Wall ties are used in cavity walls to hold the two walls together, without wall ties the two walls can bulge or bow. In older properties steel wall ties were used which rusted and corroded away due to the moisture and condensation in the cavity. Stainless steel wall ties or sometimes galvanised steel wall ties are used, which are far better as they do not rust.
Often in older properties the wall ties will need replacing as the old ones have rusted away, there are companies which specialise in this and should give a ten year guarantee or perhaps longer. Full structural surveys often pick up corroded wall ties, normally a hole is drilled into the cavity and a camera is used to survey the cavity. Work like this needs doing or it can effect your mortgage decision. Sometimes pieces of slate were used to tie the two structures together but new building regulations have deemed this unsuitable.
Wall ties 
