What are the symptoms of wall-tie failure and, once identified, what remedies are available?
The corrosion of zinc-coated cavity-wall-ties is now recognised as a major building defect not only in areas subject to high risk, such as industrial and coastal locations, but throughout the UK. Corroded metal expands to take up many times its original volume and, even if the ties themselves do not fail, this expansion will result in cracking which will open up the surface to direct rainwater penetration, which further accelerates the corrosion process.
The main problem for the surveyor is that the defect is not usually evident on the surface until the damage has been done. There is normally no means of inspecting the condition of the wall ties without opening up the structure — not a practical proposition during most types of standard survey.
When it is considered that the majority of post-war domestic building uses cavity-wall construction and that the method was widespread in the inter-war period, this is not a restricted problem. In fact, there are some 12m houses in the UK which have been built using cavity-wall construction. Surveys have estimated that 900,000 of these are already affected by wall-tie failure and the remaining 11m are likely to be affected at some time in the future.
The use of cavity-wall construction is more widespread than once thought. In certain areas of the country it occurs mainly in post-war buildings but in other areas, especially the North, it was in common usage in the 19th century and has been identified as early as 1804.
Materials used include brick headers, hollow stoneware and cast iron. Since the 1920s coated mild steel has become commonplace and in the 1950s there was a tendency for cheaper and more flexible wire and strip ties to be used. These have limited zinc protection and are at risk of failure.
Whatever the thickness of the galvanised coating, the metal tie will eventually corrode causing the metal in the outer leaf to expand to anything up to seven times its original thickness. This results in a build-up of pressure which will eventually cause the wall to crack along the mortar joints. Wire ties are usually thin and, if bedded in thick mortar joints, may not expand sufficiently to cause cracking but, because of the thinness of the metal, there is the added risk that they may fail altogether resulting in serious weakening of the structure.
History
The problem of wall-tie failure was first noted in the 1960s in the case of a Welsh farmhouse where black ash mortar had been used. It was thought at the time to be an isolated incident. The true extent of the problem began to emerge in the early 1980s.
Most modern wall ties are made of mild steel with a galvanised zinc coating but ties have been formed of stone, slate, brick, terracotta and even timber. Metal, though, is the most common material because it is flexible enough to withstand slight movement but strong enough to withstand horizontal loading. They are not bulky and can be easily formed to produce the required drip. Cast iron, wrought iron, mild steel, copper and stainless steel have all been used. Because of cost and weight considerations mild steel is the most common.
Mild steel is a ferrous metal and its tendency to corrode is an inherent source of weakness. Early ties were unprotected or coated with bitumen to arrest corrosion. By the 1930s it was common practice to galvanise the steel with a thin covering of zinc. The thickness of the zinc galvanising tends to vary with the age and type of tie. British Standards were actually relaxed during the 1960s and 1970s but have since been toughened. The thickness of coating required by the British Standard for wall ties was increased in 1981 following research by the Building Research Establishment which showed that the ties were deteriorating faster than expected. (See BS 1243:1978, revised 1981, Amendment 3651 — Increase in thickness of zinc galvanising).
Functional considerations
The advantage of cavity construction lies in its ability to resist the transmission of moisture through the structure, coupled with an overall structural strength which exceeds that of its comparatively thin separate components.
Vertical loads are carried directly on the inner leaf only, with no direct load on the outer leaf. However, in view of the thickness of internal brick or blockwork, the inner leaf is not capable of withstanding the loads applied by the structure and some of the direct loading has therefore to be transferred to the outer leaf. In addition the outer leaf itself will be subjected to horizontal loading from wind forces. The support of the external leaf from these horizontal forces and the need to transmit some of the structural loading is achieved by tying the two leaves of the cavity wall together.
The function of the wall ties is to hold the external leaf on to the main structure. They have to be strong enough in tension to prevent wind suction from pulling the wall off, but stiff enough in compression to prevent the cavity from closing. Ties should allow sufficient sideways movement to let the two walls expand and contract independently without damaging the walls or the ties themselves. They should be provided with a drip to prevent the passage of water across the cavity. Ties have tended to be thin to allow them to be bedded into the mortar joints and to prevent mortar from collecting on the exposed part of the tie. Such design requirements have resulted in the widespread use of fish tail or thin wire butterfly ties and it is the use of these types which has tended to result in failure from corrosion of the material used.
Symptoms
As the product of the corrosion occupies a greater volume it will produce a pattern of horizontal cracking in mortar joints which correspond to the siting of the wall ties (approximately every sixth course). Where aggressive mortars, such as those using black ash and pulverised fuel ash, have been used the risk of corrosion is increased owing to the extra corrosive effect of acid materials within the mortar. Penetrating rainwater combines with sulphur in the mortar, forming a mild sulphuric acid solution which reacts with the galvanised coating and hastens the normal corrosion process. Such mortars were commonly used in the 1930s, particularly in mining and industrial areas such as south-west Wales, Lancashire, Yorkshire and north-east England.
In severe cases the outer leaf will become displaced relative to the inner leaf, resulting in bulging. This may in turn result in the outer leaf being forced to carry loads for which it was not designed.
First indications of the existence of corrosion are often evidenced by the cracking of mortar joints with a pattern of horizontal cracks following the location of ties. In some cases the cracks may have been repointed, but this will often be indicated by wider bed joints. Even where cracks exist they can be difficult to detect, especially where thin wire ties have been used, and close inspection of the walls is necessary, particularly on the higher parts of the elevation.
Outward bulging is sometimes evident but outward movement below 25 mm is not easily visible to the naked eye looking upwards. The use of a ladder may be helpful in carrying out the initial inspection. The bulging results from the expansion of the ties in the outer leaf. Typically a wall will contain around 12 rows of ties so that the expansion over the full height of the wall can be significant. The ties bedded into the inner leaf tend not to corrode at the same rate because they are not so exposed to moisture penetration so the inner leaf tends not to increase in height. Other symptoms will include the upward cambering of window sills where the movement is less constrained at openings. This will sometimes be accompanied by stepped diagonal cracks from the bottom corners of windows. Significant expansion in the outer leaf may be sufficient to cause the edges of the roof to lift and this is known as the pagoda effect. In extreme cases displacement of the roof timbers may occur.
Internally, concave bulging in the wall, usually above first-floor level may be evident. This is sometimes accompanied by the separation of window reveals. Cracks at the junctions of internal and external walls, cracks above doorways and at the junction of wall and ceiling, separation of wall from skirting and wall from stair stringer may also be evident. All these defects can of course be masked by internal decoration.
Inspection and identification of ties
Once cavity brickwork has been identified as being at high risk (coastal areas, industrial areas, black ash or lime mortars, walls built pre-1981 or where substandard ties may have been used) it will be necessary to recommend a more detailed survey to establish the condition of the wall ties. A number of methods are available. Metal detectors can be used to locate the metal ties but do not indicate the presence of corrosion so, once located, sample ties must be examined by the removal of single bricks which results in some damage. Other methods of location include radar (expensive and heavy), radiography (expensive and requires some precaution) and infra-red thermography. The latter is still experimental but it is possible that this could have the advantage of identifying the extent of corrosion without the removal of brickwork. Fibre-optic probes are also used to view the cavity through a small access hole. It should be noted that this method allows inspection of only the visible section of the tie within the cavity and is not conclusive because corrosion normally begins in the section of the tie bedded into the outer leaf. Such observations will, however, provide vital clues to the general state of the ties provided always that the cavity is clear of debris and is not filled with insulation material.
The greater part of the damage to wall ties will occur in the outer leaf which is the portion subject to the greatest water penetration. So sample observation will be necessary and this should give a good idea of the general state. Remember there is likely to be a difference between elevations, only those most exposed to driving rain may be affected.
The ties, once exposed, should be examined for type and condition. Are they still protected or has corrosion begun? Note the distribution of ties. Sufficient ties should be provided, especially at openings. They should be staggered and evenly distributed — 4.9 ties per m2 where one leaf is 66-90 mm thick and 2.5 ties per m2 where both leaves are greater than 90 mm thick. There should be a row of ties every sixth course of standard brickwork. Horizontal spacing should be no greater than 900 mm (450 mm for cavity widths of 50-75 mm) and ties should be placed at each block joint, intervals of 300 mm, within 225 mm of all openings. Ties should be horizontal or fall to the outer leaf, with drips in the centre of the cavity and no mortar bridging the cavity. Ties should be of sufficient length to be properly bedded at least 50 mm into both leaves. (See BRE Defect Action Sheet 116, June 1988.)
In addition, it will be important to note the construction and thickness of both leaves, the cavity width, the type of tie and the type and condition of mortar. These are all factors which will have a bearing on the remedy to be specified.
Interpretation
If there is no physical damage to the structure and the ties are not corroded it may be sufficient to recommend reinspection in 10 years’ time. If corrosion is present but there is no damage it may be appropriate to reinspect in two years’ time. If there is physical damage and this is the result of corrosion then immediate action will be necessary.
New ties
A great deal of development has been undertaken within the last 10 years into the design of new types of wall tie which has resulted in the use of a wide range of alternative materials including coated mild steel, stainless steel, copper, aluminum, phosphor bronze, polypropylene, glass-reinforced resin, resin-coated galvanised steel and epoxy-coated steel. In addition, new designs have been marketed, some of these specifically in response to the need for a tie which can be installed as replacement for existing corroded ties without undue disturbance to the surrounding structure. Fixing mechanisms include expansion, gluing, friction and screwing.
Stainless steel ties have been developed for a wide variety of applications. Typical replacement ties for general application consist of a bolt in a metal sleeve fitted with plastic or stainless steel outer sections at both ends. The ends are fixed mechanically to the drill hole formed from the outside of the structure by tightening the central bolt. Tightening causes the outer sections of the tie to expand and grip the material of the wall. The advantage of this type of tie is that it can be fitted from the outside with a minimum of disruption. The actual method of fixing will depend on the nature of the materials used but broadly this falls into three categories:
- Mechanical-to-mechanical fixings, used where both leaves are of sound brick, block or stone.
- Mechanical-to-resin, which utilises a resin bond to the inner leaf where this is not sound.
- Mechanical-to-timber, using a screw fixing where the inner leaf is of timber-frame construction.
In all cases the ties, once inserted, should be pull tested to ensue that the fixings are capable of withstanding a force of two kilonewtons for at least one minute.
Remedial work
Any repair strategy will depend on the structural condition of the wall and it will be necessary to identify the materials of both inner and outer leaf and whether they are loadbearing or not. It will also be important to establish the condition of both leaves, whether or not they have been damaged by movement or cracking, the type of tie provided, the condition of the ties and whether adequate ties have been provided.
Solutions range from simple repointing in order to repair the cracks caused by corrosion to the complete demolition and rebuilding of the wall. Where the problem is severe, resulting structural damage, bulging or distortion in either leaf or even subsequent movement of roof timbers will also have to be dealt with. Defective ties must be replaced and old ties removed or isolated. This may entail the removal of bricks which is both time consuming and unsightly. New products have been developed to limit the disruption by obviating the need to remove bricks. As a consequence cheaper and more aesthetically pleasing results can be obtained. It is normally necessary to remove all existing wall ties, although in the case of thin wire ties, set in wide bedding joints, it is unlikely that expansion will be sufficient to cause further cracking but it may be necessary to isolate the ties in the outer leaf to prevent further distortion. This can be achieved by the insertion of a PVC sleeve, treated with a rust inhibitor, around the outer end of the existing tie.
Replacement ties should be of stainless steel. Other non-ferrous metals can be used but tend to be more expensive. Mild steel can also be used provided it is coated in at least 1mm of durable resin (see BS 1243).
In some cases it is possible to effect repair by pumping high-density polyurethane foam into the cavity. This has the added advantage of improving thermal insulation and operates by improving the bonding between inner and outer leaf. It is not suitable for use with timber-frame construction as it will not provide an adequate bond with breather paper or other sheathings. It should be noted that this approach will tend to increase the risk of water penetration so is not suitable for use in exposed situations. It should also be noted that the long-term performance of such materials has not been tested.
Conclusion
The life of an original tie is unlikely to extend beyond the nominal 60-year life of the wall in which it is fixed, and metal ties have been known to deteriorate after as little as 13 years. Where a cavity wall is identified the surveyor should always warn the client of the inherent dangers of this form of construction. In any circumstances where cavity-wall-tie failure is suspected owing to symptomatic presence or high-risk factors such as location, age or type of materials, it would be wise to recommend a survey to establish the condition of the ties so that remedial action can be taken if necessary. Many firms specialising in the replacement of wall ties offer a free survey service and this will establish the state of the ties by sampling.
If replacement is deemed necessary, fortunately this will rarely result in the demolition of the outer leaf as a number of alternative methods are now available to ensure the reasonably painless replacement of damaged existing ties. Typically, work on a small semi-detached house might cost in the region of £1,500 where no consequent defects are present. However, if the condition is not identified early enough the consequences can be disastrous. In the case of low-rise domestic buildings complete collapse is unlikely but in high-rise buildings it is not unknown for external leaves to collapse totally and without warning. In all cases, early identification of wall-tie corrosion may save considerable expense on associated remedial work.
Further reading
Building Research Establishment Information Papers:
IP 28/79: Corrosion of steel wall ties: recognition, assessment and appropriate action
IP 29/79: Replacement of cavity wall ties using resin-grouted stainless steel rods
IP 4/84: Performance specification for wall ties BRE Defect Action Sheets:
DAS 21: External masonry cavity walls: wall tie replacement, March 1983
DAS 115: External masonry cavity walls: wall ties — selection and specification, June 1988
DAS 116: External masonry cavity walls: wall ties — installation, June 1988
BRE Digest 329: Installing wall ties in existing construction, February 1988
R C de Vekey, “Ties for cavity walls and masonry cladding” Structural Surveys, April 1990. Henry Stewart Publications
