# Concrete or mortar on plywood



## warmsmeallup (Apr 3, 2008)

I overheard a mason say that you never want to put a stone floor with mortar or concrete directly onto a plywood sub. You always use backer board first. I've seen hundreds of stone/tile floors installed directly on plywood as long as the mortar below the stone was at least 1-1/2" thick for support (dry pack or the like.)

What are your thoughts and why?


----------



## JazMan (Feb 17, 2007)

If you're setting the tiles on a 1 1/2" thick mortar bed, you're not tile directly on plywood. 

Tiles can, but should not be installed directly on plywood. Even then you would require a double layer of plywood. It's one of the methods recognized in the industry, but only in dry areas and with many precautions. High failure rate.

In your question you mentioned stone tile. Stone tiles required a much stiffer subfloor system than regular ceramic/porcelain tiles do. Your typical house is rarely built to meet these higher standards unless stone tiles were planned. 

Jaz


----------



## warmsmeallup (Apr 3, 2008)

Let me clarify..and I'll fill in the blanks...

12" o.c. joists, house definitly prepared for the weight, 1" -2" thick stone on a 1-1/2" mortar bed.

We installed an electric heating element on the 3/4" plywood (as we have done many times before) and the mason is saying "You can't just lay the mortar (concrete, actually) right on the plywood. It will absorb the high volume of water and warp, rot...etc. He said that I have to remove the element so he can put down a tar paper (or backer board) to make it waterproof first, then our element and then his concrete and stone floor.

Isn't this design creating a "floating stone floor"? The concrete doesn't bind with the plywood...right. Or, am I wrong and his is the right way to install a stone floor?


----------



## Bud Cline (Mar 12, 2006)

He is correct.


----------



## warmsmeallup (Apr 3, 2008)

Bud:
Would you clarify why you don't want the mortar to bind with the plywood sub. I've been told that was the objective, a solid substrate to support the tile/stone...whatever.

Also, how then would you apply an SLC (50% water) over plywood if it isn't going to bind?


----------



## Bud Cline (Mar 12, 2006)

> Bud:
> Would you clarify why you don't want the mortar to bind with the plywood sub.


Did I say that? Where? That's not what I said. I said I agree with the mason.



> He said that I have to remove the element so he can put down a tar paper (or backer board) to make it waterproof first, then our element and then his concrete and stone floor.


The above statement is correct. It has nothing to do with bonding the mud to the substrate, it has everything to do with the moisture in the concrete mix ruining the substrate.



> Also, how then would you apply an SLC (50% water) over plywood if it isn't going to bind?


SLC's do bond to the substrate.. But, all SLC's require that a primer be used prior to the installation of the SLC. This primer separates the wet SLC mix from the vulnerable substrate.

Shouldn't you know all of this by now seeings how you do what you do?


----------



## warmsmeallup (Apr 3, 2008)

Actually, not at all. We don't install flooring. We install the electric (low voltage or line) radiant system that is integral. The design is up to someone else but in all of the installations where we have installed a cable (exclusively), solidly 1/2 are on plywood and the rest is on concrete/backer or some other type where a mortar, thinset or SLC is applied after we're in place. My apprehension came in when a roofing tar paper was being asked to go in before us.

Then, backer board was suggested instead and what I couldn't understand is what's the difference between backer board and the mortar or concrete itself. I "thought" that if you use a mortar or concrete over BB, the moisture will surely wick through to the plywood below...and doesn't Durock require a thinset under it to maintain their warranty...or is that old info?

So, I was back at, if that was the case, why not just apply the mortar right on the plywood!? I do understand that if you wet the plywood over long periods it will damage the wood, naturally. But I have been in many new construction homes where the plywood sub is open to the elements for long periods, getting wet or snowed on...It dries out, no? So, what's the difference??? 

Keep in mind that I'm here seeking information about something I'm not sure I understand. I am not looking to lay blame.


----------



## Bud Cline (Mar 12, 2006)

There are many things in this industry that are arguable for sure.

All backerboard is to be placed in a bed of thinset. I assume this is OK because the the backerboard will immediately begin to wick moisture so that all of the moisture isn't there to attack the substrate be-it plywood or OSB.

I can see where a thicker mud-bed could be a little different and offer up more moisture for a longer period and ruin inexpensive plywood and OSB substrates. Plus the mud bed requires the roofing felt to slow the drying and curing process and make the finished product a lot stronger.

But what about when using WEDI Board or KERDI Board, then what? They aren't going to wick anything.

Then it is true that plywood sub-floors and more-so OSB can sit exposed to the elements for long periods during new construction. Other than a few proud seams nothing really happens to those products under those conditions. How can a little cement mix or thinset be any different or any worse?

If exterior grade plywood or sealed OSB is used then that helps. But, the truth is, not all builders will spend the extra money. Most will buy the absolute cheapest panel product available that day.

So I guess where I'm coming from is knowing and following (and preaching)
the industry recommendations. This way if something goes to hell it won't be me paying to fix it.

I can honestly and proudly say that in almost thirty-five years I have never experienced a single installation failure of any kind. Oh sure a few minor issues now and then but by being anal and following the rules set forth by the industry I have managed to stay out of trouble and keep my money in my own pocket.

What would be wrong with you guys talking to some principals in these projects and getting an idea of what is going to be done before you charge in like bulls and do your thing. Why not work together.

I never start a big job without first talking with the customer, the builder, the plumber, the electrician, the tile supplier, even the painter and HVAC guys from time to time. I install a lot of floor heat systems and SLC and have never had an issue with any of it.


----------



## warmsmeallup (Apr 3, 2008)

As you can probably see here, I'm big on communication. However, it seems that every tile guy (as compared to the "true mason") all have their own ways of doing things. Since we work in 4 states, it can be quite varied. You could certainly combine the similarities but in the end and in most cases, we have to work with them, as compared to working together.

We have ongoing projects where masons were brought over from Italy, the Chzek republic..etc where they do things entirely different. Super high end stuff. Then we have this project where the HO is the GC. SHE told me that the floor was ready for us. We went in, installed the elements and left. Then I get a phone call from the mason installing real stones ...and you know the rest. I didn't argue. We went in today and pulled out 1600' of low voltage cable. Done. But it brought up all these questions in my head.

We have another project ...let's just say prominant guy in NYC. A 22k sf new build where we installed low voltage in 13 bathrooms, 700sf foyer, 2300sf kitchen, 900sf membrane roof and 1000sf hallway, etc...all of the interior work was with our element on 5/4 plywood with SLC (gypcrete, if that's different) poured directly on top of our element...no sealer/primer of any kind. It cured and then the 1" drypack and then 1" marble went on top.

Needless to say, it leaves one boggled as to what is the "right" way.


----------



## Bud Cline (Mar 12, 2006)

Gypcrete is different. Gypcrete is a proprietary product offerd by Maxxon and they pretty much do what they want. You can not apply tile directly to Gypcrete. There are special methods available for those installations.

However you can dry-pack mud over Gypcrete. Actually the minimum thickness of the dry-pack is supposed to be 1-1/8 to 1-1/4".


----------

