Do I need mortar between 1x6 deck boards and cement board?

Question

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.

Answer 1

A membrane won’t be adequate. If you want ‘waterproof,’ think shower pan. Not literally but by making this area semi-vitreous, there must be a drain or drains and a slope.

If you’re trying to avoid the disaster of the previous owner, I wouldn’t recommend the travertine, unless you put in a drain.

Have you considered Saltillo tile, limestone, or another type of stone? You can make wonderful, colorful installations indoors and out.

Regardless of materials used, the water will have to go somewhere and if you post some photos of the area, better suggestions can be made to make a beautiful and practical new flooring solution.

Answer 2

Your addition of cement backer is a good idea, I usually do put down with thinset and screws. If you use a membrane it should be fine even with travtrine , I would make sure their is no standing water on the occasional rain showers. And use a sealer on top, even something like Thompson’s water seal will be helpful , my step dad did an overlay and water sealed it in 99 and it has held up well although he has resealed because the stone is soft and the main walk path was was showing wear so he re seals when he notices the wear.