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Unread 10-02-2007, 09:01 PM   #1
Bill Anderson
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Question Using unmodified thin-set with 18x18 marble

Howdy gang, my wife is sensitive to the additives in modified thin set so we would like to install 18x18 marble tile on durock using MEGABOND without the liquid additive.

Is this practical? Are there any pitfalls?

Thanks for your advice
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Unread 10-02-2007, 09:17 PM   #2
Tool Guy - Kg
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Welcome, Bill.

We're happy to help, but sometimes we ask more questions of members before answering so we give 'em spot on advice. Frequently we'll back 'er on up to the beginning.

I'm going to do just that. Have you run your floor structure through our Deflecto? It's a calculator that helps determine if your floor structure can handle the extra stiffness needed for natural stone. Click here and enter the requested data. Then come on back with your floor rating and we'll continue helping you out.

By the way, can you tell us what kind of marble you have there?

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Unread 10-02-2007, 09:32 PM   #3
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Thanks for your quick reply ToolGuy!

I tried to use the calculator but it is not applicable since we have an engineered joist system on 16" centers. The subfloor is Advantec with .5" Durock screwed and glued.

The marble is called Polished Paradise Beige (similar to Crema Marfil) and is 1/2" thick 18"x18"
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Unread 10-02-2007, 09:35 PM   #4
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"Screwed and glued"?......that phrase has my attention in a hurry.

Does that mean your Durock has been glued down with beads of construction adhesive gunned over the floor in a random fashion? Or was it set with a layer of thinset combed-out with a notched trowel?

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Unread 10-02-2007, 09:47 PM   #5
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Sorry, I missed spoke -- it was set in thinset with a notched trowel.
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Unread 10-02-2007, 09:54 PM   #6
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Ok, good to hear.

Onto the next thing. About your I-joists: Take a look at this article in the Liberry on I-joists for span tables.

Now, onto the subfloor. The single layer you have isn't up to snuff when using natural stone tiles. I'm afraid that your single layer of plywood and Durock is suitable for man-made tiles like ceramic or porcelain, but not so for natural stones that are weaker. Natural stone requires a second layer of plywood for stiffness. You're likely to experience cracked tiles over the course of time if you install over a single layer of ply.

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Unread 10-02-2007, 09:56 PM   #7
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All of the approved methods of installation of stone tile requires two layers of plywood. You still have time to fix that. Were the trusses designed for stone installation? If not, then they rarely have enough strength for stone tile...normal seems to be to design them for L/480, and you need at least L/720 for stone. Would work fine for ceramic, though. The second layer of ply needs to be minimum of 3/8" on new construction, but with the cleanup and all of the screw holes from the cbu, 1/2" or more would be better. A good quality dryset mortar should work for the marble. To keep the height reasonable, you might consider a membrane instead of cbu. The stuff you have down might come up, but it probably won't where you want to reuse it.
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Unread 10-02-2007, 10:11 PM   #8
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The joist system is not an I-joist but a light commercial style open web style that was engineered/spec'ed to support a marble floor (by the architect) with minimal deflection.
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Unread 10-02-2007, 10:20 PM   #9
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Hi Bill, you still need another layer of ply for the stone.
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Unread 10-02-2007, 10:23 PM   #10
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What Mike said!
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Unread 10-02-2007, 10:34 PM   #11
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How thick is your AdvanTech subfloor?

Did the architect spec it for Marble or just ceramic tile? Seems strange that he would spec it for marble and then only spec out a single layer of subfloor.
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Unread 10-02-2007, 10:44 PM   #12
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The subfloor is Advantec 7/8" thick.
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Unread 10-02-2007, 10:47 PM   #13
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I would still put in another layer. TCA wants a minimum of 1 1/8" of subfloor for stone.
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Unread 10-02-2007, 10:57 PM   #14
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Ok, I have the 7/8" Advantech on commercial open web joist with 1/2" Durock which totals 1-3/8" thick. Are you guys saying that I need to rip up the Durock, install another sheet of plywood and replace the Durock? Is there any alternative to this nightmare scenario? BTW: The marble is actually 5/8" thick if that makes any difference. Thanks again for weighing in.
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Unread 10-02-2007, 11:07 PM   #15
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Yes Bill, the Durock isn't doing diddly for stiffening the floor. If you opt to set the stone as is, there could be and most likely be problems.
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