I am remodeling my half bath and there is 5/8" plywood subfloor over 7 1/2" joists that are 16" on center. The bath is small - 3' x 5' floor area so the longest joist run is 5'. I am going to install 9" x 9" porcelain tiles. I bought hardiebacker ez grid 1/4" thick cement board and the instructions say that there should be a minimum of 5/8" plywood underneath. I have read websites saying that 1 1/4" total subfloor thicknes is generally recommeneded. Should I add another 3/8" layer of plywood over my 5/8" plwyood?

Would another option be to put 2 layers of the ez grid 1/4" cement board over the existing 5/8" plywood. All things being equal, I would rather go with plywood as it seems easier to work with and less dusty when cutting.

I bought premixed thinset - Mapei type 1 tile adhesive and wondering if this was a good choice for underneath the subfloor and tile? I read a few forum posts about premixed thinset not being as strong as powedered? Thank you!

Dave