We've finished the god-awful task of ripping up the old bathroom subfloor (1/4 vinyl underlay, 3/4 plywood, 1/4 masonite, 3/4 plywood (with a "wood putty" patch that was 2 X 3' and 2 inches thick in spots!!) before we finally got to the ORIGINAL porch floor). No wonder the room sloped so badly!!! They never even attempted to level it.
The original floor featured 2X6 beams spaced about 14 inches apart with a 12 foot span. We left the porch floor intact and leveled the floor with new joists set 14" apart and screwed into the original porch flooring and wall beam (after making sure the original floor was solid). To that we secured 3/4" plywood. It wasn't ideal, but we were already 5+ inches below the floor with the tub and short of blowing up the room, I wasn't sure we could even sister joists to provide enough stability if we went deeper. Life is good for now.
HOWEVER, now I'm left with some questions. Everything I've read recommends Ditra for underlayment but I can't find it in our area (NW Iowa). I'm left to choose between hardibacker/Durock and Denshield. If I were to use the Denshield, which thinset should I use? (We have a Menard's nearby but all other shopping options are hours away. I'm a little limited in my options here- I think they only carry Mapei!!). I'd read recommendations for Kerabond with Keralastic. Any thoughts? What kind of waterproofing would I need to do for the floor prior to setting the CBU?
I am not sure how your joists and the entire subfloor system is now built. Did you say your 2x6 joists span about 12'? If so, that is way too much.
If you can't find Ditra, I recommend either Hardie or Durock. You would use a modified thinset for any of the three CBU's, both to install it and to install tiles over it. Mapei is a world-class company with great products. You shouldn't need any waterproofing for a bathroom floor. But if you were to use one, it would go over the CBU, not under it.
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