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Unread 01-28-2007, 09:46 PM   #1
jeffdeline
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Underlayment for Marble/Travertine

It's been awhile since I've posted here but it's good to be back. I have a question that I realize has been hashed over many times but with my new project I need to solicit opinions again.

We will be installing either marble or travertine floors (12, 16, or 18 inch tiles) in about 900 sq ft of the new home we are building. Our subfloor is 3/4" Advantech over 14" I-Joists. The I-Joists have 1 1/2" by 3 1/2" solid flanges and are spaced 16" o.c. in two rooms and 8" o.c. in the third room. I am not at all worried about the deflection in the joists are are worst case for the 3 rooms is L/865 total load.

I have been discussing our project with the tile man that laid the marble in our last house (very similar arrangement). In our last house he installed 1/2" plywood over the 3/4" OSB as that was the requirement of the builder. He did a fantastic job, the marble looked good when he was done and still looks great today. Installing a 2nd layer of plywood is not his preferred method. Other than the one builder he always installs cement board over plywood or osb subloor for all of his other jobs whether it is stone or ceramic. He says his biggest concern is the deflection in the floor joists. He sets the plywood in thinset and screws it down making sure to avoid the floor joists. I have made a decision to use him for the ceramic and marble in this project. He is very adament about his preferred method, he said he has never had a call back for cracks in tiles or grout when the joists are sized properly but at the same time will install what I specify. We did discuss Ditra. He has never used it in a residential application but a couple of different times in commercial applications over concrete.

I'm not quite sure what direction to take here. I know the consensus on this board is to have a minimum of 1 1/8" of osb/plywood. The tile guy I am going to use is very comfortable with his preferred method of installation. I've talked to a number of builders that he does work for and haven't gotten any complaints. Any suggestions here?
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Unread 01-28-2007, 10:52 PM   #2
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Hi Jeff, I wouldn't take any chances. I would add another layer of at least 3/8" ply then go with Ditra. It's the deflection between the joists too.
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Unread 01-28-2007, 11:18 PM   #3
cx
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Welcome back, Jeff.

Your builder was correct in installing the additional plywood in your last job, but I hope he didn't have the stone installed directly to the plywood. That what was done?
Quote:
Originally Posted by Jeff
He sets the plywood in thinset and screws it down making sure to avoid the floor joists.
I hope that's a typo. You meant he installs the CBU in that manner, perhaps?

Your tile man is not following industry standards in installing natural stone tiles over single-layer subflooring. You must have double-layer subflooing, plywood or OSB (depending upon substrate manufacturer), correctly installed, then a CBU or decoupling membrane.

The between-joist deflection is usually of more concern even than the joist deflection, as MMike points out.

I'd ask your tile guy to point to the industry standard he is relying upon for his method and let the discussion go on from there.

My opinion; worth price charged.
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Unread 01-29-2007, 05:23 AM   #4
jeffdeline
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CX, yes the previous builder always had ceramic or stone installed directly to the plywood. And yes, it was a typo, I meant to write that the tileman sets the CBU in thinset.
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Unread 01-29-2007, 08:20 AM   #5
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Installing ceramic tile over double-layer plywood, properly installed in new construction can work and is an industry accepted method, Jeff, but I've never known anyone who would do it with natural stone.

I don't even think our resident, die-hard Tile Over Plywood pro would do that, but I'll axe him.

Yes, thinset under CBU is good for stone.........over the second layer of plywood.

My opinion; worth price charged.
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Unread 01-29-2007, 11:06 AM   #6
Tim_4_Tile
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As far as Ditra is concerned, Schluter's Installation Manual will give you the exact requirements...

http://www.schluter.com/english/prod...32)_052005.pdf

(3.65Meg PDF file)

In a nutshell, your 3/4" Advantech w/ Ditra would be okay for ceramic/porcelain, but you'd need another 3/8" ply (minimum) with the Ditra for stone.
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Unread 01-30-2007, 04:35 PM   #7
Bill Vincent
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I don't even think our resident, die-hard Tile Over Plywood pro would do that
Ummmm....er... Sorry, cx. Technically, there's nothing that says it's okay, but I have done this many times in the past, and wouldn't have any problem doing it again. However, as I've said for years now, This is not something I'd recommend for DIYers, or even pros who aren't very familiar with what's required to do an over plywood install-- even with ceramic.

Jeff would be much better off going with one of the more "mainstream" installation methods, such as the ones that havev been suggested here.
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Unread 01-30-2007, 05:22 PM   #8
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But he might.
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