Most properties constructed after 1920 have cavity walls. These walls are made up of a brick or stone outer skin, a cavity, and an internal skin that is usually brick or blockwork. The outer skin of the wall is secured to the inner skin by metal ties that span the cavity. Many early cavity wall ties had no protective coating or coatings that have deteriorated over time. Without protection, the metal corrodes and this creates two distinct problems: firstly, the wall ties can fail making the walls unstable; and, secondly, the corroding metal expands and this creates horizontal cracks along the mortar joints.
Cavity wall ties are generally spaced horizontally at 900mm centres and vertically, four to six brick courses apart (300 – 450mm), in a diamond pattern. Expansion of the corroding metal causes the horizontal mortar joints to crack, and this regularly spaced, horizontal cracking is one of the distinguishing features of cavity wall tie corrosion. There are other reasons why walls can crack, though, and a careful assessment is required to identify the precise cause and appropriate remedial work.
There can also be multiple causes of cracking
There are different types of cavity wall ties that are used in buildings. Those that cause cracking damage are often of a mild steel 'fishtail' type, and each can expand in excess of 5mm. Because there are around ten rows of cavity wall ties in a typical house, it is possible for this combined expansion to lift the roof by 50mm and cause the walls to bulge. Cracking damage tends to be worse on the upper parts of walls because there is less weight, above, to resist movement.
Mild steel wall ties affected by corrosion can lose section, and the metal comes apart as it expands.
Not all cavity wall ties cause cracking damage. Galvanised 'butterfly' wire ties are made of metal that has about the same thickness as a coat hanger and, unless the mortar joints are especially thin, they can corrode without causing visible damage to the mortar joints. In 1981, the specification for the thickness of the galvanised coating of wire ties was tripled, so zinc coated ties used before this date are particularly at risk from corrosion. This particular problem is made worse because the protective coating was easily damaged by bricklayers as they used their trowels to tap the ties into the mortar beds as the walls were built.
Butterfly wire ties do not usually cause cracking damage unless the mortar joints are very thin. In the absence of this visible evidence, corrosion of these ties can go unnoticed, and, because the metal is relatively thin, they can fail completely without revealing any apparent evidence of an underlying problem.
Cavity wall tie remedial work involves three distinct stages:
1. Remedial wall ties are installed to stabilise the wall;
2. The existing wall ties are assessed and treated, if necessary, to prevent any further cracking damage or movement;
3. The base of the cavity walls should be inspected and, if necessary, cleared of debris.
There are various types of replacement cavity wall ties that can be used for repair work. Most are stainless steel and are secured mechanically, using a screw or bolt action, or with epoxy resin. Ultimately, the choice of remedial cavity wall tie is determined by the particular property type and construction.
Here are three examples of stainless steel, remedial cavity wall ties. The top two are mechanically fixed, but the tie pictured at the bottom can be screw fixed or resin bonded. |
After new remedial cavity wall ties have been installed and the outer skin of the wall has been secured in position, it is not always necessary to treat the thinner, butterfly wire ties, but there are circumstances when this work is necessary. Treatment is always required for thicker, mild steel ties because cracking damage will otherwise continue. This treatment usually involves removing or treating the outer end of the existing wall ties, but, in some cases, they have to be completely removed. This is because some types of construction, for example houses built with black ash mortar, aggravate cavity wall tie corrosion, and this can cause the horizontal mortar joints to crack in both the outer and inner skin of the wall. The only solution to this problem is complete removal of the corroded wall ties.
As with the selection of suitable types of remedial cavity wall tie, the decision to treat the original wall ties and the method of treatment is influenced by the conditions at each particular property. Britannia Preservation's surveyors carry out a thorough assessment before recommending appropriate action based on specific site conditions.
The final part of the remedial work--inspecting and clearing the external cavities--is very important but is often overlooked. Drilling to install remedial cavity wall ties and the treatment of existing ties allows mortar debris to drop down into the base of the cavities. This debris can bridge the damp proof course and it allows rainwater to pass from the outer to the inner skin of the wall, which can cause dampness, deterioration, and decay on the inside. It is also common to find debris and mortar droppings in the base of cavity walls that have been there since the wall was built.
This is a thermal image of a cavity wall. The yellow coloured area at the base of this wall is where debris blocks the cavity. If this debris is not removed, rainwater can pass through the wall and cause dampness on the inside. The debris also reduces the temperature of the wall, and this makes it more susceptible to condensation. We recommend that the bases of cavity walls are inspected when cavity wall tie remedial works have been carried out. Any debris found in the cavity should be removed and replaced with insulation where required. |
The problem of debris in cavity walls is made worse if the cavity is filled with insulation, because the debris prevents even distribution of the insulation. Instead a void is created which is much cooler than the surrounding insulated part--this is referred to as a thermal bridge. These cooler parts of the wall are prone to condensation, and, for these reasons, it is essential that the bases of cavity walls are inspected and debris removed when cavity wall tie remedial works are carried out.
Cavity wall tie remedial work is a major structural alteration to a building and is not a task to be undertaken lightly. Britannia Preservation are Chartered Building Surveyors, Corporate Building Engineers, and members of the Property Care Association, qualifying us to undertake this work. Cavity wall tie corrosion can be dealt with, but always ensure that any person or organisation you engage to undertake structural works has appropriate qualifications and experience. Do not be afraid to ask for recommendations and seek genuine feedback from previous customers.
© Britannia Preservation 2011