Designing Kitchens and Bathrooms - new tile floor

Im gonna lay 12' x 12" tile in the bathroom...I have a subfloor and 5/8 underlay already down. Can I tile directly to this? I'm assumming not due to flex issues....whats my next step? :confused: Thanks for any help.

Can you be more specific about what kind of layers are on the floor now? What size joists and their spacing and unsupported span? Can you tell if the 5/8" is square adge or tongue and groove?

Can you be more specific about what kind of layers are on the floor now? What size joists and their spacing and unsupported span? Can you tell if the 5/8" is square adge or tongue and groove?

Not sure about the subfloor (looks to be 1"x8')....the house was built in 78...the 5/8 underlay is square edge, not tounge and groove...the joists are 16" on center. The floor feels very solid. There is one weak spot that gives just slightly when walked on, but it will be under the jacuzzi tub. Thanks for your time tileguy, I really appreciate any help!!!

Put a 1/4" cement board down over the 5/8" ply. Embed the board in a layer of thinset combed with a 1/4" square notch trowel. Use 1 & 1/4" galvanized roofing nails (hot dipped are best) or backerboard screws. Tape the board seams with mesh tape and coat with a thin layer of modified thinset, then set your tile.

Put a 1/4" cement board down over the 5/8" ply. Embed the board in a layer of thinset combed with a 1/4" square notch trowel. Use 1 & 1/4" galvanized roofing nails (hot dipped are best) or backerboard screws. Tape the board seams with mesh tape and coat with a thin layer of modified thinset, then set your tile.

can I buy this cement board or do I make it?

Cement board is a generic term that cover such marketing names as Hardibacker, Durock, Permabase and Wonderboard. All can be bought at various home centers and tile retail stores

just curious: Can I just screw the backer board down to the underlay and avoid the thinset? Whats the thinset doing for me?

I don want to meddle with an expert like TGB, but since he's not responded...

The thinset under the board fills any voids you might have between the cementboard and the plywood. If you just screwed it down, you'd get small air pockets that would eventually flex and deteriorate your nice tile work.

I've been told: Screws hold the cementboard down, Thinset holds the cementboard up.

It's needed.

I don want to meddle with an expert like TGB, but since he's not responded...

The thinset under the board fills any voids you might have between the cementboard and the plywood. If you just screwed it down, you'd get small air pockets that would eventually flex and deteriorate your nice tile work.

I've been told: Screws hold the cementboard down, Thinset holds the cementboard up.

It's needed.

Thanks....Makes perfect sense. My rational thought was that my wife will get bored and want new tiles eventually. If I could just screw the backer, then It wouldnt mess with the underlay next time Im down there:thinker:

Thanks for the help guys, you've been invaluable....:)