My surveyor thinks my house suffers from wall tie corrosion. Does this means replacing all the ties ?
Not necessarily, this can only be determined after a specialist survey to examine the true condition of the ties on each elevation. This will involve use of an endoscope and the physical removal of sample ties.
The condition of the ties may vary from elevation to elevation particularly in exposed locations.
If the installation of remedial wall ties is recommended, the old ties are usually isolated to avoid any further corrosion but left in place because their removal would be costly, time consuming and difficult to achieve without necessitating wholesale repointing of the masonry.
We have had a structural engineer to check a house we are looking at buying. He said that there is an issue with the wall ties as they were set in black ash mortar.
Black ash mortar was made using a low cost by-product from power stations and used quite widely until it was realised that it was a highly corrosive medium in which to bed mild steel wall ties, even if they were galvanised. Wall tie corrosion starts to become visible in the form of horizontal cracks in the brickwork at regular course intervals. The ties expand as they rust and this is sufficient to raise the brickwork at each tie course
A survey involves gaining access to take out sample ties to assess their condition and if necessary, replacements will be installed in stainless steel with the originals isolated from further decay.
How does Peter Cox isolate old fish tail ties?
My mortar bed is about 10 mm thick
Because they will continue to corrode, old ties need to be either removed or isolated from further exposure to water and air. Physical removal usually˙involves taking out bricks which makes it a disruptive and more costly approach.
If the ties are heavily corroded, it is sometimes feasible to snap off the end of the tie in the outer leaf which is the section most susceptible to corrosion.
The alternative, and˙more commonly adopted, solution is isolation. This involves˙raking out˙the mortar joint thoroughly to expose both sides of the tie and then either sliding a greased plastic sleeve over the tie or using polyurethane foam to encapsulate it.
The joint is then repointed, taking care to match the existing mortar as far as possible (although this is often difficult). It is much easier if the wall is being re-rendered.
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