# OSB to durock install



## acfrqflyer (Oct 24, 2008)

hi there,

I'm re-doing my bathroom floor which has a 23/32" OSB sub-floor 16" o.c. I had to cut some pieces out of the floor due to rot and replaced it with new OSB. The sub-floor now has a few seams. Now their is a mix of new exterior grade osb and old OSB (i thinks its interior grade). I brought 1/2" DUROCK and planning to place ceramic tile on top of the durock. My question is : the mortar bed that goes in between the OSB and DUROCK should be what type of thinset? (unmodififed or modified?) Also, do I have treat(seal) or protect the OSB before applying the thinset??? The room is only 35 sq. feet..

Thanks for the info...


----------



## Bud Cline (Mar 12, 2006)

Use unmodified thinset, do not treat the OSB with anything.


----------



## Termite (Apr 13, 2008)

Also be sure to "mud" and tape the seams of the durock with thinset and mesh backerboard tape.


----------



## acfrqflyer (Oct 24, 2008)

when tapeing the seams of the durock do i also you the unmodified thinset??

I appreiate the info. thanks


----------



## RippySkippy (Feb 9, 2007)

Mud the tape when you set the tile, what ever you use on the tile will work fine. By doing it at the same time, you'll lessen the chance of creating a bump at each seam.


----------



## acfrqflyer (Oct 24, 2008)

thanks everyone for the info. hopefully, everything works out.


----------



## JazMan (Feb 17, 2007)

First of all, .....because words mean things, thin set mortar should not be referred to as "mud", especially on a tile forum. You do NOT mud the seams and tape them. Mud is sand/cement and is applied an inch thick or so as the base for tile. Thinset is the correct term.:thumbsup:

You can use unmodified thinset to install your Durock, but Durock specifies the use of modified thinset for under it and to install all tiles. 

Jaz


----------



## acfrqflyer (Oct 24, 2008)

thanks jazman,

what is the difference between modified and un-modified??

What would you recommend?

Thaanks again


----------



## JazMan (Feb 17, 2007)

I think it's always best to follow the manufacturer's directions. Durock and most other manufacturers want you to use modified so......?

Unmodified is standard thinset. It can be used over most cementitious substrates to install many types of ceramic tiles. It is not used to install tiles to wooden surfaces, which we don't recommend anyway. Modified or polymer modified thinset is used to install practically all types of ceramic tiles including porcelain and glass etc. It gives a much stronger bond and in most cases has some flex to it. ALL backer board manufacturers want us to use modified thinset to install tiles to their boards. 

When used under backer boards, modified thinset laminates the two materials together, yet allowing a bit of lateral movement because it's latex/polymer ingredients. I believe this extra bonds makes a more solid substrate. Do you know how to install, fasten, tape the board?

Jaz


----------



## acfrqflyer (Oct 24, 2008)

I was going to follow what the manuf. recommends. I never done this before . 1st time. any hints??


----------



## Termite (Apr 13, 2008)

JazMan said:


> First of all, .....because words mean things, thin set mortar should not be referred to as "mud", especially on a tile forum. You do NOT mud the seams and tape them. Mud is sand/cement and is applied an inch thick or so as the base for tile. Thinset is the correct term.:thumbsup:


Oh give me a break. Re-read my post. I used the term mud with quotations around it and then said to use thinset for the application. That is pretty clear, especially to a DIYer. There was no room for confusion in that post, and I realize there is a big difference between mud mix and thinset.


----------



## JazMan (Feb 17, 2007)

Hey Termite, you misunderstand my criticism. I know that you know what you're going to use, I'm thinking of all the people that don't know what we're talking about. The industry is full of hacks that say they are going to apply "mud" on certain installations when all they do is spread thin set mortar. I think they just picked up the the term from drywall installers? 

So, it makes a difference. There's no comparison between mud and thinset.

Jaz


----------



## Bud Cline (Mar 12, 2006)

http://www.contractortalk.com/showthread.php?t=9321


----------



## andynicks (Nov 23, 2008)

Hi all,

Thanks for the good information in this post. I found it using Google.

I have a 25sqft floor space in a bathroom I have renovated. I replaced the subfloor with 3/4" OSB and will be laying 12" tile. I purchased modified thin-set mortar on which 1/2" Durock cement board will be installed. Before mixing I read the back of the Versabond thin-set mortar that I picked up at Home Depot. It reads under the heading "Acceptable Surfaces", "*Do not bond directly to* hardwood, Luan plywood... plastic or *OSB panels*".

Interesting. I'm on my way over to Lowe's in a few minutes where I will be reading the back of their modified thin-set mortars. Has anyone experienced this same issue?!

Thanks,
Andy


----------



## jerryh3 (Dec 10, 2007)

andynicks said:


> Hi all,
> 
> Thanks for the good information in this post. I found it using Google.
> 
> ...


http://www.custombuildingproducts.com/docs/tech_bulletins/TB12%20Bonding%20to%20OSB.pdf?USER=pro\&LANG=en
Is it done? Yes. Will it be covered under warranty if it fails, no. Glue and screw a piece of CDX and you'll be covered. It's one of those things you'll get a lot of different advice about. The schluter install guide shows ditra going over OSB using a latex portland cemnt mortar. There are a few tile guys on here that maybe be able to give you a more definitive answer.


----------



## Bud Cline (Mar 12, 2006)

Thinset *CAN BE USED* to install CBU over OSB. It's not there to bond to begin with.
Thinset *CAN NOT BE USED* to bond tile directly to OSB.

Read all the bags you can find they all say the same thing.


----------



## JazMan (Feb 17, 2007)

Andy,

There is no contradiction in using thinset to install backer board or even bonding Ditra to OSB and the directions not to "bond direct to....... They are talking about not bonding ceramic direct to the OSB, not that modified thinset will not stick well to it. Modified thinset bond great to OSB, it's just that OSB is not stable enough to use as an underlayment. Installing a CBU or membrane changes all that. 

Jaz


----------



## slg1285 (Jan 5, 2014)

Just to clarify, I can apply modified thinset to the seams of durock? I am going to redgard before tiling. 

Or would unmodified thinset be fine since I am using redgard?

Thanks!


----------



## oh'mike (Sep 18, 2009)

I suggest you use modified thinset for any filling and taping---this is what you will be using to set the tile over the Red Guard any way--so why buy a second product---

Be sure you use a thin coating---creating a hump will cause lippage when you set your tile---


----------



## slg1285 (Jan 5, 2014)

Thank you! That is what I thought, but I wanted to be sure. Appreciate the quick reply!


----------



## JazMan (Feb 17, 2007)

Sig,

Sounds like you're doing a tub enclosure or stall shower, yes? Did you happen to have some unmod left over from another project?

Jaz


----------



## slg1285 (Jan 5, 2014)

Jaz, 

Yep, going a stall shower install, I don't have any unmodified, but you can buy the small bags of it, just to do the seems for the durock, I wasn't sure if it was acceptable. I need to buy modifed anyway for the tile, so I just got it as well. 

Thanks for the reply!


----------



## JazMan (Feb 17, 2007)

> Yep, going a stall shower install


OK, good. Most showers are not built completely right, (even by the so-called pros), are you up to speed on the details? The floor, lower walls and curb are the most critical for a shower and where 95% of the mistakes are made. 

Wanna share your plan in detail? Tiled floor or molded pan floor? 

Jaz


----------



## slg1285 (Jan 5, 2014)

Jaz, 

There was a corner shower unit installed prior. The unit started coming off the walls, it was just glued onto the drywall, this is when I first started to realize I had a problem. Upon removal and further inspection, the drywall behind had rotted away and the got underneath the plastic shower pan and started to get to the sub floor.

I decided to rip every thing out and start fresh. I was down to the studs and subfloor, cleaned it and let it dry for about two weeks (thankfully the house is only 6 months old and I noticed it in time). The subfloor was still sound. 

I installed Durock in the shower surround from the floor up 7 feet. I used tile-ready inserts for the shelving unit. I am using modified thinset for the seams, and will apply 2-3 coats of RedGard at 30 mils. I will install and pvc based (Oatey) pan liner to the sub-floor then do a pre-slope pour for the shower pan, redgard, final slope, redgard, then tile over the redgard on the floor and walls using modified thinset.

On the floor outside the shower, I have removed all existing tile, thinset and cement board, I am going to use an UNMODIFIED thinset to apply Ditra then tile over the Ditra with a modified thinset.

On a side note, I also siliconed all the screws in the durock and let dry prior to sealing the seams with modified thinset.

Think that sounds okay, any suggestions?

Thanks!


----------



## slg1285 (Jan 5, 2014)

As far as the curb is concerned, I used 3 2x4 wrapped in pvc pan liner and used Mark-E-Industries Kirb-Perfect plastic form to provide the appropriate cement backing the tile.


----------



## JazMan (Feb 17, 2007)

sig said:


> (thankfully the house is only 6 months old and I noticed it in time)


What? 



sig said:


> I will install and pvc based (Oatey) pan liner to the sub-floor then do a pre-slope pour for the shower pan, redgard, final slope, redgard, then tile over the redgard on the floor and walls using modified thinset.


Where did you get this procedure? Too many things going on here. 

Why use the pvc shower pan membrane and also Redgard? Gonna create a moisture sandwich. 

The pvc pan does NOT go right on the subfloor if you wanna do it right. You want the pre-slope first, then the pvc membrane, and finally the finish deck mud for the tiles. This is the traditional method.

If you want to use a liquid membrane you do not also use the pvc. Does someone now recommend that? Where did you get that info?

I only use the Kerdi Drain & Kerdi Membrane method, but 10 years prior used the traditional method. Kerdi and surface waterproofing is the best way. 



sig said:


> On the floor outside the shower, I have removed all existing tile, thinset and cement board, I am going to use an UNMODIFIED thinset to apply Ditra then tile over the Ditra with a modified thinset.


By your description the floor is plywood or OSB. You've got the types of thin set required backwards. You'll be very unhappy if you use unmod for Ditra over ply or OSB. 

No reason to silicone screw heads or seams. Now the Redgard may not stick to it. You may not know it but, paints, adhesives etc, do not bond to silicone.

Jaz


----------



## slg1285 (Jan 5, 2014)

You're correct about the thin set, error in typing, modified to bond to osb. I suppose I could just preslope onto the osb, figured it would give me a barrier to protect the floor.


----------

