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Unread 09-19-2008, 10:46 AM   #1
FCCC
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Mobile Home Floor

I am helping a friend do some upgrades to his mobile home. He is redoing both bathrooms and the master bedroom. Both bathrooms were tiled by the owner a few years ago. He tiled directly over the 5/8" osb subfloor and it has held up everywhere except near the joint where the halves come together.
We replaced the tile in the guest bath a few months ago using Ditra following all the manufactures recommendations. This was on the recommendation of the local tile store. The grout is beginning to crack. The new tile was a 12 x 12 while the old tile was 8x8. Other than that, there is little difference in the installation method other than the Ditra.

We are going to replace the guest bathroom floor and they definitely want to tile the master bath.

I guess my question is what is the best solution to this problem. The existing floor is 5/8" osb on 2x6 joists 16" on center. The floor joists are over steel beams 8' on center.

I am open to all suggestions, but am leaning toward 1/2" plywood screwed and glued over the osb with 1/4" hardi backer over that. My second choice is 3/4" plywood screwed an glued, red guard, and then tile.

Thank-you for the help.
Steve
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Unread 09-19-2008, 11:56 AM   #2
koihito
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I believe your problem lies with the 5/8" OSB. While some underlayments will spec 5/8 exterior ply, none, to my knowledge condone OSB under 3/4". I would also like to know if the OSB is exterior grade and T&G it's often not in modular housing, depending on the age and region.

Add to that the fact that OSB was probably not 100% after tearing out the previous floor (which had already failed) and you've got plenty of potential for problems.

I would like to see you remove the OSB altogether and go back with 3/4 Exterior T&G and 1/4" CBU or Ditra.

If you did add ply, no glue, screw or ring shank nail every 6", but NOT into the joists. Red Gaurd over ply, to my knowledge is not an approved method, you would be better off with Ditra.
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Last edited by koihito; 09-19-2008 at 12:04 PM.
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Unread 09-19-2008, 01:24 PM   #3
FCCC
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Mobile home floor

koihito,
The tile had not failed in the bathroom previously. There are extra joists in both bathrooms and maybe that is the reason. It makes sense to me that removing the old floor may have something to do with tile movement now.

I am not sure why not gluing and not nailing into the joists is an advantage. It seems to me that adding plywood and making sure it is well adhered to the entire floor structure would provide a less flexible surface for the tile.

I understand that the osb expands and contracts differently than the plywood, but would imagine that applying 3/4" over the osb would be as good as removing the old osb and replacing with plywood.

I am a bit weary of using ditra again due to conversations with two tile installers I have spoken with who have had similar issues with Ditra and when the floors were redone with backer board they held up.

I have tiled directly on 3/4" plywood many times without issue. Just thought the red guard would provide extra waterproofing and protection.

Steve
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Unread 09-19-2008, 05:49 PM   #4
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Quote:
it has held up everywhere except near the joint where the halves come together.
To me, that sounds like failure...

If you got full coverage with a wood glue (like tight bond) than glue would not be bad, but most people mean liquid nail which leaves voids meaning the second layer is not supported by the first layer. If you used 1/2" liquid nail (or other construction type adhesive) would be bad.

You could go with 3/4" and pretend that the OSB doesn't exist, but why raise the floor that much when an hour or two of work gets you a fresh start?

As to Ditra, I really gotts wonder why so many problems, were do you live? I bet if you polled Tile Pro's who have used Ditra your going to find 99% believe in it. But 1/4" CBU will serve you well if it makes you more comfortable. While it is possible to tile over plywood successfully, doing it right is not simple and going directly on #/4" is not the way. You may have been lucky (it happens) or you may not have the same definition of without issue. The reality is tile directly on a 3/4" plywood floor 10 years down the road has a VERY low chance of being in good condition.
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