Cavity Wall Ties 

Did you know that over half the building stock in the UK is of cavity main wall construction?

It is surprising how many people have no idea how the property they live in is constructed, let alone the property they wish to purchase! In broad terms, most houses built before 1935 have solid brick external main walls. After this time the trend, increasingly, was for main external walls to be built in 'cavity' form (a wall with two 'skins' and a gap/cavity in the middle). Initially, both 'skins' or 'leafs' would have been of brick material. As building regulations changed over time, the inner 'skin' or 'leaf' was built using blockwork so as to improve the thermal (heat retention) qualities of the structure. Sometimes the gap/cavity is filled with a variety of additional insulation materials such as foam, blown glass fibre or polystyrene chippings/sheets. To hold the outer and inner skins/leafs together (and maintain the engineering integrity of the wall) metal wall ties would have been inserted at the time of construction. In more recent years, metal ties have been superseded by stainless steel and plastic wall ties.

Did you know that recent building research (carried out by the Building Research Establishment (BRE)) has identified that some cavity main walls built before 1981 MIGHT be prone to metal wall tie corrosion?

The problem with some metal ties is that, over time, they corrode. Eventually, if left unnoticed/untreated, the ties will fail. If too many ties fail in the same area of a wall, partial/total collapse occurs. To complicate things, a variety of wall tie types have been used in the building industry over the years. Are they all at risk of corrosion? Realising there was a possible problem the Government commissioned the Building Research Centre (BRE) (an independent buildings research organisation based at Garston,near Watford) to investigate.

Do you live in a house that is 'at risk' to wall tie corrosion? Are you proposing to buy a house that might be at risk? What are the causes of cavity wall tie corrosion? What are the symptoms? What are the remedies and the costs involved?

The results of the BRE research may be summarised as follows:

(1) Cavity walls built before 1981 are potentially at risk because the metal wall ties often lacked protection against corrosion. After 1981 metal wall ties were more thoroughly protected and stainless steel/plastic alternatives introduced.

(2) Some pre-1981 metal cavity wall ties are more prone to corrosion than others. Generally, the older the cavity wall tie the greater the risk.

(3) Cavity wall tie corrosion is exacerbated by environmental factors.  Proximity of a property to industrial (airborne) pollution or exposure to salt air (cliff-top/seaside) will increase the risk of wall tie corrosion.

Put simply, if you are purchasing a property with cavity main walls which was built before 1981 and is located in an industrial town or close to the sea there is a potential risk of wall tie corrosion. Unfortunately, the first signs of a problem usually happen at a later stage, when wall tie corrosion is well-advanced and the structure is beginning to fail. Horizontal cracking occurs along the brick courses. Bulging in the wall increases too.

If a problem IS diagonosed, the remedy will take the form of inserting new steel or plastic wall ties next to the old ones. If the old wall ties are the 'vertical twist fishtail' type, they will have to be removed. This will double the cost. If the defective wall ties are of the 'butterfly wire' type they can be left in place and allowed to corrode merrily away.

And what is the cost  if ALL the wall ties need to be replaced? The repair of a typical 3-bed detached house with, say, 120 sq m total floor area, will  be in the region of £4,000 (or £8,000 if older wall ties need removing).

Martin and Mortimer will advise, always, on the issue of cavity wall tie failure when carrying out a survey on a building for a potential purchaser. We will not carry out any excavation through the wall structure as this is not something we can do on a property under separate ownership. However, we will assess all the facts and use our experience and expertise to decide whether, before exchange of contracts, it would be advisable to have a CCTV inspection carried out by an approved specialist.

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