Wall Ties and Cavity Wall Tie Failure

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. 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.

Cavity 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 capability.

Over time mortar joints, which host the wall ties, undergo a chemical change through carbonation.  The mortar becomes aggressive to the base steel and its protective coatings, reducing the life expectancy of 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 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 volume is accommodated within the mortar bed as the cavity ties erode. In such cases there may be little sign that the outer facade is free-standing and the walls not supported, save that the wall may appear bulged.

In other cases, particularly where sheet steel or vertical twist wall 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 tie course.

Wall Tie Replacement Strategy

Where any type of cavity tie failure has been established the walls should be immediately stabilised with new stainless steel remedial wall ties. Where each leaf is at least 90mm thick, installation of the remedial brick ties is undertaken at a density of 2.5 per sq. metre, tie centres measuring 900mm horizontally and 450mm vertically, in a staggered 'domino 5' pattern. Additional retrofit ties are installed adjacent to open reveals. Installation of wall tie replacement systems is a simple process that can be carried out by general builders and those competent in DIY.

Where wall tie corrosion and expansion has resulted in horizontal cracks to a brick facade, a qualified engineer should be asked to assess the potential for continued expansive damage and to weigh this against the damage that could result from efforts to remove or isolate all the corroding wall ties. Remedial work to remove or isolate existing cavity ties is a separate and delicate task requiring extreme care if the masonry is not to be unduly damaged.

Remedial Wall Ties - Selection

There are a number of corrosion resistant stainless steel remedial wall ties that are designed specifically for cavity wall tie replacement. in domestic housing and for ease of use by professionals and for DIY alike. The most common retrofit brick tie types are:

Mechanical Wall Ties - An expansion type wall tie that is inserted into pre-drilled holes in masonry. Preferably these remedial cavity wall ties have radially expanding neoprene sleeves, which are sequentially tightened to form a mechanical grip connection without excessive point loadings. A setting tool is used with an electric screwdriver, to ensure rapid fixing rates. Mechanical ties are suitable for use on domestic dwellings having good quality brick walls.

Helical Wall Tie - A profiled and twisted remedial wall tie that is driven into a pilot hole. Work hardened blades cut a spiral interlock into brick, block and concrete walls as the cavity ties are driven by a series of impacts. Tiny 6mm pilot holes enable extremely quick wall tie installation with minimal disruption to the facade or to cavity insulation. Use helical ties in all masonry types. A setting tool in a hammer drill facilitates the easiest and fastest retrofit replacement wall tie system available

Grouted Wall Ties: - Remedial cavity wall ties that are pushed into resin or grout filled holes in one brick leaf before more bonding agent is pumped around the other end. The resin or grout sets around, and keys to, major deformations located at each tie end. Grouted wall ties are well suited to domestic buildings having inconsistent or suspect quality masonry. Whilst these replacement ties are the cheapest of the three, installation is more intricate, demanding more time and care.