The lads in my Topps store also said that 6mm backerboard is now to be used on floors and they don't stock the 12mm anymore, can you confirm this Gaz@Topps?
You talking about ply or backerboard? With ply the drill/driver will act as a self countersink so you can screw down flush. Backerboard - can't help as I have never used it yet.🙂
Grumpy
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Oh, also with ply, it is a good idea to pilot drill wider than the shank of the screw. If you just drive screws straight through ply it can have a trendancy to lift the ply slighty so you get voids underneath. If you pilot drill, the head of the screw clamps the ply down tight to the boards underneath.