Is 1/2 inch plywood over 3/4 inch subfloor adequate for 12x24 ceramic tile?

Question

We took up old ceramic tile and found no cement board or other underlayment material underneath. However, I did find that the previous installer had put another sheet of 1/2 plywood over the 3/4 inch plywood subfloor. No nails or screws in that top sheet of plywood. Guessing they just glued it to the 3/4 inch subfloor.

Is this adequate to install new 12 x 24 inch porcelain tile if I screw that top plywood onto the subfloor, ensuring the screws are every 6 to 8 inches apart and not screwed into the joist?

Or, do I need to get rid of that 1/2 plywood sheet and install cement board, or something like Ditra?

Thank you. Kicknback1

Answer 1

I suppose if the floor won't get wet, it would be perfectly fine to just lay the porcelain tiles directly over the plywood with thinset.

If the 1/2 top layer plywood was glued to the 3/4 plywood, then it's effectively a 1-1/4" plywood.

Answer 2

Ditra works fantastically well, when you follow the instructions... and it demands slightly less plywood than some other underlayments. I've done a few bathrooms, a laundry room, and a couple kitchens with ditra. Not a single crack, even in locations where you used a little less ply than called for.

There's a ditra install manual on the web, just plug in your values and you're good to go.

Answer 3

I would not install porcelain tile on plywood. I'd only use a cement board.

I would not use thin set if it needs to be waterproof.

However, deflection is the key element in the performance of tile. Too much deflection and the tile (and/or grout) will crack. Therefore, I'd check the size, spacing and span of the floor joists before starting.