Most people order the cheapest cavity wall ties that the builders merchants can offer. The only criteria being the length, which depends on their cavity width. But the cost of using the wrong tie is enough to send shivers down your spine. Using the wrong ties could ultimately mean one of the walls potential caving in or falling down completely.
The main code for wall tie specifications is Euro6 design or masonry BS EN 1996-1-2-2005 and EN 845-1 2013
Manafacturers state that ties should adopt a minimum of 2.5 per square meter. The very common HRT4 is a lightweight stainless steel tie, but is only suitable in areas with a maximum ambient wind of 27mph and a maximum altitude of 150m above sea level. The tie is only recommended where the two cavity walls are of a similar stiffness. The maximum permitted height using the HRT4 is 10m, the RT2 is very similar but has a maximum height limit of 15m.
The euro code 6 part 2 recommends movement tolerant ties between the leaves of a cavity wall to prevent masonry movement.
When choosing ties for your project, make sure instead of going for the cheapest, you get the right ones for the job.