Cavity Wall Ties

What are Remedial Wall Ties?

Wall ties are an essential element for the stability of a cavity wall structure, tying its weather protecting brick facade to the main body of a building. Cavity walls have been a common feature of buildings in the UK for over one hundred years. Two leaves of brick or stonework are held together using metal straps or "wall ties". The cavities between the two leaves of masonry provide an affective barrier to water penetration and also acts as an insulation layer. An effective brick tie system transfers static and live loads across the cavity, enabling load sharing by both inner and outer walls. Typically cavity wall ties are bedded in a mortar bed joint as a building is constructed.

How do they fail?

In some circumstances these wall ties can corrode or fail. Failure can manifest itself as cracking in the external leaf of masonry, structural movement or in severe cases the collapse of sections of masonry.

Cavity wall tie failure can be a consequence of a construction defect; for example where the original 'built in' cavity wall ties have been omitted, incorrectly fixed or fitted with brick ties that are too short. Alternatively failure may be a result of a buildings aging process, whereby wall tie corrosion may have compromised the walls load sharing capabilities.

Over time mortar joints, which host the wall ties, undergo a chemical change through carbonation. The mortar becomes aggressive to the steel base and its protective coatings, reducing the life expectancy of the cavity tie systems to as little as 26 years. the design life of the building is typically much longer than this period and it therefore follows that at some point a remedial retrofit replacement of the brick ties may be necessary if the stability and load sharing capacity of the wall is to be maintained.

As wall tie corrosion sets in, the mild steel brick ties generate a build up of iron oxide layers which occupy a greater volume than that of non-corroded steel. In most cases, particularly when wire wall ties have been used, the increase in the volume is accommodated in the mortar bed as the cavity ties corrode. In such cases there may be little sign that the outer facade is free standing and the walls not supported, save the wall may appear bulged.

In other case, particularly when sheet steel or vertical twist ties have been used in less forgiving mortars, the iron oxide build up may have the effect of lifting the masonry above a line of insitu brick ties. If expansion is widespread and present along several rows of corroding cavity wall ties, a tell-tale pattern of horizontal cracks may be produced along each wall tie course.

Remedial Work

Remedial work involves installing new stainless steel ties to tie together inner and outer leaves of brickwork. Old wall ties are then removed or isolated to prevent further damage.