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Old 03-13-2014, 10:34 AM   #1
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adhesion on weathered OSB?


Hi All,

I am finally getting restarted on a tiling project, the problem is that the OSB subfloor has received a lot of foot traffic and some moisture. It's not trashed by any means, but it definitely has some surface weathering and fine grime. I covered a small area with Red Guard and let it cure. I then got my fingernails under an exposed edge and was able to peel up much of the cured Red Guard in strips, some with small bits of wood clinging to them. I'm guessing it shouldn't come up this easily...

I need to coat the subfloor in Reg Guard and self-leveling cement before tiling and am wondering the best way to prep the wood to insure proper adhesion.

Belt sander?
Wood hardener?

Thanks!

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Old 03-13-2014, 10:39 AM   #2
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adhesion on weathered OSB?


I'm sure the tile guys will chime in, but is prying at the edges of a product an expected wear feature?
prepare yourself for span/joist size/wood species questions.

OSB not ideal underlayment

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Old 03-13-2014, 10:48 AM   #3
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adhesion on weathered OSB?


Back up, tile is never installed directly over OSB!!!
As already said, what size floor joist, free spans, spacing.
What's the thickness of this OSB? Was it tongue and grooved?
What size and material tile are you using?
Tile is a one shot deal, what's going on under the tile is far more important.
If it's flexing it's going to fail.
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Old 03-13-2014, 11:25 AM   #4
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adhesion on weathered OSB?


Backing upů

13" porcelain tile over Ditra over self-leveling cement with metal lath over Redguard over 3/4" T&G OSB over 2x8's 16" O.C. (with a span of just 50".) It's quite stiff: L / 2604 according to John Bridge Deflecto-lator.

I've been told that an additional 1/2" of plywood would be good insurance, but is not required. I know that OSB doesn't get along with moisture so well.

Sorry for lack of details. I was feeling pretty good about the rest of the install, and was mainly hung up on the adhesion issue.

Thanks for your help.
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Old 03-13-2014, 12:40 PM   #5
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adhesion on weathered OSB?


I don't understand the Redguard or SLC application. If you're looking for a moisture barrier under the tile, you would kerdi-band the seams and edges of the ditra. And if your floor truly needs to be leveled, I've never seen a SLC manufacturer suggest wire mesh. That sounds almost like a Jersey mud job which never worked well. What's the reason for not going with Ditra directly over the OSB?
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Old 03-13-2014, 01:10 PM   #6
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adhesion on weathered OSB?


1. I've read in numerous places that properly supported OSB is acceptable (though perhaps not idea.) I assume it's due to moisture, swelling & degradation. Got it - I'm good with that.

2. Embedded lath in SLC is a common recommendation for SLC installed over wood substrate. (Custom Building Products Level-Quick, etc) I've seen tons of references to this approach.

3. I have seen Custom Building Products recommend Red Guard for OSB as a means of priming it before installing SLC. It seals the OSB and bonds well with the SLC.

4. Peeling up the edges - I assume that the stuff should bond well to the substrate, otherwise any work above (Ditra, mud, tile) could still lift up from the subfloor over time. If this stuff can normally be peeled off in sheets, please tell me, because I'll stop worrying about it. If it shouldn't peel up that easily, then I better re-prep my subfloor.
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Old 03-13-2014, 08:43 PM   #7
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adhesion on weathered OSB?


I would choose a system and use the products that manufacturer recommends. Which SLC will you be using? Each company makes a primer for it. I don't see the need for Redgard, unless you also use Custom's SLC and they recommend Redgard as a primer.

I've used diluted Redgard to prime Gypcrete, but not to prime OSB for SLC. Is Custom ok with your plan?

Otherwise as I said, I see no reason for Redgard. As for your peeling test. There will not be the amount of stress that you are subjecting it by pulling a corner like that in real live use. Pulling and peeling applies at least 5 gazzilion times more stress than it's intended use. People do that with Ditra and Kerdi too, means nothing. Think about it. By peeling there's tons of pressure on the leading edge which keeps moving as you pull.

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Old 03-13-2014, 10:03 PM   #8
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adhesion on weathered OSB?


I read a Q&A from a Custom Building Products Tech. Support person who said that RedGuard should be used over OSB to protect the tile install from the increased expansion/contraction that occurs with OSB vs. plywood.

I understand what you are saying about peeling the RedGuard back being an apples to oranges comparison. At the same time, I have to think that a weathered dirty surface might have poor adhesion without first abrading the surface. Do I get a thumbs up for the belt sander as prep for either RedGuard or SLC primer?
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Old 03-13-2014, 11:06 PM   #9
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adhesion on weathered OSB?


I don't know of anyone that would recommend installing tiles directly over OSB with Redgard on it. OSB is not stable for tiles, you have to install a suitable underlayment or mat over it.

But you're not doing it that way, you're using Ditra, which is a good thing. If the OSB is so bad, you're better off installing an underlayment. Normally the addition of SLC would be enough. So I guess the question is, should you prime with Redgard or prime with the primer that goes with the SLC.

On that 50" span. That's not the width of the room right? It's the actual span of the joists?

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Old 03-14-2014, 11:36 AM   #10
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adhesion on weathered OSB?


Right, Redguard may help with expansion/contraction, but so does Ditra, so maybe Redguard is overkill (belt + suspenders) and I should just prime.

50" is the length of the joists and the depth of the room - it's a small entryway and adjacent 1/2 bath. 50 or 54" front to back, and 12 feet from side to side. It's an old addition that was partially reframed.

DEFINITELY NOT trying to put tile directly over OSB. At least I've got that figured out!
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Old 03-14-2014, 01:04 PM   #11
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adhesion on weathered OSB?


I wouldn't call Redgard over kill necessarily. It does what it's designed to do is all. But a piece of wood has 6 sides. Applying Redgard coats one side.

I'm not apposed of using a diluted Redgard as a primer for the SLC, if Custom says it's ok for that. Redgard may be cheaper, plus has more than one use. Primer just has one purpose.

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Old 03-14-2014, 01:55 PM   #12
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adhesion on weathered OSB?


Makes sense, thanks for your help.

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