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t majchrowski 10-09-2004 09:53 PM

Thanks to all

Really appreciate all the input.

The general contractor made all the decisions on the subfloor. He said he checked with his architect and everything was fine. He also claims to have done this type of installation many times without a problem. Maybe our eyesight is better than others.

I don't understand all the deflection issues, but do realize that the 19.2 inch spacing between joists and 3/4 inch plywood doesn't meet specs.

Now to try to get the problem fixed. Wonder if we should go with a good ceramic tile instead of all the work to upgrade the subfloor for stone tile. Oh, well.



Bill Vincent 10-09-2004 10:14 PM

Tom, your contractor might want to read the following article:


Quite frankly, I don't think ceramic would make it on what you have now, either. Not at 19.2" spacing. If he's done this before with no problems, he's been lucky.

cx 10-09-2004 10:47 PM

Well, I'm sure don't nobody wanna hear my architect opinions again, but...... :rolleyes:

One or both of them shoulda known better than to do that installation, Tom. If a competant architect was involved with the designing, he would have known the use of CBU over 3/4 inch plywood over 19.2" joist spacing was not an industry approved procedure even for ceramic tile, let alone natural stone. Couple that with an incorrect CBU installation and I agree with Bill, your chances of even ceramic tile surviving are poor.

If you want your stone (as I presume you were promised by the builder in your contract), you'll need to take up the existing floor at least down to the plywood subfloor, add another layer of plywood, correctly installed, then a substrate of one kind or another, correctly installed, then tile. If that's gonna cause finished floor height problems, we can discuss that, too.

The article Bill posted, by Dave Gobis, is a good one for you to present to the builder and architect. It's simple, clear, and athoritative. Dave has tested a lot of floors in his time and knows whereof he speaks.

My opinion; worth price charged.

t majchrowski 10-10-2004 06:10 PM

Thanks for all the info. I have the article by Dave Gobis. I will pass it on to the contractor and tile installer. See what they have to say. I will hold back on all your advice until I get a response. Wish me well!!


t majchrowski 10-25-2004 10:21 PM

Natural Stone Cracked Tile III
Continuation of a long story. First, thanks for all the fine inputs to my first two posts. I have given info to the general contractor. I'm the guy with cracked tile running linear accross the width of two rooms and a hallway, with the joists 19.2 inches apart, 3/4 inch plywood, thinset with backerboard. Deflection is calculated using standard techniques as over 1000. Your past comments say to look at the deflection in the plywood between the joists and to make sure the backerboard is screwed, not staples down. Also should look into supports under the stone between the joists and adding another 1/2 inch of plywood. The contractor says he has found people who fix the problem by just taking down to the plywood and adding two layers of backerboard perpendicular to each other. Does this sound right? In our earlier discussions, there were concerns that the subfloor was not strong enough for the stone. Wouldn't adding another layer of non-supporting backerboard over the existing subfloor just add more weight and maybe make the problem worse? Or, is running the backerboard perpendicular in two layers okay despite the added weight.

Hope I am getting to the end of this adventure.


cx 10-25-2004 11:31 PM


I merged your new thread with the previous one. Please keep all your questions for this project on this thread so anyone who wants to comment will be able to follow the history. Your thread will pop up to the top of the queue every time you make a post to it, no matter how old it gets. :)


Would you please invite your GC to come by here and talk with us?

Tearing out everything down to the subfloor is a good start. Adding two layers of CBU makes no sense at all. He is obviously unaware even that CBUs have no grain and it makes no difference which direction they are laid - they still don't count appreciably toward the strength of the subfloor, although common sense recognizes you gain something. Why would he go to all the trouble to correctly install two layers of CBU instead of adding a sufficient second layer of plywood and then an approved substrate? That's if, in fact, any CBU manufacturer will specify a correct method for that second layer - but, hell, first you gotta get one to approve using their CBU over that joist spacing at all.

Try to 'splain to him that he needs a subfloor that meets the 1/720 deflection standards between the joists before the installation of a tiling substrate, and the substrate hasta be approved for the application. If you have joists with a three inch wide top chord, that's a deflection of no more than .022 inches with a really, really fat guy standing on one foot right inna middle. That's a very stiff floor, and he ain't gona get it with only one layer of 3/4 plywood.

And I think I still had a question about how y'all determined the L/1000+ rating on that joist structure.

I think you're still being led down the primrose path, Tom. Don't go.

My opinion; worth price charged.

Steven Hauser 10-26-2004 07:34 AM


Hi there,

First and foremost, it doesn't matter what the deflection calculation is along the truss because as it has been clearly delineated the primary issue is the fact that backerboard manufacturers don't advise using their products on truss systems. nor floor joists spaced wider than 16" O.C.

That aside, I think it is time to print out the responses you have and present them to the contractors.

My .02

t majchrowski 10-26-2004 12:35 PM

Deflection Calculations and Subfloor Info
CX and Folks,

I had a fellow from Trus Joist run the deflection calculations on his own program. I believe this is the industry program. Also had an independent engineer run the calculations on his own program. Both came up with about the same results. I saw the independent engineer enter the info for his program. The inputs were type of trus, length of trus, separation of trus, width of plywood, number of supports under the trus, including the end walls, and maybe something else. I believe these calculations just gave the deflection of the trus. Anything else, way beyond my comprehension!!

Hope this is what you asked for.

Also, just found some info indicating my subfloor is not 3/4 inch plywood, but either 23/32 inch OSB or 1 1/4 inch 1.3E Timberstrand LSL. These are listed as accessories on my floor plan. Don't know which goes where. Does this make any difference?

I will give this entire thread to the general contractor and the tile contractor. Hope they see fit to ask questions on this forum.



cx 10-26-2004 07:34 PM

Well, we'll feel better about the joist deflection now. :)


Also, just found some info indicating my subfloor is not 3/4 inch plywood, but either 23/32 inch OSB or 1 1/4 inch 1.3E Timberstrand LSL. These are listed as accessories on my floor plan. Don't know which goes where. Does this make any difference?
Yep, that could make a difference, but only if we know qual es qual. The OSB option is still just a 3/4 inch subfloor. The LSL would be substantially better. You'd still hafta contact the manufacturer to determine the deflection, though. But first you need to find out what's actually on the floor, eh?

And then, even if the deflection is better than 1/720, there's gonna be at least one guy jump in here and say it don't matter on acoounta you still gotta have double-layer subfloor for that stone. But that will be a fun argument. :D

My opinion; worth price charged.

T_Hulse 10-26-2004 07:54 PM

You still gotta have a double layer subfloor for that stone. :)

Bill Vincent 10-27-2004 06:15 AM

Absolutely. AS well as having L/720 along the framing, you also need it between. Double layering is a must. Even if using a membrane, all membranes, including Ditra, require that the floor meet the deflection standards PRIOR to installation of the membrane.

cx 10-27-2004 07:15 AM

Aw, c'mon, Vincent, I promised the guy a fun argument. :rolleyes:

If he has that injineered flooring, he'd have 1 1/4 inches of subfloor between joists, likely meeting 1/720, but it will be a single layer.

Now, would you kindly enter into the argument as presented? :shades:

Bill Vincent 10-27-2004 07:49 AM

Well, as "presented", the floor should do fine, then, for Ditra or 1/2" cementboard, however, I'd still prefer the protection that double layering the plywood gives.

T_Hulse 10-27-2004 10:48 AM

All right CX you finally baited me into it. :stick:
There is a 2 layer subfloor requirement for all stone floors. When you add your final underlayment layer (CBU, membrane, or mud) that makes 3 layers total. The reasons you need two layers of subfloor are that stone is super sensitive to movement, & because deflection is not our only movement concern. There is also expansion/contraction. The overlapping 2 subfloor layers limit the effects of this by tying the whole floor together structurally. Imagine you found a subfloor material as thick & stiff as concrete. Let's just say you could put concrete up there. You still have a cold joint every 4x8 feet, and none of the underlayments let you just plow through & ignore cold joints. So a super stiff 1 1/4" panel is great for deflection, but it does nothing to tie the floor together structurally.
Tom please make sure your builder understands that cement board is not a structural panel. They will sag, crumble, & pull right through screws when subjected to structural stress because that's not what they were designed for. They're just an underlayment made to help isolate the tile from floor stress (not reduce the stress) and to provide a good glueing surface.

You need 2 layers of subfloor, 1 layer of underlayment for stone. :)

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