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I am laying 12mm plywood on existing timber flooring (before laying Hardiebacker board & ceramic tiles). Apart from screwing the ply to the sub-floor, should I also use grab adhesive or thinset mortar?
The reasoning for thinset (I'm told) is that it helps to fill & support the plywood over any low spots in the old timber floor (18mm tongue and groove on 400mm c/c joists).
I would glue and screw down the plywood. If you use hardy backer board I would use a dash patch or leveling compound on that before you set your tile. A good alternative would be to do a mud job on top of the plywood that would take care of the leveling problem. A mud job is tar paper stapled to the plywood with a layer of stainless steel mesh nailed to that and a mixture of Portland cement and sand set on top of that. The cement is leveled and suitable for thin set application. This should last a very long time.
