# thinset b4 cement board on OSB?



## deputy (Sep 11, 2012)

a couple of my buddies, who often do flooring work, told me not to put the cement board in a bed of thinset before tiling. they said to glue the cement board to the OSB then screw it per instructions. they all said the OSB will absorb the water out of the thinset and swell like a sponge and start popping tiles. true? if so, what glue do i use? ive got 2X10 floor joists then 1X8's then 3/4 OSB...am going to use 1/2 in backerboard.

second, what size trawl do i use on some 6X24 wood plank porcelain tiles? .25 or .5?


----------



## oh'mike (Sep 18, 2009)

Use the thinset----they are wrong----the thinset is there to fill any voids and keep the backer from flexing----

To verify what I said --read the tcna manual---or look at the instructions for Durrock---


----------



## garlicbreath (Jun 25, 2012)

deputy said:


> a couple of my buddies, who often do flooring work, told me not to put the cement board in a bed of thinset before tiling. they said to glue the cement board to the OSB then screw it per instructions. they all said the OSB will absorb the water out of the thinset and swell like a sponge and start popping tiles. true? if so, what glue do i use? ive got 2X10 floor joists then 1X8's then 3/4 OSB...am going to use 1/2 in backerboard.
> 
> second, what size trawl do i use on some 6X24 wood plank porcelain tiles? .25 or .5?


I read on JB's site that you should not use OSB when tile is involved. Can someone confirm that is the case?


----------



## cleveman (Dec 17, 2011)

I would try a .375 trowel frist, but have a 1/2" handy.


----------



## JazMan (Feb 17, 2007)

deputy said:


> a couple of my buddies, who often do flooring work, told me not to put the cement board in a bed of thinset before tiling. they said to glue the cement board to the OSB then screw it per instructions. they all said the OSB will absorb the water out of the thinset and swell like a sponge and start popping tiles. true? if so, what glue do i use? ive got 2X10 floor joists then 1X8's then 3/4 OSB...am going to use 1/2 in backerboard.


Sorry, but your buddies don't have a clue in this case. But, is this 3/4" OSB subfloor grade or cheapo generic, what brand? All CBU are required to be set into fresh thinset mortar, fastened and seams taped. 



deputy said:


> second, what size trawl do i use on some 6X24 wood plank porcelain tiles? .25 or .5?


Neither of those. How about .375. So, 1/4x3/8x1/4. 




garlicbreath said:


> I read on JB's site that you should not use OSB when tile is involved. Can someone confirm that is the case?


Perhaps you miss understood what someone was saying. OSB as a subfloor is fine. You install a backer or Ditra then the tiles and you're good. What they were saying, (or should have been saying), is that you can not install tiles over OSB.

Jaz


----------



## deputy (Sep 11, 2012)

its going on top of 3/4" OSB t&g screwed down to the joists through the 1X8's with 3" screws. the floor is about 160 or so sq. ft. and i used almost 5lbs of screws. the markings on the OSB is Blue Ribbon 22/32" THIS SIDE UP...got it from lowes.

Looking at my sub-flooring job, i accidently have one board flipped over...does it matter?


----------



## JazMan (Feb 17, 2007)

Deputy,

It's best to fasten the underlayment only to the subfloor and not the joists too. This allows a bit of separation from the twisting of the joists. Not the end of the world if you already installed the 3/4" ply since you'll have another layer over it. 

We normally have you tell us how the framing is built and with what materials. Give us the; type and size of the joists, (2x10 in your case), their species and grade will help a lot, on center spacing, (usually 16"), and the span, to the inch. We will refer to a span chart and let you know. 

I wouldn't worry about that one sheet much. BTW, can you confirm exactly which Blue Ribbon OSB product you bought? 

Jaz


----------



## MLB1961 (Dec 7, 2016)

I am confused about how to proceed with preparation of my floor for tile.
Background:
About 150 sf floor to cover in upstairs bathroom with separate toilet area. Construction is OSB (unknown Exposure Rating) over wood I Beams (I Beams are 12 inches wide (deep?) and seem to be spaced 20 inches apart). Longest span within the room is 11 feet – it may run out to 15 feet before there is a wall (support) below. Curiously it appears that the OSB is only nailed at joint with another sheets (not field? nailed) – I know they used adhesive on top of I Beams).


We had a leak (small drip – while gone for a week) and floor got wet. The OSB subfloor is a bit uneven now (and may have been since new). It was mostly covered with carpet except the toilet area and a “landing” area for the tub/shower. – Both of these areas were tile over ½ inch Durock. The Durock was nailed directly on the OSB (no thinset). These two small area have held fine for 16 years (age of house) – but it still seems incorrect. 


I want to do the floor prep correct as reasonably possible. 


It seems a given that the cement board (all types) have to be bonded to the sub floor. This is 1st area of confusion. I have read (somewhere) not to put thinset on top of OSB – Drock website states to use latex modified thinset or Type I Organic Adhesive. And notes the OSB should be Exposure Rating 1 or better).


I guess some will say go ahead and thinset cement board to the OSB. If that is the case any thought to covering subfloor with tar paper or rosen paper? - I do not know the OSB exposure rating…


I was thinking I would cover subfloor with Luan. I like the idea that (a long time) down the road you could remove the floor “cleanly” after the Luan is removed – back to the original subfloor. But now I have read to never use Luan between OSB and cement board… - Is that true?


Next thought is to use ¼ plywood screwed to the OSB and then thinset/screw ¼ cement board- I would think that would be a very good foundation for the tile. Of note here is that I would rather not have the height build up but I am will to make that trade-off for a trouble free install. 


Thanks in advance for your valuable knowledge and experience. I am OK spending a bit of extra time/money to “do the job right”. I really want to eliminate any possible future problems…


----------



## MLB1961 (Dec 7, 2016)

I am trying to delete my post #8 as i posted this on a new question
how do i do that?


----------



## JazMan (Feb 17, 2007)

What is the status of #8 now? 

Jaz


----------



## MLB1961 (Dec 7, 2016)

It is still there - I posted the same questions in another thread and was trying not to double post.

If you can take it down thanks 
Also I would value your opinion as to my best options
thanks for your time, you seem a wealth of knowledge


----------



## MLB1961 (Dec 7, 2016)

#8 is still there - I post this a a separate thread and wanted to delete this one - so as to not double post.


----------

