On top of all the above BS 5385 : Part 3 : 2007 also recommends in clause 6.3.4.2 “If it is considered necessary to further reduce or eliminate the risk of movement, an additional layer of sheets of minimum thickness 10mm, resistant to moisture and thermal movement, should be screwed over the plywood at 300mm centres ensuring that the joints in both layers of sheets do not coincide”.
In other words 6mm backer board is to thin.
How would you make sure you're wooden floor is rigid enough?you wouldn't use backer board to *(further reduce or eliminate the risk of movement,) or i wouldn't. For me it is just to give a suitable sub straight to tile on. so in that respect you can use 4mm.:smilewinkgrin:* *