Exterior Tile Installation Over Plywood . This involves screwing a sheet of plywood over the top of the existing floorboards / chipboard or plywood to provide extra rigidity and a flat surface onto which to tile. One common misconception concerning tiling over plywood is that you can use any latex modified thinset (a portland. Ditra Over Plywood ⋆ 🌲 from theplywood.com Thinset is the wet base that you first trowel on the plywood to make the. We show you how to apply redgard elastomeric waterproof membrane to. Make sure your deck is clean of dirt, grime and other objects before installing wood decking tiles.