insulation to increase thermal efficiency.
The purpose of a wall tie is to retain the outer skin wall to the structural inner skin and prevent movement. Primarily ties were constructed from metal (wrought iron / cast iron & mild steel) which were protected from corrosion by bitumen or a galvanised coating. Time has shown, however that the level of protection applied to ties installed prior to the 1980's was often insufficient and as a result, in some circumstances, ties corrode.
When metal ties corrode, they expand and where multiple ties corrode together in a single mortar joint, this expansive force can be enough to cause the wall to crack. As ties are placed during construction in purposeful locations these cracks may appear in the walls, where the ties are positioned, and increase with the height of the building as the weight of the wall reduces. Internal cracking as a result of wall tie corrosion is rare however, can happen in severe cases.
Corroding ties become brittle, often up to the point where they are no longer capable of providing the restraint needed to hold the outer wall to the structural inner wall. The cracks created by the corroding wall ties also reduce the integrity of the brickwork and leave weak and brittle ties in free and oversized joints. Cracks also allow rainwater to penetrate into the wall accelerating the corrosion process. Overall, the effects of wall tie corrosion reduce the integrity of the wall which can lead to the wall becoming unstable and in the worst-case scenario even collapse.
The regular presence of horizontal cracks, increased mortar joints and bowing walls may suggest a wall tie problem.
Severely corroded bitumen fishtail tie exposed in outer leaf wall.
Severe ferric oxide corrosion occurring to galvanised fishtail tie.
Severe horizontal cracking due to wall tie corrosion.If you suspect a problem and would like to request a survey please contact us.
