Backer Board Required for Slate Tile [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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01-16-2003, 08:30 PM
:confused: 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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01-16-2003, 08:54 PM
I believe Schluter Ditra is about 1/8" in height and would work in lieu of the CBU.

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


John Bridge
01-17-2003, 04:11 AM
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.

01-17-2003, 07:07 PM
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.:bow:

01-17-2003, 08:13 PM
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.

01-18-2003, 08:55 AM
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.

01-18-2003, 09:51 AM
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.:confused:

01-18-2003, 10:02 AM
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.

John Bridge
01-18-2003, 08:09 PM
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?

01-19-2003, 10:01 AM
Unless there is another option that might be easier, that is what I plan on doing.

01-19-2003, 10:09 AM
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.

John Bridge
01-19-2003, 06:23 PM
Or you might consider a stone look-alike ceramic tile. That would solve all the problems, it seems. :)

01-21-2003, 07:59 PM
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.:bang:

01-22-2003, 06:38 AM
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.

01-22-2003, 07:37 AM
Again, thanks for the helpful information. You people have saved melots of time, money, and headaches.

01-24-2003, 09:56 PM
Hi BB, what would be the best solution in this case if sistering the joists were not an option and the span was longer than 11 feet?

01-24-2003, 10:05 PM
Randy, Is Your plan to install slate or another natural stone product??

01-24-2003, 10:19 PM
No todd, I'm just trying to learn the best way to do everything. I'm getting back into installing and want to know the best way to do every type of job. There is "SO" much to learn.........its totally mind boggling.......:confused:

01-25-2003, 09:09 AM
If you are not planning to use slate or other natural stone, then your joists are good to go! :)

With your 3/4" OSB, Ditra would be the underlayment of choice, since it contributes only 1/8" or so to the height of your floor.

01-25-2003, 09:51 AM

Perhaps you should re-think the perceived problem with increased floor height. If cutting down some doors is the only consideration, it's not really much of a consideration at all, especially if these are all interior doors as I would suspect (an exterior is probably high enough already).

How many doors of what kind are we considering here?

01-25-2003, 05:06 PM
Many are hallow core doors. Those would be no problem to cut. However, I have an entry way door and a mud room door that are solid wood. And it is those doors that I fear on trimming.

However, on another note, I just visted a tile shop and one of thier installers agreed that the Ditra would not work in my situation with the natural slate. However, they adivised to use the skim coat method in addition to sistering up the joists. The skim coat would utilize a wire reinforcement and a latex based compound fortified with portland cement. Being an civil enginerr, this sounds reasonable. They claim this is the best route to use in lieu of the backer board and the ditra. Can someone tell me if this method is adequate, or shed some light on this method? Your advice is greatly appreciated.

01-25-2003, 05:35 PM
The ditra that is being reccomended needs a substrate of l/360 for tile l/720 for stone. If your floor does not meet this then it is not recommended with out first fixing the substrate. There has been debate in the past as to this, however if you are not sure what you have test it.

01-25-2003, 05:49 PM
What exactly is the l/360 or l/720 equation? Is to calulate deflection. If so, how much deflection is tolerable for natural stone, if any? You advice on the skim coat method would be greatly appreciated.

01-25-2003, 07:19 PM
L is the length of your joists between supports, in inches. Divide that by 760 (no, make that 720; thanks cx) and you will get the maximum allowable deflection for the floor. As for the skim coat, there was a recent discussion on that. Hit search, and type in the keywords jersey mud job and read the thread that comes up. :)

John Bridge
01-25-2003, 07:21 PM

Yes, it is deflection that we are concerned with, both along the length of the joists and in the subfloor between the joists. For stone the minimum is L/720, which equates to 1/720 of the overall length in inches under a "300 lb. concentrated load." In 15 feet, for example, the maximum allowable deflection would be 1/4 in. overall.

Please keep in mind that these are minimum standards and that more rigidity is better. You can go engineer to engineer with our chief engineer, bbcamp, and you two guys can have a great time talking about engineer stuff. Shoot him an email or a private message. ;) Just kidding.

On the "skim coat" method, we more often refer to it as the "scratch coat" method. It entails nailing down metal lath over the plywood and skimming it over with thin set (you referred to this as some sort of latex portland cement arrangement -- modified thin set). The method has been repeatedly tested at the TCA lab and has repeatedly failed under both load and stress testing (Robinson Floor Testing Machine). There are hundreds of guys around the U.S. and Canada who swear by it, but they are not in positions of responsibility.

Although we may not agree with everything that is developed within the tile industry as a whole, we at this site try our best to steer people into methods that will produce long lasting results. We try to stick with stuff that has been proven either through responsible testing or through decades of practical experience.

That's not really my answer to you. That's my answer to the guy who is recommending "scratch coat." :)

01-25-2003, 08:08 PM

DaveM meant to say, divide by 720, rather than 760. Please accept his humble appology. :D

The only difference betwix the hollow core door and the solid wood doors is a little weight, they're all solid at the bottom and all quite easy to cut with a skill saw, straight edge, and a little instruction. Don't let that dictate your method of constructing your floors. Really.

What JB is too polite to tell you is that the guy who recommended the "scratch coat" method is not your friend. Pay heed - or pay later. ;)

01-25-2003, 08:57 PM

The solid doors are no more difficult to cut than the hollow core version. They are alot heavier to remove and re-hang but the cutting aspect is no different.

Should you decide to do so...make sure you put a board on the under the door side opposite your circular saw. This will prevent tear out when you cut.

I just cut 8 of these doors with a 18V cordless Milwaukee circ saw with no problems.


01-26-2003, 11:01 AM
Let me add this about the defletion. You will want your floors to meet building codes, regardless of the floorcovering you later install. Most building codes specify 40 pounds per square foot live load, and deflection less than L/360 under live load. The live load will most certainly be over 300 lbs.


01-26-2003, 02:06 PM
There are alot of nice porcelain tile out there that looks exactly like slate. Could this be an option?

01-26-2003, 04:01 PM
Thanks for the information on cutting the doors. It seems now that raising the floor height won't be a problem. But I wouldn't like to raise it any more than 3/4". The ungauged slate tile has a max. thickness of 1/2". However, in my situation, could I use a 1/4" hardibacker board product in addition to sistering up the 2x10 joists at the 12' span. Since there is many replies, I would like to remind you again that I have 3/4" T.&G. OSB plywood with 1/4" inch plywood on top. This is all screwed and glued to the floor joists which are 16" o.c.

01-27-2003, 08:03 AM
The hardibacker people suggest calling their tech support number for natural stone tiles 12" and over. I think you should do that.

01-28-2003, 01:40 PM
So jasonamy, I guess after reading all of this, what are you going to do??

01-28-2003, 09:28 PM
Just one more question, bbcamp. I ran across Dura rock (pardon the spelling) and Wonderboard at a tile supply shop and they had indicated that these are the best products for laying natural stone on in my situation. I have done some research on the floor joists and buliding codes here and the calulated deflection that my floor joist would be less than the maximum deflection for my floor joist. This is based on the properties of the wood type for the floor joists, spacing, etc. and the 1" total thickness of wood above joists. However, I still think that I should sister up the joist for additional support. What is your experience with either material and would either be appropriate in my situation? How much deflection is tolerable for stone tile? I realize that zero deflection would be desireable, but it only sounds good in theory. The thickness of either product is 1/4". I would like a second opinion.


01-28-2003, 09:38 PM

I think you need to 'splain this to at least some of us:

"I have done some research on the floor joists and building codes here and the calculated deflection that my floor joist would be less than the maximum deflection for my floor joist."

The deflection you're lookin' to be less than for your stone tile is L/720. That's a very rigid floor.

Others will tell you about the different CBUs. If you do a search under either of those names you may find some discussions about the merits - and lack thereof - of the different types. We've had lots of discussion about that.

01-29-2003, 07:06 AM
It would not surprise me at all if you went to your building code and found that your joists met their standard. Most codes specify deflection less than L/360 under live load, which is usually 40 pounds per square foot.

Generally, when I evaluate a floor for tile, I use 50 pounds per square foot to account for the tile, backerboard, etc. It's a little insurance.

For natural stone, like your slate, the deflection criteria is L/720, or twice as stiff as code requires. Again, I use 50 psf, although the stone installation will likely be heavier than tile. If you double all the floor joists in the tiled area, you will meet this criteria for the floor framing.

You also must meet a stiffness criteria between the joists. That's where we worry about how thick the plywood is. For ceramic tile, the minimum thickness of plywood is 5/8" if 1/2" CBU is used, or 3/4" if 1/4" CBU is used. Some CBU manufacturers recommend "double wood" for stone. I always do. "Double wood" means 1 1/8" total plywood in 2 or more layers, overlapped, glued and screwed.

Despite the previous paragraph, CBU is assumed to add no structural strength to the floor. The CBU manufacturers test their products using different plywood thicknesses. Their installation instructions reflect the minimum plywood that makes their product work.

As far as the CBU brand, I have no preference, as long as you follow their installation instructions. Go to the store, look at each product, price it per square foot, and take home what's available. You will find some differences in surface texture, edges, crumbliness, etc. These have no effect on the final installation. Do a search here for other's opinions on cutting, weight and handling. You'll still take home what's available at the cheapest price.

Ditra is another story altogether. We've mentioned it before as a way to reduce the height of your floor. It's a little more expensive per square foot, but is an excellent way to deal with problem floors. In your case, you've decided that you can trim your doors, so height is no longer an issue. Getting a stiff subfloor and joist system is.


01-29-2003, 01:56 PM

All your information is well appreciated. You have saved me lots of time and money. I am now going forward to install the stone tile. Thanks again. However, I may have some questions during the installation. Thanks again.