Whatever you do don’t use “crapboard” of any grade; if you must overboard with anything use WBP ply. What sort of tiles are you fitting? If ceramic/porcelain/stone, why are you overboading anyway? as Chri5 days, it's normal to tile directly onto the concrete screed even if your fitting vinyl tiles.
I also agree it’s not the right time of year to be tiling a cons. with the current weather & all, especially if the screed has only recently been laid; don’t lay tiles of any variety until it’s dried out or it will most likely end in tears.
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