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Unread 01-16-2003, 08:30 PM   #1
jasonamy
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Question Backer Board Required for Slate Tile

I have goteen conflicting answeres to the following question, so can someone please help.

I am planning on installing natural slate tile within the dining room/kithen areas, but I have concerns on raising the floor height. Currently, my floor consists of 3/4" OSB Plywood overlain by a 1/4" plywood subfloor. This systen is glued and screwed to the 2x10-inch floor joists that are 16"o.c. The natural slate is 12x12-inch, if that matters. My first option would be to install the tile directly to the plwood subfloor. However, if a 1/4"backer board or an equal is required, that would result of the trimming of all my doors, which is not a preferred route. Is there an equal material to a backer board that is durable and is thinner, possible 1/8" thick, or can I install the tile directly to the pywood subfloor. Any information would be greatly appreciated.
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Unread 01-16-2003, 08:54 PM   #2
Jason_Butler
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I believe Schluter Ditra is about 1/8" in height and would work in lieu of the CBU.

www.schluter.com

In any case, do not attempt to set the slate directly over plywood. The wood is too unstable to ensure a problem free install

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Unread 01-17-2003, 04:11 AM   #3
John Bridge
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Hi Jason and Amy, Welcome.

Jason B. is correct about the Ditra. It is 1/8 in. thick and will work well in your situation.

We do need to know what the span of the joists is. How long are they between supports? This information is critical. Stone tiles require exactly twice the stiffness that is required for a ceramic tile floor.

Also, is the 1/4 inch plywood luan? If that is the case, we'll ask you to remove it.
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Unread 01-17-2003, 07:07 PM   #4
jasonamy
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The floor joist span is 12-ft. However, I am not sure if I have a laun subfloor. It is a tad bit difficult because the flooring is presently covered by vinyl. In addition, what is exactly laun plywood? In the interim, I have the house floor plans here and I will check if there is a bill of materials. Thanks for your continuing advice. By the way John, you were recommended to me by a tile manufacturer, and he had indicated you the most knowledgeable installer. Thanks again.
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Unread 01-17-2003, 08:13 PM   #5
tileguytodd
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Well he may be the most knowledgable ,but thats cause he uses alberts brain for information storage.

He runs a perty darn good club though

Luan is a Mahogony plywood.Underlayment grade 1/4" is Birch.
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Unread 01-18-2003, 08:55 AM   #6
bbcamp
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The maximum span for 2x10 joists under stone is 11 feet.

And, I think we need "double wood" for the subfloor too, especially for Ditra.

What's under this floor, an unfinished basement or crawlspace, I hope?


I hate being the wet blanket, but is this another job for ceramic slate look-a-like? Everything is in place for a ceramic tile floor.
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Unread 01-18-2003, 09:51 AM   #7
jasonamy
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Beneath the floor is a finished basement that is generally dry. Would it be possible to double up the floor joists with additional 2x10's if this is a concern for natural stone? I, must say that the floor at the present seams to be fairly soild. The house ifs fairly new, if that matters. However it seams again that I am receiving conflicting answers. 1st, John Bridge has indicated that the flooring components is sufficient, if a luan subfloor is not present. I do anticipate on using DItra, since it is sold here locally in Milwaukee. Some more guidance would be helpful.
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Unread 01-18-2003, 10:02 AM   #8
bbcamp
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I'm not sure that you've gotten conflicting advise here, yet. John questioned the joists, and remarked that you need twice the strength for stone as for tile.

I checked the joists, and doubling them is certainly a reasonable solution. It will give you the stiffness you need.

Sometimes we overlook some detail in answering your questions. Easy to do with so many coming in every day, and everyone here is doing this as an extracurricular activity. Not much time to reflect on the whole problem. We do tend to focus on the question asked.

However, this is all balanced out by the number of experts that help out. We often get another perspective from a passerby, including newbies. So, hang in there with us, we will try to help you out.
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Unread 01-18-2003, 08:09 PM   #9
John Bridge
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Jason and Amy,

Yes, I did say that Ditra will work as far as the substrate is concerned. That's one issue. The structure is another. I do hope we are not confusing you, because that's not why we're here.

Now, what Bob is saying is that from what you've reported, your floor is not up to a stone tile installation, but it is fine for ceramic tiles. If you want to install the stone, yes, you will have to do some beefing up. Sistering the joists would be the way to go. Are you prepared to do that?
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Unread 01-19-2003, 10:01 AM   #10
jasonamy
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Unless there is another option that might be easier, that is what I plan on doing.
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Unread 01-19-2003, 10:09 AM   #11
bbcamp
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The Schluter website says that you need "double wood" to use their Ditra under stone. They mean that a total of 1 1/8" plywood is required. You will need to add 3/8" exterior grade plywood over the OSB before you install the Ditra.

This is in addition to sistering all of the joists.
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Unread 01-19-2003, 06:23 PM   #12
John Bridge
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Or you might consider a stone look-alike ceramic tile. That would solve all the problems, it seems.
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Unread 01-21-2003, 07:59 PM   #13
jasonamy
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Well, it sounds like placing natural stone tile is going to be out of the question. I just don't feel like raising my floor that high and trimming all my doors. I greatly appreciate all your advice. One last question. Could I lay ceramic tile directly over the plywood subfloor or should I use the Ditra or backer board? Thanks again.
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Unread 01-22-2003, 06:38 AM   #14
bbcamp
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You can do either, but I recommend the Ditra, because of your desire to minimize height, as well as because Ditra is an excellant product.


If you go the plywood route, there are very specific requirements for the plywood, the way it's nailed down, the thinset and grout. There is a discussion going on over at the Pro's Hangout. Dave Gobis from the Tile Council of America provided this:

t is the conclusion and recommendation of the Engineered Wood Association and after many years of both independant testing and field experience that you can install successfully over plywood. HOWEVER, the requirments for such an installation are very specific and include the requirement the subfloor be a min. of 5/8 CC Plugged,T&G, glued and fastened to the joist, nailed 12 in the field and 10" on the edge followed by an overlay surface of 15/32? minimum (19/32 commercial) C-C plugged exterior grade, non-fire treated plywood fastened every 8" in the field and 6" on the edge. Underlayment fasteners should not penetrate the framing below. A gap of 1/8th? shall be left between sheets of plywood and ΒΌ" at all other surfaces which they abut to allow for expansion. Many manufacturers have their own specific recommendations on spacing and treatment of plywood joints. Problems can occur in plywood substrates when subjected to moisture. Installation of ceramic tile using an ANSI A118 .11 EGP thinset mortar and A118.7 grout is recommended only in interior dry or limited water exposure areas. All wood floor structures should be designed with proper ventilation on the underside and thorough protection from water vapor or moisture. In the case of homes built with crawl-spaces, an effective moisture barrier of atleast 8 mil thickness must be provided. All joists, supporting structure and plywood surfaces shall be dry prior to installation of ceramic tile as any shrinkage occurring after the installation will result in bond failure. Moisture of framing members should not exceed 19%.

At this point, you may not be able to determine if all the requirements have been met.
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Unread 01-22-2003, 07:37 AM   #15
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Again, thanks for the helpful information. You people have saved melots of time, money, and headaches.
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