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Old 09-09-2017, 05:13 PM   #1
dan1976
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What goes on plywood subfloor?

Hello, I'm redoing a bathroom and the subfloor looks to be exterior grade plywood. I'm planning on doing a Kerdi shower with a mortar base and was wondering about what goes on top of the plywood both inside and outside of the shower.

In the shower: The video shows 4mL poly and metal lath right over the plywood. Is that what people do or is there are any other preferred method out there?

Outside the shower: Any recommendations for outside the shower in the main area of the bathroom. Do you need to lay cement board or Ditra before you tile? I'm using 30" by 30" porcelain tiles.

Thanks in advance for any advice!
Dan
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Old 09-09-2017, 05:23 PM   #2
chuck stevenson
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Welcome Dan
Have you checked your floor system with the Deflecto tool in the dark blue banner above?
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Old 09-09-2017, 05:48 PM   #3
jadnashua
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Assuming your joists are adequate, for a tile that large, your subfloor will need to be VERY flat to work. While there is a method to install tile directly on TWO layers of plywood, you are generally better off using something on top of the ply. At a minimum, the subfloor must be 5/8" T&G or thicker OR have all joints blocked so it locks them together so you won't get a sag along the joint. Two layers of ply gives you a bit extra margin, but one, properly installed and in good condition, will work.

Your choice on Ditra or to use a cbu...Ditra is, IMHO, easier, to get home, easier to install.
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Old 09-10-2017, 11:41 AM   #4
dan1976
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Thanks. So I'm not entirely sure what type of wood the 1st layer of subfloor is, but it looks like it's 3/4" and then the 2nd layer of plywood is 1/2". I don't have all the info I need to use the deflecto tool, but I'm hoping I'm safe with that 1.25" total.

I agree with your point about it being very level. Would it be wise to do a self-leveling membrane over the plywood and then the ditra? If so, are there any specific directions for the type of thinset on top if the tile is porcelain? Not sure if it matters, but my big tiles are also rectified.
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Old 09-10-2017, 03:53 PM   #5
rmckee84
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Do you know if your joist structure is adequate?
Is the first layer tounge and groove?

If I were you I'd make sure everything checked out structurally, then proceed with either a cement backer board or Uncoupling membrane like ditra.
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Old 09-10-2017, 04:29 PM   #6
jadnashua
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Download and read the Ditra installation handbook...it will answer all of your questions regarding prep and materials.

You need a modified mortar to anchor Ditra to the plywood, and generally, an unmodified on top of it. The thinset on top does not stick to Ditra, it's locked in place because of the shape of the pockets, but that pocket is flexible, which gives it the flex. Therefore, the bond to the tile is not stressed, and an unmodified works fine IF you install things properly. Schluter does have a modified thinset they approve for use on top of their membranes if you insist on using a modified there, but in most cases, it just adds cost unless you've got an unusual situation. You can't just use any modified on top of the membrane, or you risk problems.
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