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-   -   Slate Deflection in Condo and 1/2" vs 1/4" CBU? (https://www.johnbridge.com/vbulletin/showthread.php?t=129979)

risotttto 10-21-2020 04:21 PM

Slate Deflection in Condo and 1/2" vs 1/4" CBU?
Hi Everyone,

I kept getting really good info from searching google on this forum, so thought I would ask a quick question...

I'm re-tiling the kitchen in my condo, I want to do Natural Slate, 12x24x3/8" (can cut down to 12x12 if it's necessary)

It's a sealed condo floor, so no way to get under the subfloor to take a look at what the joists are made of. It was build in 1991 in Queens, I do know that the joists are wood or more likely engineered lumber, 19.2" on center, with 3/4" ply on top. The kitchen is around 90"x94", so not a huge span. Unfortunately the Deflectolator is not terribly helpful here without joist details.

Doing an IRL Deflectolator with my laser level, I see pretty much no difference in the laser height on the wall setting it on the middle of the floor with no load vs. 2 people standing next to it (400lbs!). Maybe 1/32?, 1/64 or less than 1/2 the width of the laser beam. Valid test? Seems pretty solid just by feel.

The old floor had 15/32" underlayment which had rotted from leaks and was covered in glue from a a terrible tile job, so I pulled it up and replaced it with 19/32" CDX, glued and screwed every 8" in the field and 2" on the perimeter with expansion gaps. Going by TCNA Handbook F250-16 for natural stone as a guide....

Next step is concrete board...

So my questions are

1. Is there any reason to go with 1/2" CBU vs 1/4"? Seems like most people say it does not add rigidity. I have 1 3/8" of plywood on top of the beams, and will already have 3/4" to transition to the wood floor using 1/4" CBU. I'm also thinking Hardiebacker for sentimental reasons.

2. Still safe to go with the 12x24 natural slate tiles? Is there any huge advantage to avoid future cracking if I run them parallel vs. perpendicular to the joists? Or cut them in half to 12x12?

3. The floor is pretty much flat, but is not level, sloping around 3/8" over 8 feet. Is this a major issue other than having to shim the cabinets up after installing the tile wall to wall?

4. Any other advice?

Thanks in Advance!

jadnashua 10-21-2020 04:38 PM

First off, CDX plywood is a no-go for tile installations! Industry calls for at least C-faces as the worst ones. The D-face, aka voids, is a red flag for any tile installation.

Industry standards call for two layers of ply, properly installed for any natural stone tile.

The size of the tile has no effect on how strong the structure needs to be or how it is oriented relative to the joists. Nor does the thickness of cbu which is a tile compatibility layer, not an indication of strength.

cx 10-21-2020 04:50 PM

Welcome, S. :)

If you don't put that geographic location into your User Profile the information will be lost before we leave this page. And it's frequently helpful in answering some types of questions.

The requirement for joist deflection for a natural stone installation is L/720, exactly double the requirement for ceramic tile. If your structure was built without natural stone in the original plans, it's very un-likely that your joists meet that requirement. If you want to install your stone tiles anyway, that's entirely up to you. The information you gained from your deflection test tells you nothing at all useful in determining the design deflection of your joist system

The Deflectometer will not help you with LVL joists even if you could see them. The size of your room to be tiled has no bearing at all on the joist requirement. It's the unsupported span of your joists that matters and you cannot determine that unless you are on good terms with your downstairs neighbor.

For you subfloor: Method F250 requires joists spaced no wider than 16 inches on center. In fact, I think you'll find the Natural Stone Institute doesn't have an installation recommendation for any wider joist spacing. Once again, you wanna install over what you've got, that's up to you.

Your CD grade plywood does not meet the requirements of either the ceramic tile or natural stone industry requirements. We'd also rather that you had not glued the subfloor layers together.

Did you use a full spread of wood glue or something out of a tube?

Did you orient your second layer of plywood with the strength axis perpendicular to the joists? That would be the same direction as the first layer.

1. There is never a reason to use the thicker CBU in a floor installation unless you just want to raise the finished floor height. I find it difficult to get sentimental over Hardiebacker, but...........:)

2. See above. No, it makes no difference in which direction you orient your tiles.

3. Your tiles don't give a rat's patooti whether your floor is level, they care only about flat. The larger the tiles, the more they care. The industry standard for flatness for tiles the size you have is no deviation of more than 1/8th" in ten feet nor 1/16th" in two feet. That's a very, very flat floor and you'll be glad to have it come time to set tiles.

4. Consider re-thinking your tile selection. Lots of stone-look porcelain tiles these days that are not unpleasant to the eye and would be a safer choice in your application. But you can certainly install what you have if that's your choice.

Your Condo Association is OK with your new floor covering?

[Edit] Jim's not really faster, he just typed less than did I. :D

My opinion; worth price charged.

risotttto 10-21-2020 05:17 PM

Thanks for the detailed and quick replies!

I'm on the condo board, so that's not an issue, before there was just 12x12 ceramic tile mastic-glued onto the 15/32" CD plywood, with a layer of sticky vinyl on top of that with assorted small insects stuck in between.

I looked at a few of slate-lookalikes, nothing quite looked right next to the real thing!

1/4 CBU it is!

Used Loctite 3x PL and the CD at the recommendation of a contractor friend.

Is it really worth ripping up 2 sheets of ply to replace CD (there was no X, just looked at the stamp) with BC or AC? It involves removing 300 screws, forcing and scraping off the glue, buying 2 more sheets of ply, ripping it, then putting in another 300 screws. The wife says no way we are doing that, am I totally screwed? (pun intended)

jadnashua 10-21-2020 05:43 PM

Two things (other than the possible joist and spacing, which could also be big):
- while that construction adhesive works fine on the top of a joist where you can nail/screw it down and expect it to spread out and fill in gaps, that WILL NOT happen when spread out underneath a sheet. A liquid wood glue, spread like thinset so you get full coverage CAN work, but it's tricky getting things together before the glue starts to set
- D-faces are known to create problems in tiling situations.
- maybe a third, if the glue in the ply itself is not rated as Exposure 1 or EXT, moisture can cause it to delaminate.

Your call, your home. How long do you want it to last? No guarantees either way, but the industry doesn't agree with your choices so far for reason, supported by testing.

cx 10-21-2020 07:23 PM


Originally Posted by S
Is it really worth ripping up 2 sheets of ply to replace CD...


Originally Posted by CX, Post #2
Did you orient your second layer of plywood with the strength axis perpendicular to the joists? That would be the same direction as the first layer.

I didn't see a response to my question and that would influence my response to your question.

risotttto 10-21-2020 09:30 PM

yes, the overlayment sheets strength axis runs perpendicular to the joists, same as first layer offset 1 foot without overlapping any seams, used 1 5/8 ceramic coated deck screws screws avoiding the joists.

cx 10-22-2020 07:33 AM

The gluing with a gun-tube material is a potential problem, as is the CD grade plywood with large voids both interior and exterior. The use of a CBU in that application will at least be better than a sheet membrane as regards the voids in the plywood, but it's still recommended against by the ceramic tile industry.

We can't guarantee failure any more than we can guarantee success, we can only tell you what the tile industry and product manufacturers recommend and where the smart money will be betting. :)

My opinion; worth price charged.

risotttto 10-23-2020 05:39 PM

my hand hurts
Just spent 8 hours ripping up one sheet of glued down ply! I am feeling the pain of my mistakes! Would have been 10 minutes without the tube glue. Lots of voids in the ply and beneath, that PL 3x sticks like a mother!

I figure better to pay now then try and fix it after CBU, tile, cabinets, counters and appliances are in!

Going to fill in any gouges in the subfloor after removal, check flatness then go for 3/4 AC or BC (X if I can find it), just screwed like crazy into the subfloor.

My contractor friend was obviously not a reliable source of info. He also mentioned PL 3Xing the CBU to the plywood, which seems odd as well.:scratch:

risotttto 10-23-2020 07:21 PM

underlayment joints?
Ok, one last silly question, the underlayment sheets are supposed to be gapped by around 1/8" according to Stone F250 and this Article , but when you lay down the mortar to hold down the CBU, won't that fill in the gaps when you key it in? Any reason to put some caulk in it to preserve the flexibility of the gap?


jadnashua 10-23-2020 08:03 PM

SOme people are adamant that that is a requirement, the thinset manufacturers seem to realize that any thinset that gets in there will pulverize if the sheets move. Gaps on the subflooring are generally required prior to closing the building in, as they can be rained on and the moisture levels can spike while the joists are also drying out. The plywood association actually calls for no gap on underlayment that ends up being installed after the building is after it was closed in and dried out. Subflooring goes in at the beginning, underlayment gets installed just prior to finished materials go on, much later. A small gap doesn't hurt, but it is not critical on a closed building later on.

cx 10-23-2020 08:17 PM

That second layer of plywood is structural subflooring, S, not underlayment. The CBU will be your tile underlayment. Yeah, yeah, I know what that article says, and what's in some of the TCNA Methods, but the second layer of subflooring would be an underlayment only if you intended to install tile directly onto it.

Another point: There is no CDX grade of plywood. Nor ACX or BCX. There are AC and BC and CD grades. The X we all see now in the big box stores is, I suppose, to represent the exposure rating, but that rating should be found in the grade stamp and expressed as EXT or Exterior or Exposure 1, all indicating an exterior glue was used in the manufacture.

End of rant. :)

And I'll point out again that the Method 250 for Stone in the TCNA Handbook us for 16" joist spacing. You don't have that. Nor the requisite L/720 joist deflection. Also, I think you'll not find a method in that handbook for using CBU as a tile substrate with any joist spacing exceeding 16" on center. Individual manufacturers' of those products may have differing recommendations.

All that said, you install your CBU per the CBU manufacturer's instructions. If they don't indicate doing anything with the subfloor panel joints, don't do anything. Just pretend they're not there. No, it's not intuitive to intentionally create those joints and then fill them with mortar while installing the CBU, but it's done all the time. I'm told there are some membrane manufacturers out there that now recommend bond-breaker tape over the joints, but I've not see any of them in writing. I know at least one has always said to pay no attention to the joints; don't fill them with mortar, don't try not to fill them with mortar.

If you think it better to tape over them or fill them with a flexible sealant, I suppose you can do that and I don't know that it will make any difference at all. I do know that if you have a tile installation failure, the CBU manufacturer is gonna say, "We didn't tell you to put that caulking in there!" That'll be right after they say they never said you could install stone tile over 19.2" joist centers with their product. Or somethin' else. :D

But if you think it's all gonna work out well, I'll just hope you're right.

My opinion; worth price charged.

risotttto 10-23-2020 08:58 PM

Good to know about the AC/BC/CD/X, I figured it was mostly BS, wood and water don't mix no matter what the label says!

The L/720 is unfortunately not possible to calculate in my situation using the Deflecto, which does not work for engineered lumber or account for second layers of ply above the sub-floor. I'm trying to beef things up and re-enforce best I can where possible. like adding 3/4" as a second layer above the subfloor, more than the 15/32" required by F250.

Before I ripped it up, the floor was holding 12x12 ceramic tile, on an 15/32" CD underlayment nailed to the subfloor (<20 nails/sheet?), with no CBU or mortar of any sort, just some sort of yellow glue holding the tile to the ply for 30 years with no cracks.... I hope to improve in this!

I figure adding 3/4" quality ply will help stiffen the floor with lots of screws (no glue; learned my lesson there), plus joists being engineered or LVL, and 19.2 OC is not as far off of 16" as say, 24"

.... basically, my joists aren't 16" OC but 19.2; I can't change that, so I add an extra 1/4" of plywood thickness over the F250 minimum to compensate.

Is there a better IRL test I can do for deflection other than putting the laser level in the middle of the floor, marking the wall, then adding a 400lbs of live weight in the middle? (Also did the string version of this, could not see a noticeable change in the laser/string)

Any thoughts on self-leveling "liquid backer board" self-leveling type products? I'm not quite convinced they can replace CBU, but would make cabinet installation and tiling a lot easier if the floor was truly level!

jadnashua 10-23-2020 10:39 PM

Ardex's liquid backerboard is one of few that can be feather edged over a wooden subfloor...most require some minimum thickness, often 1/2" above the highest point and metal lath over their proprietary primer. SLC does not really self-level, either...just like a pancake doesn't spread to fill the pan...to make say a crepe, you have to spread it out manually, you can't just pour it. SLC is the same way. While you can have success using it, if you don't follow all of the rules, it can be a very expensive mess. The side kick to this is SLC starts to set fairly quickly, in some cases as little as 10-minutes after you add the water, in the longest, about 30-minutes. If you're not done by the time that starts, it's sort of like trying to get a partially frozen puddle to lay flat after you walk on it. IOW, you have to have a very focused plan and execute it very quickly.

cx 10-24-2020 08:27 AM


Originally Posted by S
Is there a better IRL test I can do for deflection...

Sorry, but what is an IRL test?

The ceramic tile industry requirements for subfloor structure are based upon design deflection standards. The requirements for the joist structure are based upon lumber industry data for various species and grades of wood for joists of various size and span and based upon standard live loading of a particular weight per square foot - usually 40 pounds per square foot for most residential applications - plus a dead load based upon the structure and anticipated floor covering.

The requirements for the subflooring are based upon actual testing of floor assemblies including the subfloor, a particular underlayment or substrate, tile and grout. The assemblies are then published as an installation method. There is no reasonable way to duplicate such testing on site after the floor is installed.

You want to construct, or modify, your floor structure to meet the published guidelines if you want a reasonable expectation of long term success. That is not to say other structures won't work, just that there is a much better chance of success if you use a structure known to meet the industry guidelines.

My opinion; worth price charged.

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