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Unread 11-18-2019, 11:25 AM   #1
tracism
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What Thickness of Floor is Needed?

Hi I am tiling 12 x 24 tiles in a 4 x 6' bathroom with a deflecto rating of L/514.

I have 5/8s sub floor on 10" joists/16" centers.

Should I add 1/2" plywood, then 1/4 cement board?

Or with such a small space am I safe to use 1/2" cement board (on top of sub floor) and tile on that?

Of course following all industry standards for application process.

Thank you!
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Unread 11-18-2019, 11:32 AM   #2
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Welcome back, Traci.

5/8" plywood is the bare minimum thickness to use for subfloor. If it was new and in perfect condition, I might consider it under certain circumstances, but otherwise I'd use 1/2" plywood over the existing subfloor, followed by a tile backer and tile
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Unread 11-18-2019, 11:35 AM   #3
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So 1/2 plywood and 1/4 cement backer...then my heat mat and tile?
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Unread 11-18-2019, 11:35 AM   #4
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thank you for your quick response
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Unread 11-18-2019, 11:42 AM   #5
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If I put a 1/2 inch plywood down, do I absolutely need a cement board? There is no water that will be splashing or draining?
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Unread 11-18-2019, 11:42 AM   #6
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Yup, 1/2" ply, then backer board , then heat mat.

You're gonna love the heated floor.

Don't forget that you need to set the backer in mortar.
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Unread 11-18-2019, 11:49 AM   #7
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Traci, using a tile backer board is the safest bet to a long lasting tile installation over plywood. The backer isn't for water proofing, it's to provide a very stable substrate to set tile on.

What tile warming system are you using?
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Unread 11-18-2019, 11:55 AM   #8
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If height is a concern, just use a tile membrane like Ditra or Stratamat. They're only 1/8" thick.
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Unread 11-20-2019, 04:34 PM   #9
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The ditra looks complicated...like there are a number of ways for it to fail, i.e., not pressing hard enough to embed the mortar underneath. The cement backer is straightforward. Can any of you advise on that point? Height is an issue, transitioning from my hardwood floor.
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Unread 11-20-2019, 06:42 PM   #10
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Traci,

If a heated floor is still in your plan, and height is a consideration, Ditra Heat mat applied directly to clean plywood is the way to go, and is an approved Schluter application. You can ease the DH mat installation by purchasing the individual sheets instead of a roll.

You can of course install a cement backer board first and then bond the DH mat onto that, but that'll add about 5/16th to 3/8th" in height. The regular Ditra heat mat alone will add about 5/16th.
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Unread 11-20-2019, 07:09 PM   #11
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The tricks to installing any membrane is to first wet the plywood so it doesn't suck moisture out of the thinset when you spread it out, making it thick and hard to embed the material, and then, spreading out too much that you can't cover it before it skins over. You need a quality thinset and mix it right. If you pull up the mat when you think it's good, if it's fully covered, just press it back done. If it's not, you need to readjust your technique.
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Unread 11-21-2019, 07:39 AM   #12
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Okay, I am going with Ditra over the 2 layers of plywood, the last being the newly installed. Do the screw heads need to be below surface or can they be at surface level?
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Unread 11-21-2019, 07:58 AM   #13
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A good go/no go test is if you can scrape the blade of a putty knife across the heads without it catching.

Are you all set on the proper way to install the 2nd layer of plywood, Traci?
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Unread 11-21-2019, 08:33 AM   #14
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I screwed in a 6 inch pattern, 90 degrees to subfloor. What next?
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Unread 11-21-2019, 08:34 AM   #15
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and the 3' level stays in the bubble.
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