I’m a contractor looking for advice about installing slate tiles on an exterior wall. Our subcontractor did this 10 years ago on exterior grade plywood with thinset mortar and the tiles started falling off within 1 year. The advice we got at the time was that we should use galvanised expanded metal lath on the ply wood first which we did. That  lasted longer, but the tiles are falling off again.  The wall is exposed to the weather, and the freeze thaw cycle in the Toronto area. Should the substrate be cement board like durock or is this just a bad application for slate tiles?  I don’t want to change materials because the slate is also on an adjacent interior wall, so it looks like they run together.