I’m building a porch and, while I’m pretty confident about the framing, the contemplated stone tile floor is new territory for me. The floor will be partly over an existing concrete slab and partly over a new wooden joist system. I’ve gotten conflicting advice about the subfloor:
Option 1) Run the treated plywood subfloor right over the joint between the two substructures, tying them together. Then lay down cement backer board and stone tile.
Option 2) Make the plywood/cement board sandwich flush with the edge of the slab and put a control joint in the stone floor over the junction.
What does the communal/conventional wisdom say? And does a control joint in stone tile work better with some softer substance in it, or should I just score the grout and let it crack down the seam?
A sub-question: what thickness of stone tile is sufficient? The finished floor will only be about 8″ off the ground, so I’m limited to 6x joists. I’m making them 16″ o.c. and only 5′ long, with a slight crown along the lengthwise centerline of the floor, topped with 3/4″ treated ply. Will this be rigid enough to support 1/2″ stone tiles, or do I have to go for 1″ or even more?
Replies
Build the wood structure so that it is VERY sturdy. Deflection of 1/720 is what you're looking for -- nothing less is acceptable. (For ever foot of span, 0.01666 inch of deflection.) Be certain that you check the span charts and do this right.
Make the sub-floor heights match by whatever means you want.
Cover the entire area with Kerdi membrane -- this is what will separate the tile layer from the substrate layer, reducing the possibility of a major failure at the wood/concrete joint. Follow the directions on their website.
Then lay the stone tile.
Politics is the antithesis of problem solving.