• Why ply anyway? I would replace the current boards with P5 chipboard then use a decoupling membrane or thin foam-core tile backer with ‘flexible’ tile adhesive. Should end up around 22-27mm from the joist.

  • Cheers both (@Sheppz). I am only repeating what a terrazzo reseller told me, which no one else has been up for challenging, although they do sound a bit puzzled when I tell them.

    Sub 27mm from joists is perfect, and would result in a 20mm step up from board to new floor which I am fine with (almost like the idea).

    I'm toying with the idea of 18mm ply + 9 mm concrete board think. Will ask the builder what he would do normally and try and work out some sort of rational middle ground.

    This sounds sensible: http://www.ultimatehandyman.co.uk/how-to­/tiling/tiling-a-floor


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