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Old 01-14-2009, 02:16 PM   #1
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Subfloor Prep for Tile


I'm installing a glazed ceramic tile floor in my bathroom and my current floor consists of a 3/4 subfloor and an additional 1/2 layer of underlayment for a total floor thickness of 1 1/4". I believe that this is thick and sturdy enough to support the tiles.

The old floor tiles that I removed were thinseted directly to the top plywood layer. Can I just set the new tiles directly to the plywood since the thinkness is already acceptable. I am unable to remove the top underlayment layer and replace with CBU. Can I seal the underlayment with Redgard or Blue Seal to ensure that water won't damage my subfloor?

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Old 01-14-2009, 02:59 PM   #2
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Subfloor Prep for Tile


the subfloor and underlayment will both need to be plywood. Yes that is thick enough if your joists are 16" on center.
I would add on Ditra underlayment to be for sure about deflection.

good luck

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Old 01-14-2009, 08:27 PM   #3
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Don't wanna step on Floorwizard's toes, but I don't like going direct to plywood with mortar. The water in the mortar can be absorbed into the wood, causing swelling/delamination. Even Ditra needs to be mortared down to the subfloor.
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Old 01-14-2009, 09:09 PM   #4
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Subfloor Prep for Tile


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Originally Posted by Mudd View Post
Don't wanna step on Floorwizard's toes, but I don't like going direct to plywood with mortar. The water in the mortar can be absorbed into the wood, causing swelling/delamination. Even Ditra needs to be mortared down to the subfloor.
What makes you think one would not follow the installation instructions for Ditra.. thus over mortar.
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Old 01-14-2009, 09:25 PM   #5
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Subfloor Prep for Tile


Wood and water don't mix. That was my point. With or without Ditra, you're putting water on the decking and that can cause problems. I like 1/2" cement board. The best of all is creating a mud bed with lath inset, but that's over the head of most DIYers.

How is the existing 1/2" layer applied? Can it be pulled without too much difficulty (not glued or ring-shanked, just screwed, for example)?

I just measured a job that someone else did where they put a beautiful tile job on plywood that needed replacement. Wet feet from getting out of the shower let water get on the grout, down into the plywood subfloor and the surface layer of the plywood delaminated; the tiles started sounding hollow in that area and the grout lines cracked. Now an expensive tile job has to be replaced.

Start with a good foundation.
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Old 01-15-2009, 06:30 AM   #6
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Originally Posted by Mudd View Post
Wood and water don't mix. That was my point. With or without Ditra, you're putting water on the decking and that can cause problems. I like 1/2" cement board. The best of all is creating a mud bed with lath inset, but that's over the head of most DIYers.

How is the existing 1/2" layer applied? Can it be pulled without too much difficulty (not glued or ring-shanked, just screwed, for example)?

I just measured a job that someone else did where they put a beautiful tile job on plywood that needed replacement. Wet feet from getting out of the shower let water get on the grout, down into the plywood subfloor and the surface layer of the plywood delaminated; the tiles started sounding hollow in that area and the grout lines cracked. Now an expensive tile job has to be replaced.

Start with a good foundation.
Removing the 1/2" plywood is important to do. But thinset over the plywood is what is re
recommended for Ditra installation. The issue you point out with moisture is a good one also and ties in with why the 1/2" plywood needs to be removed. You need to install over exterior grade plywood (this can handle the water in the thinset and some seeping though the grout) Not AC ply of even CDX.
I had a tile guy that I got rid of because he always tiles over plywood subflooring. His unbelievable reason was this allowed the tiles to be removed easier when they cracked. He did it this way for 25 years. Sometimes experience is not substitute for knowledge.
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Old 01-15-2009, 11:19 AM   #7
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Subfloor Prep for Tile


If you really want to be "safe", remove the 1/2" layer. Now install exterior grade 5/8" ply. For the underlayment use Ditra. The Ditra is a waterproof layer. If you use Kerdi-Band on the Ditra seams (and even edges, 2" up the wall), you will have a fully waterproofed installation.

Sounds like you're not going to remove the 1/2". Just so you are aware, the old thinset needs to be removed (scrape, sand). Now use the Ditra.

RedGard can be used but the same conditions apply; old ply needs to be free of thinset or any other contaminant that will interfere with the bond to the ply. Must pre-fill any gaps/holes in the ply or ply to wall spacing for waterproofing. Follow directions carefully. I am not a fan of RedGard.

As for tile over plywood, it is an acceptable method by the TCNA. However, I don't recommend it because of the steps needed....no room for error. It does not sound like your situation will allow for installation directly over the ply. The Ditra method is completely DIY-friendly, no special cutting tools to buy and will help with deflection, uncoupling and moisture penetration. This is the preferred method to many tile professionals.
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Old 01-15-2009, 12:02 PM   #8
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Quote:
As for tile over plywood, it is an acceptable method by the TCNA. However, I don't recommend it because of the steps needed....no room for error. It does not sound like your situation will allow for installation directly over the ply. The Ditra method is completely DIY-friendly, no special cutting tools to buy and will help with deflection, uncoupling and moisture penetration. This is the preferred method to many tile professionals.
That really says it all.
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Old 01-15-2009, 09:54 PM   #9
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Subfloor Prep for Tile


I am with Angus on this one, Ditra is the answer.
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Old 07-25-2009, 09:30 AM   #10
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Originally Posted by Bob Mariani View Post
Removing the 1/2" plywood is important to do.
I'm also looking to do some tiling and need to build up a subfloor. I'm confused as why you (and Mudd) say that the 1/2" needs to be removed. Do you mean replaced? I believe 1 1/4" is the suggested subfloor thickness for tiles, right? If you remove the 1/2" then you're back to 3/4".

Thanks,
Nick
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Old 07-25-2009, 04:57 PM   #11
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Subfloor Prep for Tile


nickdu,

You are resurrecting a thread that is more than six months past. Two of the posters offering advice have limited knowledge of the matter.

Why not start your own new thread so that this one doesn't confuse you or influence others?

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