been thinking about this thread all day.... simply put... i have never seen a tile lift from a well installed, primed section of plywood with a decent adhesive and a well laid tile. as i said, the bastard is in the number of screws you need to go through to make the ply overboard totally inflexible... and in response to aussie tiler.... you are effectively creating a cross ply section with overboarding which makes it vastly stronger. like anything, if it is done well it will last.