Wall Tie Report
External walls to most buildings constructed after 1920 are cavity walls. Typically there is an outer leaf of 100mm thick brickwork, a 50mm wide cavity, and an inner leaf of 100mm thick blockwork. The outer and inner leaves are tied together with wall ties. Wall ties are usually 900mm apart horizontally and 450mm vertically. They are usually galvanised steel. It should be noted that older ties often had a relatively thin layer of galvanising, not adequate to stop the steel corroding.
There are two problems with corroding wall ties. The ties can corrode in the cavity and break such that the outer and inner leaves are no longer tied together. If this occurs, there is a distinct risk of the outer leaf collapsing, especially under high winds which cause suction on the face of the wall.
The more common problem is when the part of the tie embedded in the outer leaf corrodes. This tends to occur on south and west facing elevations which are exposed to driving rain and which are often wet. Rust occupies a greater volume than the original steel and therefore the expansion of the tie in the bed joint forces it to crack. Cracks running along the bed joints, every 450mm up the wall, are a definite indication of wall tie corrosion. In severe cases we have seen the outer leaf being lifted by about 50mm at roof level.
A wall tie report comprises detecting the locations of wall ties using a metal detector and then either removing a brick or the mortar or drilling a small hole and using an endoscope to examine their condition. If ties are corroded or if there are insufficient ties, the report will recommend a suitable type of replacement. Corroded ties will need to be cut out of the outer leaf to prevent further cracking.