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Wall Tie Reports
Wall Tie Reports
At a glance...
- Wall tie problems are usually shown in cracking or bulging of external walls
- Endoscope inspection within cavities
- Costings included for any repairs needed
- Replacement schemes
- Report format acceptable to lenders
Often, failure of wall ties is shown by a series of horizontal cracks in the external brickwork of property (commonly this happens at high level). The problem is the result of the wall ties corroding. As they corrode, they can swell which results in cracking in the regular patterns where ties are inserted into the wall.
Wall ties were used to connect the outer skin to the inner skin of cavity walls. The linkage provided structural stability for the wall and retained the external leaf (often the weather resistant cladding) in place. Corrosion of the ties leads to them failing and the two leafs of the brickwork become separate. Buldging of the outer leaf can occur or in extreme cases even need rebuilding.
Most wall tie reports are recommended because there are signs of distress in the external leaf of a building. This is a problem, however, which tends to affect houses of cavity construction within particular areas and a good surveyor may well be aware of wall tie failure in nearby houses. This local knowledge can prompt him to recommend a wall tie report.
Walls have been constructed in two leafs for a long time. Several different materials have been used to link the two leafs together to give structural stability and added strength to give additional height to walls of more slender thickness. Slates and bricks, for example, have both been used for that purpose. Metal wall ties became popular in the mid 1900’s – relatively light and easy to use, steel ties were later galvanised to help extend their life and in particular to resist corrosion.
The ties are placed at regular intervals typically at every fourth brick course. Over relatively recent years, however, problems have been discovered with the earlier wall ties.
Ties are bonded into the mortar of the outer brickwork and the inner brick or blockwork. Where the ties meet the external leaf, they are particularly exposed to moisture and are likely to corrode in this area first. Corrosion causes the metal to expand which has consequences for the integrity of the wall.
First, the mortar joint cracks. This reduces the strength of the tie as it can become loose and offer less restraint. Secondly, the presence of cracks in the external pointing can allow water penetration. This enters the crack and when it freezes, the water itself expands, causing the crack to widen. This ‘freeze/thaw’ action accelerates the widening of the opening to further water penetration and can lead to a loss of bond between the tie and the mortar joint. In some cases, corrosion to the tie is so severe that the tie itself falils allowing the two leaves to part. This results typically in bulging of the outer leaf uses or, in more serious case, of partial collapse.
When investigating wall tie problems, contractors also discover that the ties have been wrongly installed and in particular ties are over-spaced or omitted. This has the effect of reducing the restraint of the outer leaf and leads to bulging of the brickwork together with additional stress on the ties that are present.
Another main reason why wall ties fail is when the ties that hold built-in tie insulation is fitted or designed incorrectly, which leads to failure. In both cases, remedial works are possible and, in most cases, moderately priced.
The first phase of wall tie deterioration is corrosion of the metal in the ties. This causes the tie to enlarge. One of the first signs of problems, therefore, is damage to mortar joints around the swollen ties.
- Cracking to the external wall brickwork typically follows the pattern of the ties. Hence, horizontal cracks repeated every fourth course of brickwork are commonplace, though isolated areas of enlarged mortar joints and even stepped cracks can also result from wall tie failure.
- In more serious cases, wall tie failure can become evident in bulging to the external brick, or even in localised collapse of the outer leaf (or both leafs in extreme cases).
In order to assess what work should be carried out, the contractor will check alignment of the external walls for verticality and will note any cracks or bulging. The contractor will make special note of cracking patterns that emerge which can confirm the presence of wall tie problems. The positioning of existing ties are checked by means of a metal detector and recorded.
The internal cavity of the wall is assessed with the use of an endoscope and the condition of ties is literally visually checked. Obviously, the contractor will check whether cavity wall insulation is present within the wall cavity, which affects which type of replacement ties are required.
A good contractor will survey the property to assess what repairs are required. The amount and degree to which ties are corroded depends on a number of factors – not only the composition of the tie itself, but also the exposure of the property to weathering, orientation of the external walls etc. So, corrosion rarely affects a property uniformly.
Replacement of wall ties is a specialist job that requires an experienced and knowledgeable contractor. There are a range of types of replacement tie available on the market – ranges include mechanical ties, resin grouted ties, helical screw-in ties, hammer fix screw in tie etc. Selection of the correct tie will depend on a number of factors such as the width of the cavity; the size and materials of the two leafs of the wall etc. Guidelines for replacement are described in detail in B.R.E. Digest 329.
Replacement stainless steel wall ties are usually installed at a density of five ties for every two square metres of wall. They are fixed at regular intervals conforming to BS 5628 code of practice, fixed either mechanically or with resin. If the original ties have corroded seriously, then they can be removed as part of the repair programme or new ties installed around them. This will depend on the contractor’s view as to whether leaving them in situ poses any ongoing risk of damage.
Most wall tie repair schemes will be offered with an appropriate guarantee or warranty of the work quality.