How to identify cavity wall tie defects.
Since the late 1920’s houses in England started to be built with cavity walls. This is when two leaves of masonry are tied together with metal wall-ties to form the external walls. Since the 1950’s cavity walls have become the standard form of construction for houses.
How do you recognise a cavity wall?
By looking at the pattern or “bond” of the bricks. If all the bricks are laid with the long edges (‘stretchers’) facing you, the wall is probably a cavity wall. If alternate bricks are laid with the short edge (‘header’) facing you it’s probably a solid wall.
If the wall is rendered, a cavity wall usually measures 10½ or 11 inches and a solid wall 9 or 13½ inches, depending on the number of bricks in depth.
Since the mid-1980’s wall-ties have been in stainless steel, but early forms were not and have a tendency to corrode and expand when exposed to moisture. This can force apart the bricks and can lead to defects such as:
- Bowing of the brickwork
- Diagonal cracks through mortar-pointing
- Horizontal cracks along mortar-beds.
Because of their light weight, the corrosion of butterfly wire ties produces almost no detectable external symptoms. The tie can corrode right through without producing any evidence externally of what is happening.
But fishtail and vertical twist ties with their greater bulk of metal do expand enough to cause a visible splitting of the mortar bed in which they are fixed. This photo shows the old type of wall-ties on the left and modern types on the right:
Typical signs of cavity wall tie defects
This photo also shows typical signs of what to look for and a little tip is to look for horizontal cracks every six courses of mortar-beds as ties are placed every 450mm in height (450mm / 75mm = 6 courses)
If a typical 3-bed semi-detached house needed 100% cavity-wall tie repairs to each of the 3 elevations the cost would be approx. £1,500 + VAT. A good building survey will identify any potential or existing problems.
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