I have a screened and roofed porch/lanai. Original floor is pressure treated posts, beams, joists and 1x6 decking. Former owner decided they wanted tile. They screwed a plywood underpayment over the decking, then laid mastic and tile. At some point, they drilled some holes through the grout so any rainwater that blew in could easily drain. Well, it drained down into the plywood and decking, causing rot.
I have removed the tile and rotted plywood and repaired damage to the subfloor (the original pressure treated decking). I want to put down a travertine floor. My plan was to screw Hardy cement backer board to the subfloor, then apply a waterproof membrane (such as Schluter), then mastic/tile. I want to ensure the floor is completely waterproof and there is no way for water to reach the subfloor or joists.
Do I need to attach the cement board with thinset in addition to screws? And if so, Do I need to put something over the original 1x6 decking before attaching the cement board? I can’t imagine putting thinset on the 1x6’s. I would hope that laying a waterproof membrane over the entire floor would alleviate the need to tape the joints.