You can tile over plywood (proper grade of ply) because the proper thinset will bond with the continuous layer of wood veneer.

When tiling to OSB, the thinset has to try to bond with wood strands or wood chips, and the resin or binder that holds the chips together. It won't get a bond strong enough to deal with the stresses of a direct OSB-to-tile installation.

That's where a membrane like Ditra comes in. It'll act as an intermediary between the tile and the OSB.

So no, you shouldn't tile directly to OSB. I don't believe and governing or guiding agency allows or recommends it.

Mongo

Thanks Mongo for such a quick reply and also for the explanation of WHY OSB isn't good to use. We'll add the Ditra now. Thanks so much for your expertise--it's very much appreciated.

Man, you can just put down ditra over osb and tile over it?!

I'll have to file that info away for my *next* house. Would have saved us a lot of time LOL

Yup.

Ditra over OSB is o-tay.

Your "next house?"

You mean your "weekend getaway" right? After all your efforts would you consider selling your place?

Yup. O-tay, Buttwheet!!

Would Ditra work over chip or press board? It's all over our kitchen and bathrooms (there's vinyl currently there). Blech. If so, any hints or tips?

Take up the pressboard. It WILL NOT work with either of those, due to the fact that they expand way too much when they get wet, which would cause the tile to pop, and it wouldn't take to long to happen, either.

I used Kerdi (instead of Ditra) in a 6 sq.ft. area since it kept floor height aligned. Hope that was ok. The tiles were 2 sq.ft. each. Then I epoxy grouted.

-david

O-tay? We're definitely aging ;)

Mongo, we aren't selling...it was more a rhetorical 'next house'. We're not planning to sell until DD goes to college (about 7 years from now), and may not then unless we find something we really like better. As I'm sure you know, after all this customizing work one tends to get very picky :)

I agree with the experts, and would always use an underlayment, but have seen it done otherwise. We tore out our foyer and it was tiled directly to the OSB. No cracks or anything--it had held up fine--but we laid Ditra before we tiled.

Exhibit 2: Mr. Weedy fixed his Chiropractor's office floor, which had some tiles cracked. He went into the crawl space and found the OSB had sagged at a seam, so he braced it up and re-laid tile over the bad spot. The Chiro didn't want the whole floor torn out, so he laid it directly on the OSB. I told Mr. Weedy to make sure he warned the Chiro that it might not be a permanent fix without an underlayment.

Now the issue we've run into is that if we lay Ditra, it will raise the bathroom floor above that of the hallway (which we'll do and use a transition strip if there are no other alternatives--adding a layer of plywood would raise it also).

A tile guy is looking into whether there is a "primer" (his words) that can be applied over the OSB to not increase the floor height. I'm wondering if that is perhaps Redguard.

What about using Kerdi like davidro1 referred to above in his post? If that's usable, how much does it raise the floor?

So, are there alternatives to Ditra that won't the raise the floor height as much? Many thanks again...

Deal with the transition. DO NOT shortcut the floor composition. Trust me on this-- it'll come back to bite you. There is no way you can safely tile directly over OSB. There is no bond coat-- not Redgard, Not Ultraset, Not 9235-- nothing-- that will help.

Kerdi, while waterproof, is not an isolation membrane. It's too thin to offer any shear resistance between adjacent materials.

Ditra is only 1/8" thick. But the structure of that thickness is what allows it to absorb stresses and decouple the subfloor from the tile.

Mongo