The role of the cavity wall tie
30th June 2017
Categories Cavity Wall Ties
Every cavity wall will have ties built into it, and they play a crucial role in ensuring its safety and stability.
Any brick-built house developed over the last century or so is most likely to have been constructed using cavity walls and wall ties. Here we take a look at what they are for, why they are so important and what can go wrong.
Cavity wall construction and the radius of gyration
As we all know, a cavity wall comprises two layers, known as the inner and outer skin. Typically, the outer skin is made of standard red bricks and the inner skin is made from either similar bricks or breeze blocks.
One of the major considerations when it comes to stability is the radius of gyration. This essentially considers the slenderness of the wall, and in layman’s terms, we can approximate this to a ratio of width to height. A house brick is 102.5mm wide, and the height of the average house is 5.7m, which means a slenderness rating in the order of 1:55.
Given that a free-standing single course ten foot wall, with a slenderness rating of 1:29, can be observed to sway and flex in a strong breeze, we can see intuitively that a ratio of 1:55 is a very slender wall indeed. In fact, without support, a single gust of wind would blow it over.
The role of wall ties
Of course, the wall of a house is not free standing. It has lateral support from floor joists and internal walls, plus load from the roof joists, which provide additional stability. The problem is, of course, that all of this bracing and support is attached to the inner skin of the house, not the outer skin.
Thus, we are presented with a very strong, rigid and secure inner construction, surrounded by a slender outer skin that has only its own weight for support and could essentially be blown over by a gust of wind. This is where wall ties play their role.
Cavity wall ties are used to connect the inner and outer skin, and thereby use the overall stability of the construction to support the outer skin on the house. They are spaced at regular intervals, horizontally and vertically, and used properly, have the effect of increasing the overall thickness of the wall to the sum of the inner skin, cavity and outer wall, meaning a far lower slenderness ratio and a stable construction that will last a lifetime.
Ties have traditionally been made from mild steel, but these days, stainless steel and even plastic and composite versions are more popular.
What can go wrong?
Ties need to be carefully installed, so that they lend stability but provide enough free movement to allow for expansion and contraction in changing temperatures. If this balance is not right, it can result in cracks appearing or ties becoming disengaged, affecting the overall stability.
After years in place, mild steel ties are prone to rust, and this is probably the biggest wall tie issue that older properties can face, as it can lead to structural problems, and ultimately catastrophic failure of the wall.
For further advice and information or if you have any structural concerns about your property and would like to arrange a survey, please contact us and a member of our team will be happy to discuss your query further.