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t majchrowski 10-08-2004 11:30 AM

Cracked Natural Stone Tiles II
Have a couple of questions. For natural stone tile, the Marble Institute of America calls for a floor deflection of L/720 maximum. Is there a area that is okay around this maximum? Reason I ask is that we have calculated a deflection of about 1000 in my dining room/kitchen area and over 1300 in the area of a hallway. Our travertine tile has cracked at each backerboard joint in these areas. Is this deflection too tight? Could this be the cause of our cracked tile? If so, any recommendations on how to fix the problem?



Mike2 10-08-2004 11:51 AM

HI Tom.

I'll send a message for our engineer bbcamp to swing by and help you out with your deflection question.

In the meantime tell us:

1. What size of joists, how are they spaced, and what is the longest unsupported span in the tiled area?

2. Describe your sub-floor in detail.

3. Was the backer board laid over thinset? And was the backer board screwed/nailed into the floor joists?

4. Was an allowance for expansion provided around the perimeter of the room? And were expansion joints placed every 20 - 25 ft. within the field of tile itself, if in fact you have that large of an expanse.

5. What size of tile and what spacing was used?

Answers to all these questions and possibly more I haven't thought of will be most helpful in diagnosing the likely cause of your problem.

bbcamp 10-08-2004 12:10 PM

The bigger the number under the L, the better your floor is for stone.

The cracks over the backerboard joints tells the story. My money is on either no tape and mud at the backerboard joints or no thinset under the backerboard.

t majchrowski 10-08-2004 07:51 PM

Info on tile placement
For Mike,

1. The joists areTJI/Pro-150, 11/78 inch product. They are spaced 19.2 inches on center. The longest unsupported span is 14 feet.

2 & 3. The sub floor is .75 inch plywood with thinset application and .5 inch backerboard. The backerboad is stapled into the plywood. I have raised our kitchen island and looked under it. The backerboard does not appear to have mud between the seams. We just have a space between the backerboard panels.

4. Since the largest length of tile is about 20 feet, I do not see where there are any expansion joints. Do not see how any allowance for expansion exists around the perimeter of the tile either. The backerboard goes under the counters. The tile goes up to the base of the counter.

5. Have 12 inch by 12 inch travertine tile that is 1/2 inch thick with about a 1/8 inch spacing .

Thanks to Mike and bbcamp for your comments.


Jason_Butler 10-08-2004 08:33 PM

Those seams should have been taped / floated with thinset.

That's my bet...


cx 10-08-2004 09:24 PM

I don't have a table available for deflection specs higher than L/480 for the TJI Pro-150 joists, Tom, but I would have guessed you to be pretty close to the max span for L720, if not a little over. Injineer Bob maybe has the specs. Did you get your L/1000 spec from the manufacturer, or some other source?

That said, even if the joists are stiff enough, the subfloor isn't. In fact, it's not even close. The only installation method I know of that would be acceptable over that subfloor would be using Ditra and ceramic tile - and that's only because Schluter will warranty it. Not gonna make if for natural stone.

Assume that your CBU was laid such that the joints fell between the joists (correct procedure) and you could reasonably expect the cracking to appear there. Taping the joints correctly may have helped, but I don't know that it would have prevented the failure. Most manufacturers don't recommend the use of staples to install their CBU; that could also contribute to the failure if it allowed movement of the CBU.

Who made all the installation decisions on this project?

My opinion; worth price charged.

Bill Vincent 10-08-2004 09:46 PM


Originally Posted by t majchrowski
For Mike,

1. The joists areTJI/Pro-150, 11/78 inch product. They are spaced 19.2 inches on center. The longest unsupported span is 14 feet.


I have a feeling that the 19.2" with only CBU over one layer of 3/4" is the culprit. You may have astronomical numbers in the deflection department along the length of the TGI's, but between them it leaves alot to be desired. CBU adds almost NO structural strength. My own feeling is that you need atleast another 1/2" of plywood under that CBU.

Just realised Cx got here first!! Well, Cx, you know what they say about great minds!! :D

cx 10-08-2004 10:12 PM

Seems we been doin' that a lot lately, Bill. You must be gettin a lot smarter. :D

Bill Vincent 10-08-2004 10:49 PM

Stranger things have happened! :D

Mike2 10-09-2004 11:36 AM

One more thing:

What backer board has been approved for use by the TCA, or is warranted by the manufacturer, for joists spaced more that 16" OC? :uhh:

Steven Hauser 10-09-2004 11:50 AM

Well it is written as an uncoupling system. Not CBU

T_Hulse 10-09-2004 01:58 PM

Hi Tom,
I agree with everything the guys have said about possible reasons, but in my experience, it won't be quite as bad as every CBU seam cracking through unless 1 particularly bad mistake was made: no thinset under the cbu. Sounds like you've got lots of things wrong, but that would be by far the worst. Get a flashlight & look down in the spaces between the CBU panels. You should at least see the trowel ridges of the thinset used. If you only see subfloor, then this would be the worst of many mistakes here.

Without having the deflection tables either, my guess is that you would need something more like 3/4" subfloor + 1/2" plywood + Ditra to make this floor fly.

Mike2 10-09-2004 02:15 PM

I noticed that too Steven.

In the TCA handbook, F144-03 specifically uses the term "Cementitious Backer Unit" which is approved for joist spacing 16" OC. On the other hand, TCA in F148-03 uses the term "speciality uncoupling system" which is approved for use in joist systems 19.2 and greater. Are these two terms, CBU and speciality uncoupling system synonymous? I don't know for sure.

I did check the data sheets for Durock and Hardibacker, both say for joist spacing not to exceed 16" OC.

T_Hulse 10-09-2004 02:19 PM

Speciality uncoupling system means Ditra or equal. Backer boards are definately excluded from that category. :)

cx 10-09-2004 02:43 PM

Yeah, Mike, that's one of them clever little wording games they use to get a procedure published for a specific manufacturer's specific product without appearing to have published a procedure for a specific manufacturer's specific product. In that particular case, you can certainly use any product on the market, so long as it's exactly like Ditra. Which, of course, includes ........... Ditra. :rolleyes:

That's my understanding of it, anyway. If you look closely at F147 and F148, don't that look a lot like a cross-section of Ditra?

Actually, I wonder if all the new Ditra knock-offs are also approved under those methods now that there are two or three other waffle-type membranes out there.

How does that work? :confused:

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