Cavity wall construction has almost entirely replaced solid wall construction in the United Kingdom. It evolved in the latter years of the nineteenth century and became common in dwellings in northern and western Britain in the early 1900s. Its widespread adoption as virtually standard in the construction industry happened throughout the building booms of the 1920s, ‘30s and ‘40s.
When identifying whether a wall is of solid or cavity construction, something to be aware of is that the presence of headers in the brickwork is not always indicative of solid brickwork. From the mid-1940s to the mid-1950s, “snapped headers” were often used to emulate English bond in cavity construction.
In the early years the skins of these cavity walls were held together by metal ties made from cast or wrought iron, mild steel or copper. More recently stainless steel has been used because of its strength and resilience to corrosion.
The corrosion of wall ties was first officially recognised in the 1960s in South Wales. At first it was thought to be due to a combination of poor tie protection and exposure to the elements. Time has shown that many of the early ties and mortars were just more susceptible to corrosion. Mild steel ties initially were either left unprotected or given a bitumen coating, but in the early 1930s zinc coatings or galvanising became accepted.
The most common type of mild steel ties (under BS 1234) are strip ties (commonly known as vertical twist or fishtail) and wire ties (commonly known as butterfly or double triangle).
It is very difficult to predict the life expectancy of ties used in the construction of houses before 1945. However, for those built between 1945 and 1964 it can be fairly accurately predicted:
The expected life of ties used in construction between 1964 and 1981 is estimated at:
ALL mild steel wall ties will eventually corrode – the only question is when!
The key here is catching the problem as early as possible. If the symptoms are recognised before deterioration has progressed too far, the walls may be re-stabilised rather than needing to be re-built. Old ties must be located using an electronic detector and replaced with a suitable corrosion-resistant remedial fixing. Finally, existing ties will need to be isolated to prevent further damage to the outer leaf. Corrosion of a tie within the inner leaf of a cavity wall is unlikely to become significant as their embedment is usually in a dry environment.
Ferrous oxide will result when embedded mild steel wall ties corrode. This will expand to several times the thickness of the metal it has resulted from, often breaking through the outer leaf of the brickwork (sometimes splitting the bed joints) and causing either lifting or bowing of the walls and damage to internal finishes. From a structural perspective, this leaves the wall vulnerable to vertical and wind loads, especially in the case of large gable and unreturned walls. The instability of the wall will eventually result in the ties corroding away completely, necessitating the rebuilding of the wall.