Can I install Aqua Tile sheets on XPS insulation fastened to the roof timbers?
In principle, yes. You have to make sure that the screws are anchored in the roof timbers. The longest Aqua Tile screws are 35 mm long. If this is not long enough to accommodate the thickness of the XPS, you should use other screws. Be sure to use screws with an EPDM washer to guarantee a waterproof seal.