You are best using a 9mm stainless steel wall tie when you have a very high load requirement or a really harsh environment. These heavy-duty cavity ties are much stronger than 8mm wall ties; however, for most wall tie requirements, 8mm ties will do the job for you at a much cheaper cost.
9mm wall ties have a much thicker central core than 8mm wall ties. They are generally used for cavity widths greater than 100mm where the core of the 8mm tie has too much lateral flex to meet engineering requirements. As brick ties for existing wall brickwork ties in high-load scenarios, the 9mm is the professional's choice.
9mm ties are also used when pinning very soft substrates (i.e. soft brick or AAC, Cellcon-type lightweight concrete blocks). Softer bricks may require pilot holes of as little as 4mm with 8mm diameter ties in order to reach BRE DD140 tensile load requirements. In this instance a 9mm tie should be specified so an achievable diameter pilot hole can be used.
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