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Old 12-26-2016, 05:27 PM   #1
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Confused on OSB Subfloor


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…

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Old 12-26-2016, 06:04 PM   #2
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Re: Confused on OSB Subfloor


Need to figure out how thick and if if that OSB is T & G and if there's anything under it such as CDX.
If it's damaged it needs to be cut out and replaced with Advantech T & G sub flooring.
1/4" plywood underlayment of any type should never be used under any tile!
NO way should there be any kraft paper or felt used over the subfloor before the tile board!!

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Old 12-26-2016, 06:21 PM   #3
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Re: Confused on OSB Subfloor


I guess I missed that detail-
the OSB is 3/4 it is on the Ibeam - no CDX below.
It is T&G.

Do you know the reason why you can't put down 1/4 ply?

I was wondering about some type of paper because it is required for deck mud shower pan...

thanks for your help

Last edited by MLB1961; 12-26-2016 at 06:24 PM.
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Old 12-26-2016, 06:44 PM   #4
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Re: Confused on OSB Subfloor


I should add (for what its worth) Insurance adjusters said the floor was fine and did not need to come out. they seemed genuine and not intent on paying out as little as possible.
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Old 12-27-2016, 12:48 AM   #5
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Re: Confused on OSB Subfloor


I'll try to answer at this thread rather than the other since you hijacked the other thread which started over 4 years ago.

Your OSB subfloor should be exposure 1. Subfloor "grade" means it's made with waterproof glues and it's t&g. If any doubt, the sheets should be stamped on the underside.

Your "joists" are I-Joists and apparently are spaced 19.2 inches apart. Could be worse if they were 24", but 16" woulda been better. I'm sure they meet specs for ceramic tiles, probably not natural stone though.

All subfloor sheets should be glued and fastened correctly. If that area was done after lunch and they missed some fasteners, you can take care of that now. Sometimes it's hard to see the fasteners, esp. if they're staples.

If the floor is uneven regardless of the cause, you gotta fix it.

Tile backer not set into thinset is wrong. The lowest bidder got the job perhaps? The reason for the thinset under the CBU is not to bond it to the subfloor, it's to support the CBU so it doesn't move, causing grout cracks or worse. Most manufacturers want you to use modified under the boards, at least one says unmodified. The instructions vary a bit, but all get thinset, fastened then seams taped etc.

No tarpaper, rosin or anything other than thinset. No luan, no ¼" sheets of anything. You could got with the proper grade of ⅜" however.

¼" CBU is made for floors use it if it's better for you. It'll be about 5/16" or so installed. Otherwise you can go premium and use a membrane such as Ditra Matting which is only ⅛" installed. To improve the floor's strength, you could install ⅜" or thicker ply, then Ditra to save height.

Jaz
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Old 12-28-2016, 07:45 AM   #6
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Re: Confused on OSB Subfloor


Thank for your time and knowledge to reply. I am now readjusting - I think the 3/8 ply and 1/4 cement board may be the way to go. but I am also looking at the mats to top the 3/8 ( i did not know that type product was out there).

It is clear not to use 1/4 ply -

I also saw (in the home center) some 5mm "underlayment board" - I think that is for vinyl?

It has me curious (although I will not) what is the reason(s) not to use 1/4 plywood.

thanks again!
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Old 01-05-2017, 02:18 PM   #7
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Re: Confused on OSB Subfloor


got 1/2 B/C plywood - getting ready to cover the 3/4 OSB.

Any recommendations on nailing or screwing the 1/2 inch?

I know to not attach ( nail or screw) into the i joist but just into the 3/4 OSB. Also do not replicate joints where two OSB pieces meet - (over lap by at least 6 inches?)

Does this mean I need 1 inch fasteners?
I assume nails should be ring shank
Screws - what type - probably not drywall screws

4 inch spacing on edges and 6 inch file spacing

FWIW - I am going to use 1/8 ditra on top of plywood...
thanks again for the help and support.
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Old 01-05-2017, 02:41 PM   #8
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Re: Confused on OSB Subfloor


Space the ply ⅛" between sheets and ¼" or more from all other solid objects. Ring shank nails is the #1 choice, but good quality screws are fine. The fastener should be a bit longer than the two sheets, 1 ¼" will expose the tip from the underside, so good.

When you use the correct length you won't have to worry about missing the joists. Yes, 4 corners should only meet where 4 certain western states meet, never ply sheets. I think with ½" ply, 5-6" at perimeter and 6-8" in the field is plenty, but I usually tend to go closer.

Ditra is my favorite, don't forget the two types of thinset required. And....don't cheap out.

So that I won't need to read all again, have we established the subfloor system is OK and the floor is flat within specs for the materials being installed?

Jaz

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Old 01-05-2017, 02:51 PM   #9
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Re: Confused on OSB Subfloor


Thanks Jaz Man,
I can go tighter on the fastener spacing -

I agree this is no time to save a buck.

not sure what you mean by "sub is ok" - it is a bit rough on the surface but two insurance adjusters said it is structurally ok and did not need to be removed for mold or any reason.

Thanks or the tip about two types of thinset on either side of the Ditra _ I will follow that spec.

I am assuming that the floor will be pretty flat after plywood but I do not know the specs of the tile - it is porcelain 6 X 32 inch wood look "planks".

I am going to check for flatness with my 4 foot level - I do know the is a dip in one spot - I have not researched but as of now I think the fix is SLC on top of the plywood before ditra...
thanks once more
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Old 01-05-2017, 05:51 PM   #10
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Re: Confused on OSB Subfloor


the dip is ~1/8 to 3/16 but in a short span - between i joist and wall, maybe 20 inches.
maybe patching compound? over the small area?
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Old 01-05-2017, 07:33 PM   #11
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Re: Confused on OSB Subfloor


I'd have to "see" where the dip is in relations to everything else. If the dip is only in one spot, you may be able to fix it without causing other problems.

For large tiles over 15" on any side; the floor needs to be flat within ⅛" in a 10' radius AND 1/16" in 24" of plane.

Plywood is not gonna make any floor flatter after installing it. You're gonna fastened it to the old and it'll follow the same contour. A 4 ft. level is not gonna tell you much.

Jaz

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