ready253:

The first order of business is determining how stiff the existing floor is. This depends on your joist thickness, span, and spacing as well as composition such as sawn or Truss Joist Internationals (TJI). The thickness and composition of the existing subfloor, fastening schedule, and whether or not it was adhered to the joists will effect floor deflection also. Stiffer is better, even it it means sacrificing being flush with the existing oak floor.

Ditra is an uncoupling membrane, not a "backing board" to add stiffness. I wouldn't lay floor tile without it.

Michael Byrne wrote a book published by Taunton press on tile. I'd get it and do whatever he recommends.

Large format tile requires an EXTREMELY stiff floor to not crack. It's not as simple as you think. Earch online for the Deflectometer to find out what type of reinforcement that the joists may need FIRST.

Or you could get a detailed estimate from a highly reputable firm and see what they would recommend were they to do the job.

I have used Ditra very successfully with a few projects using 16x16 stone tile and another with 12x12 porcelain tile. I like that Ditra is very thin compared to backerboard and provides an isolation membrane. Ditra has a great handbook that provides specifications on joist and subfloor requirements. I picked a hard copy up at a local tile store, but they also offer them in PDF format online. In some cases with Ditra you need two plywood/particle board underlayments on top of the joists (which is something I found already in my house under the carpet and vinyl flooring) for certain sizes of tile.

Here is a link that might be useful: Schluter Ditra Handbooks

Thanks for the replies everyone.
After some calculations, looks like I have deflection rating of L/380 and I'm good to go with porcelain tile :)

The current 3/4" plywood is good condition, but I would like to add another layer of plywood for it to be perfect. So, where do I go from here to get my new tiles flush with the 3/4" oak floors?

Could I use a 1/8" plywood on top of the 3/4" plywood subfloor, add 1/8" Ditra, add another 1/8" for thinset, and finally the 3/8" tile?

Thanks in advance.
Jesse

This post was edited by ready253 on Sun, Mar 23, 14 at 4:51

Jesse:

1/8" of plywood isn't going to add much stiffness, it would be more like a shim. I would add 1/4" with construction adhesive and a heavy fastening schedule. Check your blocking between your floor joists; more is better.

Don't forget, there is a layer of thinset for the Ditra too. Don't fuss too much about the floors being flush.

I reblocked all the joist today using 2x10's to make it more rigid.

What kind of construction adhesive do you have in mind? As for adding the 1/8 plywood, I only thought about adding it to the layup to give me a flush transition. I don't mind a little "hump", but it's a requirement from the wifey.

Any other suggestions?