Cavity wall ties
Cavity wall ties are an essential stabilising component of a cavity wall structure, tying the weatherproofing façade to the inner core of the building of which the internal brickwork leaf forms a part. A brick tie system joins the brickwork leaves together and spreads loads across the cavity effectively making the wall structure stronger as a whole. Wall ties are usually built into the mortar joints of the brickwork as building of the wall proceeds.
Failure of cavity wall ties can be attributed to a building defect such as the omission or incorrect installation of the ties or ties which have been specified incorrectly such as being too short. Also the ties may have become defective due to some form of corrosion where the metal load bearing component of the tie is reduced in cross section reducing its strength and commonly in older buildings the two ends of the tie completely separate. Either way the tensile strength is reduced or omitted weakening the wall structure which leads to structural movement of the wall and reduction of its load bearing ability.
Inspection of a cavity wall tie during a pre-purchase survey of a property. Due to presence of cavity wall insulation a borescope camera could not be used andso a brick was carefully cut out to reveal the corroded Butterfly wire brick tie.
From 1945 cavity wall construction became the norm for houses and many other buildings. In more recent years mild steel ties have been used and despite galvanising treatments have been found to suffer from corrosion.
Wall ties and corrosion
A British Standard was produced in 1945. The estimated life of these mild steel ties is appreciably less than the 60 year life expected. The 1945 standard was relaxed in 1964 and 1978, but in 1981 when the extent of the problem was realized, the British Standard was amended to triple the zinc coating thickness on a wire tie.
Wall tie failures reported to the Building Research Establishment (BRE) include inferior zinc galvanising on mild steel, aggressive mortars and particularly black ash and permeable mortars such as lime that permits rapid carbonation.
It is estimated that over 3 million houses of cavity wall constructions were built before the introduction of the 1945 standard. Between 1945 and 1964 an additional 3 million houses were built to the British Standard BS 1243-1945 with a wire tie life expectancy of 15 to 31 years. From 1964 to 1986 over 4 million houses were built to the lower British Standard with a tie life expectancy of 23 to 46 years.
Initial identification of wall tie position using specialist accurate metal detector
Tests have shown that the rate of corrosion of the protective coating is more rapid in the damp outer leaf than in the drier inner leaf. Average zinc loss in the case of a wire tie in the outer leaf of a wall gives a predicted life of 12-26 years, compared with 43 years in the inner leaf. In 1979 it was predicted that 50% of the wall ties in pre-1939 properties could have failed.
It is quite clear that deterioration of the coating is faster in a damp wall, without consideration to outside factors such as chemical additives to mortar, marine salts and industrial atmospheric pollution.
The other consideration is the rust lamination that can cause the volume of the steel to increase up to four times its original thickness. The effect of this is to cause splitting at mortar courses and the lifting of the outer leaf of the cavity wall, without necessarily affecting the inner leaf. The result is instability with the affected wall moving out of plumb. This effect is less significant with wire ties compared to strip ties as there is less metal to expand in the vertical plane but deterioration is comparatively quicker due to the thickness being significantly less.
Invasive inspection and isolation of a ‘Fishtail’ type wall tie prior to insertion of flexible material and pointing over.