Cavity Wall Tie Replacement

Gen Damp are experts in all forms of wall tie survey and installation. From identifying the type of wall tie through to the correct treatment, you can ensure Gen Damp will treat your remedial wall tie problems efficiently and professionally.

Cavity wall construction has been around since the early Nineteenth Century and became part of normal building practice from the late 1800’s until the present day.

The principal advantages of cavity wall construction are:

• Thermal insulation
• Sound insulation
• Prevents the ingress of moisture from outside
• To give structural strength and stability
• To allow the evaporation of moisture from the structure

While the materials and construction methods of cavity walls can often vary, all methods rely on a tying mechanism to connect the inner and outer leaves and give the wall strength and stability. Therefore cavity wall ties are the crucial structural element within cavity walls.

Historically many different types of materials were used for wall ties including in some areas wood and slate. The most common materials used since the late Victorian era have been iron or mild steel and it is the corrosive properties of these type of ties that have caused the greatest problems.

It is often incorrectly quoted that properties built post 1981 are free from the risk of wall tie corrosion and associated problems. This is because after this date wall ties were of a higher specification, usually stainless steel or galvanised wire, while this greatly reduces the risk of failure it does not completely eliminate it.

Corrosion:

The older types of tie either have no protective coating or they sometimes have a bituminous or light Zinc coating. Either way these ties are highly susceptible to corrosion in the presence of moisture and Oxygen. The extent and the rate of corrosion depends on several factors and these include

• Exposure to prevailing weather
• Type of mortar be that Black ash, weak mixes and porous lime mixes
• Condition of pointing
• Poor design and construction, blocked cavities, lack of DPC’s or membranes
• Environmental factors including costal locations.

 

Wall tie corrosion can lead to a situation varying in severity from future maintain ace to a catastrophic collapse of the structure. As tie corrode they expand and this expansive corrosion can produce distinctive horizontal cracking within the mortar bed joints, usually, though though not exclusively to the outer leaf. These problems are normally associated with the thicker metal and Fishtail type of ties.

Wire ties and other thin metallic ties are unlikely to expand sufficiently to cause horizontal cracking unless the mortar bed joints are very tight(thin) or mortars very hard. Patterns of horizontal cracking can seriously destabilise the structure of the property and accelerate the rate of corrosion by increasing the ties exposure to moisture.

Wall tie density:

The pattern of existing wall ties in most older properties is often found to be irregular and the overall density of ties is frequently inadequate. This leads to areas of major concern and weak spots in these types of properties, these areas are usually

• Sides of windows and door openings
• Verge and eaves levels

Historically there was no additional tie allowance to these areas and as previously mentioned this can lead to weak points within the structure.

Wall tie survey:

The key elements to the wall tie survey are as follows:

• Location and approximate age of the property
• Establish the construction methods, cavity or solid brick construction. Inner and outer leaf materials. The form of construction can change even within the same wall, in the North East it is common for ground level walls to change to cavity walls at first floor level. Or for cavity brick walls to revert to solid walls at eaves level on a gable end peak.
• Mortar type and condition and the overall condition of the pointing
• Cavity width
• Cavity wall insulation present?
• Note all cracks and obvious signs of movement, bowed or bulging walls
• Pay particular attention to horizontal cracks that have been repaired and repointed
• Establish the pattern and overall density of existing ties
• Establish the condition of the existing ties and the extent and severity of any corrosion
• Does the location of the corroded ties correspond with the location of any cracking already noted
• Distinguish between cracks caused by corroded ties and that caused by other corroded metal eg steel lintels, cast iron down pipes etc
• Wall tie embedment, are the wall ties long enough to suit the cavity width and are they correctly positioned

Equipment:

 The surveyor carries with him several specialist item required to carry out a thorough investigation and survey.

• Metal detector to locate ties
• Cordless hammer drill for inspection holes
• Endoscope/ Boroscope to examine ties insitu
• Mortar or mastic for making good drill holes

NB Cavity wall insulation is an additional complication in properties with cavity insulation it is necessary to remove a brick to inspect the wall ties, this will incur a charge of £60.00 plus VAT at the current rate

Wall tie stabilisation:

Once the results of the survey are evaluated this will lead to the next stage which can range from no action the ties are corrosion free and working effectively, through to the complete rebuilding of structural walls. However the most cost effective remedial solution generally lies somewhere between these two extremities.

Detailed recommendations are given within Building Research Establishment (BRE) Digests 329 and 401and British Standards Institute (BSI) DD140.

The most common remedial tratments are as follows:

• Complete wall tie replacement is recommended where the ties show evidence of extensive corrosion and where corrosion is visible during inspection
• For most walls the recommended fixing density is 2.5 ties/m² with additional ties at 300mm vertical centres at the sides of openings and up verges
• Where the existing ties are of the thin metal or wire butterfly typr which may not have caused horizontal cracking, it is generally acceptable for these to remain in place
• Where expansive corrosion of ties is causing cracking to the outer leaf, these ties should be removed or structurally isolated from the outer leaf. This can only be completed after the installation of remedial ties and is usually the more disruptive and expensive element of wall tie stabilisation
• It may be that the wall ties are in good condition but low in density, it may be in cases such as these that additional ties may be fitted to supplement and strengthen the existing ties.

Remedial wall tie installation:

 A wide selection of remedial ties are available and it is crucial the right tie is recommended for each individual property. The selection should be based on the results of the survey and in particular the inner and outer leaf substrates and cavity width.

• A single standard product will not cover all eventuallities and even with the benefit of an accurate survey, installation teams will be equipped for variations in construction
• In some cases ties may be installed from the inside of the property if extensive refurbishment or alteration are taking place. This is a good option as there are no visual signs to replacement once all works are complete.
• Where holes are drilled through the centre of outer leaf bricks it is important to achieve as close a match as possible by dyeing mortars on site, our operators are trained and highly competent in this aspect of the remedial works
• Operator experience and supervision is crucial to ensure ties are installed correctly and the works completed on time and are successful.

Treatment to existing ties:

Where the repair dictates treatment to the existing ties these must fist be located, usually with a metal detector and/or a Boroscope

• Complete removal of the ties is necessary where there is extensive damage to both inner and outer leaves. This involves the removal and replacement of individual bricks. This process is slow and labour intensive and will incur relative costs.
• Where damage is limited to the outer leaf then it is acceptable to structurally isolate the ties from the mortar bed joints. Several methods can be used including the use of isolation sleeves either plastic or foam.

NB if a property is in need of general repointing it is a good idea both financially and aesthetically to undertake these works at the same time as treatment to existing wall ties.

Remedial wall tie installation is a technical solution to a common structural and durability problem associated with cavity wall structures. It should not be undertaken by non – specialists or as a DIY fix. Like other common remedial treatments such as damp, fungal attack and woodworm eradication. One of the most important factors that the client is paying for is peace of mind and confidence the work was carried out by experts and that the property is safe to live in, and that it will retain its market value.