There is a risk that the metal ties normally built into the wall cavities of buildings constructed before 1982 will eventually corrode.
Wall ties are required to tie together the two separate leaves of masonry that make up the cavity wall, which allows the two walls to act as a single unit.
Wall tie corosion.
Different types of wall ties available.
Wall ties can be made of different materials. Early ties were made of wrough iron or cast iron. Modern ties are made of mild steel and more recently stainless steel. To prevent the wall ties from corroding the modern mild steel ties are covered with a protective coating by galvanising (coating iron or steel with a layer of zinc by immersing the metal in a bath of molten zinc at a temperature of around 450 °C ). However, in 1964 and 1978 the 1945 standard was relaxed and the galvanising thickness was reduced.
Houses of cavity wall construction built before the introduction of the 1945 standard will be vulnerable to decay.
Between 1945 and 1964 houses of cavity wall construction were built to the British standard BS 1243-1945 with a life expectancy of:
wire ties 15 to 31 years
strip ties 31 to 61 years.
From 1964 to 1986 houses of cavity wall construction were built to a lower British Standard with a wall tie life expectancy of 23 to 46 years.
In 1981 the British Standard was amended to triple the zinc coating thickness on a wire tie.
Wall Tie Failure
Once a wall tie corrodes it may no longer hold the two walls together and distortions can start to occur to the outer leaf of brickwork. This typically takes the form of the brickwork bulging outwards particularly with the wire ties such as the Butterfly Pattern and the Double Triangle Pattern. Because of their thinner sections they can lose their ability to tie the walls together quite quickly. Conversely with the thicker metal ties like the Twisted Pattern when these corrode, they expand forcing the brickwork apart and causing horizontal cracking on the mortar joints that contain the ties.
When either an outward bulging or horizontal cracking is apparent to the external walls of a property it can indicate wall tie failure. However, it can also be casued by other factors and professional specialist advice should be taken.
Author: Tony Waring is a Chartered Building Engineer (C.Build E MCAB), an Associate of the Royal Institute of Chartered Surveyors (AssocRICS) and a member of the Faculty of Party Wall Surveyors (MFPWS). He has over 30 years of surveying experience and 20 years as an engineer.
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