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Old 12-10-2008, 05:33 PM   #1
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Dry-Ply Underlayment Question/Problem


The local Lowe's recommended Georgia Pacific 3/4 " Dry-Ply plywood for use under ceramic tile. Just got done screwing to the 1/2 " subloor spacing at 8" in the field and 6" at the edges. Upon looking for thin-sets to use, I saw mentioned something about not using over sealers. The thought occured to me that the Dry-Ply must have some type of sealer on it. I was planning on using Ditra over the ply, because the tile height would be too high with cbu. Called GP and the woman in customer service didn't seem to know if it would work or not. Said she thought the ply was sealed with some type of polyurethane, but didn't know if it would take thin-set or dry properly. Hope someone can help me out with this, because tile is ordered and paid for. Don't want to pull it out and start over. Any comments would be greatly appreciated. Tim

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Old 12-10-2008, 05:48 PM   #2
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Tile thinset doesn't bond to sealers or paints. You have been given some super bad information from Lowe's. DITRA relies on a thinset bond also, so that's out of the question. This is in all the books.

Your best bet now is to use a cement backerboard screwed or nailed to the subfloor. Keep in mind that the backerboard MUST be installed in a fresh bed of unmodified thinset.

Go raise hell with Lowe's and let them provide you with the backerboard free of charge.

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Old 12-10-2008, 06:21 PM   #3
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Bud, As floor height is a real issue , would I have to use 1/2 " or could I use 1/4" CBU. Or could I nail 1/4" plywood over the 3/4". Or, should I just pull everything out and start over? ( Although I already set the Swanstone shower base in mortar, so its not going anywhere in one piece ! ) Thanks for trying to help, it's truely appreciated. Tim.
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Old 12-10-2008, 06:36 PM   #4
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Tim setting tile directly to plywood can be done but isn't always a good idea for a few reasons. I know someone will come along now and tell us they set their tile to plywood 116 years ago and it's still just fine, so I won't argue the method other than to advise agains it.

In your case 1/4" cement board would work just great. Use unmodified thinset to set it in and install your fasteners as recommended by the manufacturer of whatever product you choose to use.

I see no reason to remove anything if you add cement board, no harm done.
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Old 12-10-2008, 07:58 PM   #5
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Thanks Bud, My only concern, is if I add the cbu and don't use the Ditra, the height difference between the tile and the hardwood, would approach 1 3/8 to 1 1/2". What is the maximum transition between floors, and are there saddles or thresholds to cover it ?
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Old 12-10-2008, 11:31 PM   #6
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Hi Tim........

Jaz

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