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Wall tie Corrosion

1. Lifting of the mortar beds, where the ties have rusted and expanded, pushing the mortar.

Tests have shown that the rate of corrosion of the protective coating is more rapid in the damp outer leaf than in the dryer inner leaf. Average zinc loss in the outer of 15g/m2 compares with 6g/m2 in the inner leaf giving, in the case of a wire tire a predicted life in the outer leaf of 12 to 26 years compared with 43 years in the inner leaf.

In 1979 it was predicted that 50% of the wall ties in pre 1939 properties could have failed. There are no statistics for bitumen coated wall ties, but flaking by under film corrosion at pin holes in the coating appears to occur at a rate comparable to zinc coating.

It is quite clear that deterioration of the coating is faster in a damp wall without consideration to out side factors such as chemical additives to mortar, marine salts and industrial atmospheric pollution. If the cavity wall insulation restricts the drying of a wall by preventing evaporation, this could also contribute.

Once the steel of the wall tie is exposed to air and oxygen the all too familiar rusting cycle begins. The rate of corrosion will be governed by site conditions but in severely exposed locations the life of a twist tie could be reduced by 10 years.

The loss, by corrosion of the wall tie, is one problem but the other consideration is the rust lamination that can cause the volume of the steel to increase to four times its original thickness. The effect of this is to cause splitting at mortar courses and the lifting of the outer leaf of the cavity wall without necessarily affecting the inner leaf. The result is instability, with the effect of the wall moving out of plumb.

2. Vertical cracks in brickwork, especially under windows.
3. Belly appearing in walls.
4. Cavity widths getting larger the higher you check

An example of a corroded wall tie

Cavity wall tie corrosion and cavity wall insulation problems

Wall ties are vital for the structural integrity of a cavity wall, as they hold the inner and outer leaves of masonry together. Being made of iron or steel, they will inevitably rust eventually, but in dry conditions they should last for many years. When persistently damp, however, they can corrode much quicker, and replacing them is a costly and time-consuming process, involving cutting out dozens of individual bricks from the outer leaf. Replacing corroded wall ties becomes much more difficult in a building with cavity wall insulation, as the insulation itself has to be removed around each tie, and then replaced afterwards.
Anyone thinking of having cavity wall insulation installed should first have the condition of their home's wall ties assessed using the method described in BRE Digest 401, which specifies that two bricks should be removed on each elevation (at high and low levels) and tested for corrosion. Inspecting ties by drilling a hole in the outer leaf and looking through a boroscope is not a satisfactory way of assessing their condition, as the most serious corrosion is likely to be where the ties are embedded in the mortar. Wall tie corrosion is a growing problem, and mortgage valuation surveyors are increasingly recommending that wall ties should be inspected as a condition of a mortgage advance. It is likely that the widespread presence of retro-fill CWI will increase surveyors' alertness to the possibility of wall tie corrosion, and might therefore create delays when homes with cavity wall insulation are put on the market. Homes most at risk of wall tie corrosion are those built with wrought-iron "fish-tail" ties prior to 1920, and those built with galvanised steel "butterfly" ties between 1964 and 1981. Any house about to have its cavity walls insulated is supposed to be inspected and assessed for suitability by "a trained surveyor" (ie a salesman for the insulation installers), but I have never come across a case where the wall ties have been assessed using the correct method. In most cases the "surveyor" simply drills a hole in the outer leaf and measures the width of the cavity.

Conclusion. There are many people who have had cavity wall insulation installed, who have experienced no problems with internal dampness or wall tie corrosion, and have experienced increased levels of comfort and lower fuel bills. Unfortunately there are also people who have suffered serious problems, and who have found it very difficult - or impossible - to get these problems recognised and rectified. The cavity insulation industry, and government agencies, do themselves no favours by pretending that these problems do not exist. Our own opinion is that injecting insulation into an existing cavity wall is a bad idea, with the potential to create problems whose rectification costs will far outweigh any savings in fuel costs, or corresponding environmental benefits. The best way to add thermal insulation to the walls of an existing home is on the outside or the inside, where the installation can be carried out under controlled site conditions, and any subsequent defects easily spotted and rectified. Installing insulation in an existing cavity wall by drilling holes in the outer leaf and squirting something in - in the hope that everything will be alright, even though you can't see it - creates possibilities for problems.

Wall Tie Corrosion, indications and reasons for wall tie failure

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