Cavity wall ties keep two cavity walls together. Cavity walls consist of two “skins” in the masonry or brickwork, an inner and outer skin. The function of a wall tie is to prevent the skins from separating under the weight of the roof.
Older properties may have old wall ties known as “queens”. These queens are not the metal-type wall ties that are commonly used today. Queens are bricks that bridge cavity walls together. They are embedded into both the inner and outer skins. Queens do not corrode like metal wall ties, but they do encourage damp. This is because moisture can work its way over the queens and across the cavity wall.
Metal wall ties can cause lots of problems. The galvanised fish-tail wall tie, for example, rust when exposed to lots of moisture. When these wall ties rust, they expand. This expansion can often raise the mortar beds in the stonework of a property.
Common problems to do with wall ties include:
• The wall ties not being installed correctly
• There not being enough wall ties installed in the first place
Old metal-wire, and fish-tail wall ties are prone to rusting. This rusting can affect the structural integrity of the wall ties.
When fish-tail wall ties rust, they expand. This expansion can force mortar beds to enlarge and cause a blowing in the wall. Metal-wire ties are not as strong as fish-tail ties and are more likely to snap in the middle. Snapping metal-wire ties should not cause the mortar beds to rise, but it could lead to a wall collapse. A wall collapse becomes probable when as the inner skin falls from the outer skin.
Newer properties are usually built with stainless steel wall ties fitted. These wall ties are far less likely to rust.
Cavity wall fills are a popular way to insulate and reduce heat loss in a property, but often cavity wall fills can make corrosion worse. There is a term in construction called the “external leaf”. The external leaf protects the structure of a building from moisture penetration. Cavities allow water entering through the external leaf to drip down into the sub-earth. As the water makes its way down, it comes into contact with the wall-ties. But with cavity wall fills, the insulation can stay wet for longer. The insulation then acts as a wet blanket around the wall tie, keeping it wet for longer.
Unfortunately, not all properties are built with the correct amount of wall-ties. Some are not even fitted with the correct installation methods. These neglects, or faults, can seriously undermine a building’s structural integrity.
A professional will usually be able to determine if the wall-ties have failed in a property. In this case the professional will be looking out for:
• The coursing of the bricks
• Raised mortar beds
• Cracks around the windows and doors
A full inspection of the wall-ties will be necessary if the professional finds any of these problems. A typical inspection will identify the condition of the wall-ties and put this into a report. They may even take a sample of 1 – 2 wall-ties from each elevation of the property. This is because different elevations receive different exposures to moisture and wind. Taking extensive samples in this way is a good way to gauge the quality of the wall-ties installed.
Removal of the brick
Sometimes a cavity wall fill can make it difficult to identify the condition of the wall-ties. Endoscopes, for example, cannot “see” wall-ties if the cavity has had insulation installed, or a rubble fill. In which case, the removal of the brick itself is usually necessary to look at the wall-ties. In some cases the brick on the inner leaf may also need to be removed.
The survey is usually less intrusive if it is possible to use endoscopes to inspect the wall-ties. An endoscopic survey will usually happen something like this:
• First the surveyor will identify where the wall-ties are, using a metal detector. This is known as “isolation”.
• The surveyor will then drill a hole in the mortar, where the endoscope will pass through the wall.
• The surveyor will then use the endoscope’s camera to examine the condition of the wall-ties.
• The surveyor will make a report with the findings.
Wall-tie surveys follow a strict standard set by BRE Digest. This means that the inspections and replacement of wall-ties are always of a high standard.
If there is a good amount of “red dust” (a hallmark of advanced corrosion) then the wall-tie will need replacing.
There are two main types of wall-tie. Each one requires different installation methods.
Mechanical wall-ties: Sometimes the mortar is no longer strong enough for the replacement wall-ties. This means that they may need to go through the brick face instead. This will allow the wall-tie to fix in a good position on the inner and external leaf. These wall-ties are made of neoprene, a synthetic rubber that expands as the key of the wall-tie turns. This allows the wall-tie to grip better. The holes drilled in the brick will then be colour-matched to the brick and then filled using a method similar to crack stitching.
Resin wall-ties: This process is like the previous process, only the wall-ties are bound into the mortar itself and not the brick. Both the holes, and the wall-ties, are bound with resin. This affords the wall-ties a greater strength between the inner and the external leaf. Sometimes mechanical wall-ties can leave a “Polka dot” effect on the bricks. Because resin wall-ties are bound into the mortar and not the brick, there should be no Polka dot effect.
In both cases a “pull out test” is conducted to ensure the ties are of correct strength.
Dealing with old wall-ties
A metal detector is used to identify old, faulty wall-ties. The mortar around these ties is usually ground out and a silicone bed is then placed within the mortar bed. This process is then followed by a repainting of waterproof mortar.
Remedial cavity wall tie replacement, and wall tie repairs with Cemplast Preservation
To enquire about our remedial cavity wall tie replacement and wall tie repairs services email or get in touch with us on the phone here. Our expert specialists and surveyors will be ready to help.