Tile subfloor over osb new construction

[Unregistered]

I was shown how to prepare the underlayment for a ceramic tile floor by simply laying 1 layer of plywood on top of the OSB flooring on a new home. The plywood was glued on with PL 400 adhesive and then screwed down every 6 inches. The tiles are now all cracking and coming loose and some of them are higher than others. I am now preparing to install tiles floors in the kitchen and bathrooms of another new home and am wondering what is necessary to prepare the subfloor. Currently there is a fairly uneven osb flooring on top of studs that are at 16" centers. Is it necessary to lay down a layer of plywood and then put down backerboard on top of that before tiling? If so, what is the best yet economical backer board to use? I have also seen the membranes that come in rolls which look very easy to install. Do they also need a laywe of plywood underneath them. Although I am building this home to resell right away, I want to do it properly yet keep both cost and how much work is involved in mind. Any suggestions? BTW This home is is Saskatchewan, Canada where the winter is very cold and the summer is very hot.

 

[Rob Z]

Hi,

Usually I can't offer any advice here at this site because the methods and products are different in the UK and the US/Canada...but with your situation the work of the TCNA covers both the US and Canada, and this is what applies to your job.

The standard recommendation is to NOT GLUE the second layer of plywood over the subfloor. Unless a full-spread of glue is used, the ribbons of adhesive will likely not be compressed completely once the second sheet of underlayment is laid on top of the subfloor. The testing in the TCNA lab reveals that these ribbons of adhesive might create small voids on either side of the adhesive, where the two sheets aren't drawn tightly together.

So, the recommendation is to avoid adhesive altogether (unless a full-spread of glue is applied).

The situation is made even worse with an adhesive like PL 400, which is extremely viscous (sp??) and will retain some of its body once the second sheet is brought down on top of it (it won't flatten out completely). These voids might only be a sixteenth of an inch thick, but it is enough to put movement and deflection in the second sheet of underlayment and cause the tile installation to fail.

Also, the second layer of plywood should be 1/2" or more, because the thinner plys don't have much strength. The grade of ply is important, as well. CDX is a no-go because of all the voids in the plies in the center of the sheet.

More info is needed about your joist spacing than just the 16" OC layout. What is the unsupported span, and how tall are the joists? Are they installed properly, or have they been compromised by improper notching, boring etc?

All of the cement backer boards are roughly the same, IMO. The one that is a bit dicey to use is Hardibacker. Because Hardi is so strong , it oddly enough leads to more failures if any part of the installation isn't done correctly. The testing at TCNA and elsewhere has shown that Hardi is so strong and solid as a panel, that the stresses in the buidling will transfer around the Hardi panel and reflect on a failure pattern of 3' x 5', corrresponding the edges of the Hardi panel itself. The testing in the labs seems to indicate that a softer board such as Permabase has enough "give" in the panel that the stresses of movement in the building will go through the panel rather than propogating to the edges.

The layers of membrane that you have seen might be Schluter Ditra? Schluter says it is ok to install their product over OSB, but I am a bit dubious of that and alway put plywood over OSB rather than go straight over the OSB.

 

[DHTiling]

Cheers rob...meant to pm you this one...

 

[Rob Z]

Hi Dave,

Let me contibute to the vernacular of the trades in UK, if I may. Here in the US, we don't like OSB that much, so we call it either "Beaver Barf" or "Vertical Mulch". :lol:

Gotta keep a sense of humor, ya know. OSB is a fact of life here, so we have to do have to work with it.:incazzato:

 

[DHTiling]

OSB is a fact of life here,

I have noticed that Rob....not very often do we in the uk come across it in the tiling world...

 

[Rob Z]

Dave,

There is a product here called Advantech, which I think is technically a type of Beaver Barf , uhh......OSB, but it is somehow different and seems to be of high-quality. We just used it for the first time on a kitchen floor and we are quite impresses with it.

 

[DHTiling]

Personally Rob.. i do not think it is stable enough to tile directly to it.....overlay is way forward..imo...

OSB has it use's coz it's cheap but deffo not to tile over.......

 

[Rob Z]

Dave, We've never tiled over OSB ,nor have we installed membranes over it (even thought the mfr's say it's OK).

The advantech kitchen floor...we just yesterday screwed a layer of very nice plywood over the advantech, and we will work off of that.

 

[Unregistered]

Thanks Rob, I have read everywhere that glueing the plywood was not a good thing to do so let me confirm the right way to do it. I understand that I should attach the plywood to the beaver bart with a thinset mortor apparently with some kind of additive? So just to be clear after I have put a layer of plywood over the area I should then cover the area with a backerboard as well and attach this with thinset too? Doesn't all these layers of plywod and backerboard result in your tile floor being way higher than the rest of your flooring. I will be putting hardwood and carpet in the house as well and am trying to keep the flooring approximatley the same hight for ease of transitions.,

Hi,
Usually I can't offer any advice here at this site because the methods and products are different in the UK and the US/Canada...but with your situation the work of the TCNA covers both the US and Canada, and this is what applies to your job.
The standard recommendation is to NOT GLUE the second layer of plywood over the subfloor. Unless a full-spread of glue is used, the ribbons of adhesive will likely not be compressed completely once the second sheet of underlayment is laid on top of the subfloor. The testing in the TCNA lab reveals that these ribbons of adhesive might create small voids on either side of the adhesive, where the two sheets aren't drawn tightly together.
So, the recommendation is to avoid adhesive altogether (unless a full-spread of glue is applied).
The situation is made even worse with an adhesive like PL 400, which is extremely viscous (sp??) and will retain some of its body once the second sheet is brought down on top of it (it won't flatten out completely). These voids might only be a sixteenth of an inch thick, but it is enough to put movement and deflection in the second sheet of underlayment and cause the tile installation to fail.
Also, the second layer of plywood should be 1/2" or more, because the thinner plys don't have much strength. The grade of ply is important, as well. CDX is a no-go because of all the voids in the plies in the center of the sheet.
More info is needed about your joist spacing than just the 16" OC layout. What is the unsupported span, and how tall are the joists? Are they installed properly, or have they been compromised by improper notching, boring etc?
All of the cement backer boards are roughly the same, IMO. The one that is a bit dicey to use is Hardibacker. Because Hardi is so strong , it oddly enough leads to more failures if any part of the installation isn't done correctly. The testing at TCNA and elsewhere has shown that Hardi is so strong and solid as a panel, that the stresses in the buidling will transfer around the Hardi panel and reflect on a failure pattern of 3' x 5', corrresponding the edges of the Hardi panel itself. The testing in the labs seems to indicate that a softer board such as Permabase has enough "give" in the panel that the stresses of movement in the building will go through the panel rather than propogating to the edges.
The layers of membrane that you have seen might be Schluter Ditra? Schluter says it is ok to install their product over OSB, but I am a bit dubious of that and alway put plywood over OSB rather than go straight over the OSB.

 

[Rob Z]

Hi,

I'm sorry if I was unclear...no, thinset is not used between wood panels....only an adhesive that is compatible with both of the wood products. If it is plywood-over-plywood then a wood glue such as Titebond can be used. If it is plywood-over-OSB it should be a urethane based glue. But it is current best practice to avoid the glue altogether between the panels. Only if glue is full spread (100% coverage) should any adhesive be used.

I only mentioned the backer board because you were asking about it in your first post. For a two-layer system that you are talking about, in most cases I would use a membrane over the underlayment such as Schluter Ditra or Noble Company CIS. My Canadian tile setting friends at Harry Dunbar's tile forum tell me that Schluter is widely available in Canada, but the Noble company products are spotty on the availability.

I prefer Noble CIS because it can be installed with their proprietary glue (NobleBond EXT).

If you give me more info about your joists (span, size, species, etc) I can offer some suggestions about what might be appropriate.

The one thing that the guys at harry Dunbar's forum tell me that is rampant in Canada is the use of "Scratch Coat" (nothing more than thinset skimmed over metal lath), which they tell me is horrible method, prone to massive failures. For some reason, it continues to be in wide use in Canada. I'm bringing it up just in case someone advises you to consider the method.