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Material thickness and proper subfloor?

We want to put porcelain tile in our tiny bathroom (roughly 7' x 5 1/2'). We can see that what is currently there is the original yellow pine tongue and groove flooring, then plywood, then some tar-like vinyl layer, then tile. This all sits on 2 x 10 joists that span 13 feet, spaced 16" apart on center. We have ripped out the ceiling in the room below and will sister these joists, hopefully with 2x10s along the full length, to support a 300+ pound cast iron clawfoot tub. With the ceiling gone, we can see that there are some spots near each of the fixtures where the some of the original tongue and groove boards have been pulled out to make way for plumbing.

My first question is: Should we rip out everything including the yellow pine hardwood and start fresh from the joists, or rip out just the tile and plywood and build on top of the yellow pine?

My next question is that I am reading a lot about people having two layers of plywood. Why is this? I have always understood that we should use a plywood subfloor, then a cement board underlayment on thinset, and then the tile. Am I wrong? Please advise as to what thickness of the different materials I need depending on whether or not we rip out the hardwood and start fresh from the joists.

Thanks so much for your time!

Re: Material thickness and proper subfloor?

If you will be doubling the 2x10 joists for entire bathroom,I would tear out everything down to the original pine boards if you can. Then do whatever patching you need to the missing sections.Then put screws into the original boards and put down 5/8 plywood and the concrete backer board with the tile over top.
Mostly two layers of plwood is used when the floor is made up of 2x8 joists with a lot of flex. In your case if you double the 2x10's your floor will be very stiff so you shouldn't need the extra plywood layer.

Re: Material thickness and proper subfloor?

Thanks for your straightforward reply. If we went this route, what thickness Durock/Hardiboard would we need? Also, do I screw into the pine boards over the joists, or specifically not the joists? (I've heard arguments favoring both of these methods.) My only concern with leaving the pine boards is that when all is said and done, this will be substantially thicker and higher than the pine wood floors it meets up with in the hallway...

If we went ahead and removed everything down to the joists to make the job easier for the plumber replacing the cast iron pipes, then how many layers of plywood do I need and what thickness? What thickness cement board?

Thanks again for your help.

Beer Belly
Re: Material thickness and proper subfloor?

Sorry to hijack your thread.....what are you opinions on Hardi-Backer on top of the Plywood for Tiling ???

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