You are best using a 9mm tie when you have a very high load requirement or a really harsh environment. 9mm ties are much stronger than 8mm ties, however for most wall tie requirements, 8mm ties will do the job for you at a much cheaper cost. See here
9mm wall ties have a much thicker central core that the 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.
9mm ties are also used when pinning very soft substrates (ie soft brick or aac, cellcon type light weight 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.
Remedial cavity wall tie for replacement of corroded and failed cavity wall ties
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