Cavity walls and ties have been in existence since the 19th century and have now become the standard method of construction for housing.
Although they may seem like a small aspect of the build, wall ties contribute significantly to the stability of the structure.
Unfortunately in some instances wall ties can become prone to corrosion and need to be replaced.
How do you know if corrosion has taken place and what do you need to do to fix the problem when converting or refurbishing an existing building?
First let’s explain how wall ties work to understand why problems may arise when they corrode.
How do wall ties work?
Wall ties are installed between the inner and out leaf of a masonry wall in a uniform spacing pattern to help withstand external forces like wind suction, especially between openings and in large un-reinforced areas such as gables.
Without effective ties at the correct spacing the brickwork would not be able to withstand exertion from these forces.
What happens when wall ties corrode?
Although stainless steel wall ties are now required when installing cavity walls, in the past galvanised or mild steel may have been used which may be susceptible to corrosion over time.
Corrosion of the mild steel causes expansion of the ties, which effectively forces the bricks apart.
The key things to look out for are:
- Cracking of bed joints in mortar (typically every sixth course)
- Bulging of the external leaf
Corrosion of butterfly wire ties on the other hand produces almost no detectable external symptoms.
What remedial action needs to be taken?
If existing wall ties have corroded replacement ties need to be installed and the old ties treated.
The first step would be to recruit a specialist contractor to identify the positions of existing ties and fit replacement ties in staggered positions from the originals. The old ties would then need to be treated to prevent any further damage from corrosion.
Please note that treatment of the old ties is essential for a complete repair, except in the case of butterfly wall ties.
Installing the new wall ties should cause very little disturbance to the existing structure because it is done by inserting them into holes drilled at appropriate positions which are then fixed using chemical or mechanical anchors.
Where wall tiles have corroded to an extent that threatens the stability of the wall or building, a Structural Engineer should be appointed to determine the necessary remedial works.
For further information please refer to Chapter 12 of our Technical Manual.
By Anna Symington