Wall Ties and Wall Tie Failure

Wall ties are an essential element for the stability of a cavity wall. Wall ties anchor the weather protecting facade of a cavity wall to the main structure of a building. The wall tie system enables the static and live loads that are carried by the inner leaf to be transferred by the wall ties and shared by both walls. Typically, a cavity wall tie is bedded in a mortar bed joint as a building is being constructed.

Wall tie failure can be a consequence of a construction defect; for example where the original 'built-in' cavity wall ties have been incorrectly fitted, fitted with wall ties that are too short, omitted in part or omitted in full. Alternatively wall tie failure may be a result of a buildings aging process, and caused through wall tie corrosion.

Over time mortar bed joints, which host the wall ties, undergo a chemical change through carbonation.  The mortar becomes aggressive to the mild steel wall ties and their protective coatings, reducing the life expectancy of the brick ties 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 the wall may need replacement of its wall ties if the stability and the load sharing capacity of the wall structure are to be maintained.

As wall tie corrosion sets in the steel wall ties generate a build up of iron oxide (rust) 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 wall tie erodes. In such cases there may be little sign that the outer facade is free-standing and the walls un-supported, save that the wall may appear bulged or out of plumb.

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 wall tie positions. If expansion is widespread and present along a row of corroding wall ties, a tell-tale pattern of horizontal cracks may be produced.

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 the replacement wall ties are installed into the masonry at a density of 2.5 per sq. metre, tie centres measuring 900mm horizontally and 450mm vertically, in a staggered 'domino 5' pattern. Additional remedial wall ties should be installed adjacent to open reveals. Wall tie replacement systems are simple to install and can be fitted by most general builders and competent DIY enthusiasts.

Where wall tie corrosion and expansion has resulted in horizontal cracks to the walls 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 and other metals buried in the wall. Remedial work to remove or isolate existing wall ties and other corroding metals is a separate and delicate task requiring extreme care if the masonry is not to be unduly damaged. Such care and skill may be best provided by time-served professional tradesmen.

Remedial Wall Tie Selection

There are a number of corrosion resistant stainless steel remedial wall tie products that are designed specifically for domestic housing and for ease of use. The most common remedial wall ties are:

Mechanical Wall Tie: - An expansion wall tie that is inserted into a pre-drilled hole in masonry and is tightened to form a mechanical grip connection, sequentially at each end. Preferably these replacement wall ties have neoprene expansion sleeves to prevent point loadings. A setting tool used with a drill or electric screwdriver ensures rapid fixing rates. Mechanical wall ties are suitable for use on domestic dwellings having good quality masonry.

Helical Wall Tie: - A profiled and twisted replacement wall tie that is driven into a small pilot hole in masonry such that work hardened helical blades cut a spiral interlocking seating path into brick, block and concrete as the tie is driven by a series of impacts. The tiny 6mm pilot hole enables helical wall ties to be installed with minimal cosmetic disruption to the facade and to any insulation in the cavity. Driving the helical wall ties takes only 5-10 seconds, making them the fastest remedial wall tie product available. The deep thread of the helix also makes these remedial wall ties ideal for use as a lateral restraint tie, tying walls to the end grain of floor joists.

Grouted Wall Tie: - A remedial wall tie that is pushed into a resin or grout-filled hole in one leaf and has the bonding agent pumped around its other end. The bonding agent sets around, and keys to, major deformations located at each end of the cavity tie. Resin grouted repalcement wall ties are well suited to domestic buildings having inconsistent or suspect quality masonry. Whilst this wall tie is the cheapest of the three its installation process is more intricate, demanding more time and care.