# Subfloor Uncertainty



## MarcoPollo (Dec 6, 2014)

Hey folks,

This may sound juvenile of a question but this is something I struggle with a lot when it comes to tile. It seems basic but I always go back and forth with options and never really know exactly what I should have as a "go to" for processes.

Subfloor prep before tile. I have heard guys say "just throw 1/4 hardi backer and then Ditra and you're good."

Obviously there are variables but with your standard 5/8" aspenite in decent shape on 16" centres spanning a standard house 10-12'.

There is the option of ripping out the entire subfloor blocking between and then plywood to top of joists and then inserting plywood over everything. TCNA says 1 1/8" I believe but that makes transitions from room to room quite annoying.

There has got to be a relatively easy answer as just "laying another 1/2" and go" as that creates uneven floor heights too (I'm talking drastic not minor which is to be expected)

Would anyone care to share their "basic" approach assuming its a standard bathroom glazed porcelain tile over decent subfloor?

Hope I didn't ramble much, and feel free to call me a hack


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## Stevarino (Sep 28, 2013)

1/4" hardi AND ditra is redundant and seems like a waste. According to Schluter specs ditra can be installed over 3/4 t&g ply or OSB on 16" centers and exceed expectations. 

Personally I use ditra for all new construction and remodels where I have 3/4 t&g. Sometimes it's not worth ripping the subfloor down to the joists and installing 3/4 t&g so as long as the floor is solid and stable I thinset and screw 1/4" hardi and install my tile.


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## GO Remodeling (Apr 5, 2005)

for questionable subfloors, it's plywood. You can Ditra over that. 

There is also a woven and mesh product (I have to find the name) that you thinset down and will be as strong as ply, too.

Most guys here still use cement board because that's what they learned.


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## MIKE ANTONETTI (Jul 17, 2012)

Not much input so far so here I go.

What you are unsure of is Dynamics.

A lot of math and science can go into that answer.

Stone/ceramic/porcelain type materials do not flex, therefor they require support, movement in subfloor if not factored in will cause a failure. It can be from deflection (up and down) movement, differences in expansion between one product verse another(plywood,concrete,tile) move at different rates and different causes)
Shear strength (sideways)factors in, can setting material hold on to product when movement occurs. Tensile strength(upward) how well product holds for upward pressure.

I believe that TCNA has about 20(can't remember the number) ways to set tile onto wood, revisions occurring yearly.

Of course your responsibilities as far as materials, mixing, and all the other variables go into play(expansion joints)

Successful installations don't look much different than one that will inherently fail, there is extreme knowledge in a job that not only looks right, but performs till someone says its so 80's.

Every job will be different, you need to be well versed in types of install, and products available, both of those factors are time and money, both are never enough.


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## MarcoPollo (Dec 6, 2014)

I guess where I am having difficulty is as follows:

As a carpenter if I can parallel a deck build to a bathroom tiling job this may clarify where the differences are.

A basic deck has a "go-to" of 2x8s on 16" centres as a basic structure for most common decks. When we change and deal with composites, steel framing etc it changes which is fine you price accordingly etc.

What I am trying to nail down is the "2x8" standard for tiling bathrooms.

Around here 90% of your sub floors are 16" centre TGIs with 5/8" OSB T&G on top. Most sub floors we prep for tile are in decent shape and if the floor is even slightly spungee we cut block and replace with 5/8" plywood. 

Now 3/4" t&g is preferred but rarely done around here which then doesn't guarantee our L360 etc. 

We have never had a problem with Stone installs as part of this price as part of that price becomes restructuring as floors around here are never proper for stone installation. 

So now I arrive at what I asked before. Assuming the span and spacing of our joists are good, does a standard practice become ditra or protegga and then tile?

I have a tile guy who infuse often as he is well respected around here for high end installs tell me 1/4" hardi backer is as tough as 5/8" plywood as a subfloor. So he uses that for stiffening and then uses ditra afterword for crack isolation.

Then I come here to educate further and you guys all argue about CBU offering no structural addition to the floor. 

Then we get the discussion of other layers of plywood then ditra. By the time you do this some of your floor heights after tile must be huge (1/2" ply then 1/8" for ditra then 1/2" for tile and mortar). If you don't have hardwood as an adjoining floor covering that is quite a discrepancy in floor heights. Sure you can install a saddle but some clients don't like this.

Maybe that's clearer to explain my disconnect? I'm trying to figure out how most of you get in and out making quick decisions on the "straight forward installs" without floor height issues and knowing you aren't going to run into failure long-term.


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## Evan1968 (Dec 5, 2010)

Marcopollo said:


> I have a tile guy who infuse often as he is well respected around here for high end installs tell me 1/4" hardi backer is as tough as 5/8" plywood as a subfloor. So he uses that for stiffening and then uses ditra afterword for crack isolation.


Respected? High end installs? Ditra over Hardi? I wouldnt let that guy set my table let alone any tile. 
Figuring out if floors are suitable for tile is easy. Its called deflection. Deflection is at the joists or between joists or both. Ask your guy what L/360 means to him and how to figure it out. Do yourself a favor look for a new tile guy.


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## MIKE ANTONETTI (Jul 17, 2012)

I need some help with the acronyms and spell check, TGI means? I have a tile guy who infuse?

I think some numbers from manufacturers would make decision making clearer. You can argue OSB is a good product, but quality flooring other than carpet should have a better product. 

Cement board is flimsy while fiberboard is stiffer, again numbers would show. Anything other than carpet I would want plywood as substrate, I do like advantech as a subfloor.

Deflection in the engineering team refers to the joists with live and dead load, vertical movement of decking in between joist is also a local deflection.

I don't believe that special installer is correct in saying the hardiboard is equal to 5/8" plywood, provide some numbers, I don't have them and see how he argues against fact. Nor do I believe he is an NTCA member, who would be amongst his peers, talk like that is deflected with truth.

I'm just not positive about thickness of substrate lately as there has been many changes and I'm pro slab.

Your tile guy can get by with the procedures, some of it doesn't make sense, I agree 100% about raising the height to attain standards. Personally if I were you, I would contact NTCA and talk to Gerald Sloan, Dave Gobis, can't recall the other guys, maybe James Woefel, their designed systems work, not every job that's substandard fails, but the signs are there.


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## Shellbuilder (May 14, 2006)

Yeah my tile guy engineers wood floor framing,,,,now that funny.


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## Tom M (Jan 3, 2007)

Hardi or cement board add little if nothing to stiffen its just choice of bonding surface and decouple. Ditra has the cups in the mat to bond to and is a better at decouple. In either case your underneath wood needs to meet the spec.


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## MarcoPollo (Dec 6, 2014)

Evan1968 said:


> Respected? High end installs? Ditra over Hardi? I wouldnt let that guy set my table let alone any tile.
> Figuring out if floors are suitable for tile is easy. Its called deflection. Deflection is at the joists or between joists or both. Ask your guy what L/360 means to him and how to figure it out. Do yourself a favor look for a new tile guy.


There are probably more "politically correct" ways of putting it but yes, he has been doing high end installs for longer than I have been a carpenter. Agree or not with his methods, that is fine, but I have never had a failure from his installs nor have I heard of any through any of the circles of people I know use him etc.

I understand deflection. What I was trying to ascertain was a common practice given dynamics I provided as a "go to" starting point. I want asking to be provided with answers that make it seem that I don't have a clue what I am talking about.

The truth of the matter with all you tile folk is you're very scattered with your processes and argue even amongst yourselves as to what is right and wrong, yet seem very annoyed when someone asks you a question that you think is "easy"

I could rip out a sub floor and rebuild to way over meet deflection or I can build an entire house using 3/4" t&g throughout however that isn't the only work I do so when things are different I was looking for a more straight forward when dealing with 5/8" OSB sub floor which doesn't leave massive floor height discrepancy and still meets long-term success...

Apparently this is a large faux pas.

I spent my Christmas break reading the TCNA to try and learn as much as I can but many things theory wise don't always translate to practical so I was trying to ask this. I guess that was not a good idea.


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## MIKE ANTONETTI (Jul 17, 2012)

If you don't see failures your not looking good enough, almost every job I demo I can find something wrong other than ugly and outdated.


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## MarcoPollo (Dec 6, 2014)

Mike, I can appreciate that wholeheartedly. But I don't always get the opportunity to demo every job and don't have the background necessary maybe in order to see some things on a floor assembly that maybe I would see if it was wood related because of my background. Hence the reason for asking questions reading things and learning.

I just find when it comes to tile there are far too many "differences" in installation methods among those who do it full time. 

I use two full time tilers. One I have referenced the other far less frequently as he is a larger outfit and has focused more on commercial applications as residential (unless it's a very high end install).

And even within the two of them the "best practice" varies so much. The same doesn't exist in carpentry from my experience.

And then I come on here and Christ its yet again different.

"Get another tiler." Ok sure I'll do so but please point me in the direction of the tree where the good ones grow. Because as I can see I can learn and do a better job than most of them labeling themselves as tilers out there. And at least I know if I screw up I'll be the first one there to fix it.


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## GettingBy (Aug 19, 2010)

FWIW: 
I'd think you could use your body weight [feet together] as a known, concentrated load. 

The problem is, How do you conveniently measure existing floor deflection over 16" or so, without a helper with a straightedge and a dial gauge?

I'd think best practices should more or less follow the theory, and the measurements anchor you to theory, to the "truth".


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## MIKE ANTONETTI (Jul 17, 2012)

I agree, we are all struggling between profit, work needed, trying to do the right thing, etc. there's only 35k decisions to be made to get through the day. I have a hard time referring work knowing what I know. Keep learnin and prosper!


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## MIKE ANTONETTI (Jul 17, 2012)

Here's how I handled "deflection" in my trailer for a 3000lb. Ride on scraper. 24" o.c. I wanted my fabricator to weld 12" o.c. Metal beams and I was going to throw another 3/4" ply on top. He suggested 5/16" plate steel. Works like a charm, heavy as hell though!


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## GettingBy (Aug 19, 2010)

MIKE ANTONETTI said:


> Here's how I handled "deflection" in my trailer for a 3000lb. Ride on scraper. 24" o.c. I wanted my fabricator to weld 12" o.c. Metal beams and I was going to throw another 3/4" ply on top. He suggested 5/16" plate steel. Works like a charm, heavy as hell though!
> View attachment 211466


Any visible deflection?


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## MIKE ANTONETTI (Jul 17, 2012)

Zero, I did go back too far to align it and went off the plate, crunching of the plywood and I went forward.

The weight is distributed laterally, just an example. 

C. Harimon would have a "standard answer" but I can't see not agreeing to NTCA's recommendations and then personal responsibility on how one wants to proceed making their own decisions for materials.


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## madmax718 (Dec 7, 2012)

1/4 in CBU provides very little strength for deflection. While I understand that in your example it is over existing 5/8th, the 1/4 CBU does not substantially increase deflection ability. 1/2" last I recall is the recommendation, but again, if your worried about height, it is best to take it down to the joists. Sometimes there's some very bad stuff done below- like butchered joists. You could ignore the problem, but should you? 

Dietra is a detachment layer, but really not meant for vertical deflection. It can probably withstand a little bit, not not a whole lot. Your grout would start to pop and crack, even if your tile didnt. 

IMHO, 3/4" T&G ply is close (though not perfect) to a universal answer. It is a lot more work obviously, but a single 3/4 is more effective than two 5/8th. Even if you screw and glue like crazy, its still not a full plywood layer adhesion.


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## hdavis (Feb 14, 2012)

Just to annoy everyone, call backs are usually about cracking - tiles or grout. Different tiles / sizes can handle more or less deflection - just a fact of the material. Different grouts can handle more or less deflection, again it's a material selection.

I've demo'd tile that sounded hollow but had no obvious cracks. There was a problem for sure, but the HO didn't know it. Cosmetically and functionally it was still good. 

Go tile a floor with 1"X1" and the grout is going to be the weak spot, 12X24s are a lot different situation.


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## MIKE ANTONETTI (Jul 17, 2012)

madmax718 said:


> 1/4 in CBU provides very little strength for deflection. While I understand that in your example it is over existing 5/8th, the 1/4 CBU does not substantially increase deflection ability. 1/2" last I recall is the recommendation, but again, if your worried about height, it is best to take it down to the joists. Sometimes there's some very bad stuff done below- like butchered joists. You could ignore the problem, but should you?
> 
> Dietra is a detachment layer, but really not meant for vertical deflection. It can probably withstand a little bit, not not a whole lot. Your grout would start to pop and crack, even if your tile didnt.
> 
> IMHO, 3/4" T&G ply is close (though not perfect) to a universal answer. It is a lot more work obviously, but a single 3/4 is more effective than two 5/8th. Even if you screw and glue like crazy, its still not a full plywood layer adhesion.



I would disagree with the 3/4 vs 2-5/8" layers. No numbers to support that?


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