Flooring Tile - First time ceramic install

I'm renovating an upstairs bath, including new ceramic floors. This is my first ceramic job, & I want to be sure I have all the details straight for a proper job.

The bathroom is 58x63, with an adjacent dressing area 64" wide. They're separated by a wall, but there's none between the dressing area & bedroom. I plan to take the flooring back the length of five 12x12 tiles+grout spaces. I've already gutted both rooms, taken up all the old flooring of vinyl + luan in the bath, & carpet + padding in the dressing area,cleaned the ply, put in extra screws to eliminate squeaks, & sanded down any uneveness between board edges.

My subfloor is 3/4 square edge plywood, 2x10 joists, spaced
14" & an unsupported span of just under 12'. I plannned to put 1/2 Hardibacker over this, then tile. But after reading many posts, I'm thinking of putting down 1/2 ply over the 3/4 stuff, then 1/4 CBU. I have a bag of Flexbond fortified thinset. Suitable under both the CB & tiles?The subfloor has a few dips of between 1/16 & 1/8". How to deal with?

Thanks for all your help.
Joanne

Go with the extra 1/2" ply, it should be BC grade exterior glue, pretty common, sold at HD. Leave a 1/8" gap in the seams and fill that with a bead of cheap latex caulk. The top layer of ply should be set perpendicular to the old sheet and no seam on seam with the old, leave about 1/4" around the perimeter of the room. Set screws at about 6" intervals throughout the board and stay about an inch in off the seam.
You can use a cheap thinset, unmodified will be fine under the cement board. Use a 1/4" square notch trowel to spread it evenly before dropping the cement board into it. Use 1 & 1/4" galvanized roofing nails or backerboard screws also at 6 to 8" intervals. Hardibacker's 1/4" cement board has indicators on it for placement of the nails/screws. Tape the seams with mesh tape and a thin smooth coat of modified thinset, the Flexbond will be fine for that and use it also to set your tile
The thinset under the cement board is not there for bonding but rather to fill in gaps and voids between the plywood and the cement board. The nails/screws hold the cement board in place. It is required by the manufacturers as part of their warranty.

Soundsl like you are going to do a great job. Good luck and let us know how it turns out.

Thanks, for your finely detailed advice & encouragement. This is exactly the kind of information I need to do a good job. Most books just gloss over the details.

One more detail... the subfloor has a few small humps in it, mostly over the joists. When I check them with a 4' level, I've got gaps under it in a few spots of between 1/16 &1/8". Is this something I should be concerned about or am I just overthinking the whole thing? Will the new 1/2" ply and thinset under the CBU take care of it?

Thanks again.
Joanne in VA

the subfloor has a few small humps in it, mostly over the joists. When I check them with a 4' level, I've got gaps under it in a few spots of between 1/16 &1/8". Is this something I should be concerned about or am I just overthinking the whole thing? Will the new 1/2" ply and thinset under the CBU take care of it?

Thanks again.
Joanne in VA


Yes, the ply and thinset will take care of it. What you described is exactly the reason for the thinset under the CBU.:)

I've finished with the 1/2 ply, which I installed perpendicular to the subfloor ply, so it's parallel to the joists. I'm ready to start laying out the CBU. Does this go down perpendicular to the 1/2" ply, so that it's parallel to the original subfloor, or the other way around?

Thanks again.
Joanne