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Redoing bathroom floor. After removing tiles and underlayment and linoleum, the floor that's left is decrepit, uneven, wavy, has some old plumbing holes. It is not rotted though, the wood is sturdy.

I'm going to use Ditra.

With Ditra, is it necessary to have a plywood underlayment, or can I just install right over the floor as is (covering up the holes of course) and the thing will even itself out?

Barring that, if I do need to install a underlayment, can I use 1/8 plywood?

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    Personally, I would replace the sub-floor. Most of the work is already done (sub-floor exposed) and what's there now isn't perfect (uneven and wavy). – UnhandledExcepSean Jun 12 '14 at 14:59
  • Thanks for your reply! I actually hadn't thought of that. If I remove it, is one layer of 1/4 plywood enough, or should I also add a layer of cement board on top of that? Just to do it right. – Noob Jun 12 '14 at 15:18
  • I just got done looking it up and there are acceptable size less than 3/4" or 23/32", but it is dependent on joist spacing. I personally would recommend going with the 3/4" or 23/32" plywood or OSB for the sub-floor. It only costs a little more and the area is probably not that big. The added rigidity will be good to prevent flex in the floor which could cause your tiles to pop off the floor. As far as the underlayment, I will defer to more experienced people on this site; I'm a DIYer and not a pro. – UnhandledExcepSean Jun 12 '14 at 16:07
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    In a bathroom, use plywood, and consider using exterior/marine-grade plywood. OSB + bathroom + time is a bad combination. I'm not familiar with "Ditra", but if tiling over a solid but non-level subfloor, self-levelling gypsum cement is a common and effective treatment that does not require ripping the floor up, and results in a level surface that's a good tile base. – Ecnerwal Jun 12 '14 at 17:47
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If the subfloor is plywood no other ply is necessary; but you do need a flat surface for the Ditra. If the subfloor cannot either be scraped, chiseled, or sanded flat then self leveling compound may be required. If you use self leveling compound remember that the surface should be primed first - follow the product's prep recommendations. 1/8" plywood, in my opinion is not sufficient; unless it was set down on flat surface with troweled adhesive and stapled 4" o.c. I would recommend 1/4" min - which can be found at most flooring supply stores - specially formulated for flooring; i.e., all voids filled in the plys and water resistant glue.

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Underlayment is not required for Ditra, although you may need it to make sure your tile is at the same level as the flooring outside the bathroom. In either case your need to level and smooth the sub-floor first because Ditra mat will not fix a bad sub-floor. Self leveling gypsum like Encerwal said is a good fix but if the ripples in your decking are superficial and mild you can sand them out sometimes, and underlayment will mitigate deficiencies in the sub-floor to some degree.

Were I you I'd start by establishing a dadum (an arbitrary level line) around the room with a laser level. From this you can determine A: how far out of level your floor is. B: where the worst depression or bump is. and C: what your average height requirement is (the average height of all the spots where your tile needs to line up with something relative to the dadum). If any of the above is greater than an inch, tear up everything and start over. Less than half an inch, float with floor leveler. Less than an 1/8, sand out the bumps and go buy a lottery ticket because you're apparently very lucky. Cheers!

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Assuming the existing subfloor does not have excessive deflection in it, I would put down a 1/4" tile backerboard instead of plywood, and then put Ditra on top of that, and then tile. If you are using a 1/4" floor tile, that will give you pretty much a 3/4" final thickness which generally works well with other flooring.

If the subfloor isn't that solid, I'd pull it up and use a high-quality 3/4" underlayment.

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