I currently have 23/32 OSB subfloor with 16" OC joists. I plan to install 12" porcelain tiles. I have about 1" to play with. What should I use as an underlayment? Do I need to reinforce the OSB with additional plywood?

My choices seem to be:
1) install 1/2" external plywood to strengthen the subfloor and then tile on the ply (result - nice strong subfloor - but, no CBU)
2) put 1/4" hardibacker board or Ditra right on the existing OSB? Nice CBU but no additional subfloor rigidity gained)
3) I may be able to accomadate 3/8" ply with Ditra? Tile is 3/8" so this option may not be feasible.

Which is best? #1 or #2?

I'm getting frustrating conflicting opinions. I'm hearing additional plywood is essential. I'm also hearing that ditra or hardibackerboard is fine on the OSB (as the manufacturers websites also say). I'm limited in my height by the front door clearance.

Would love to hear if there is a concensus - or does everyone have there own way of doing things? I realize that additional plywood (e.g. 1/2") PLUS a CBU is ideal. Unfortunately,I can't do both

If you went with 3/8" ply (no glue, just screws), then Ditra (adds about 1/8") then thinset and tile (adds about 1/2"), you would be right at 1". This would also be my recommendation, regardless of the height, except I might have recommended 1/2" ply instead of 3/8". But you'll still have 1 1/8" of subfloor and underlayment, which is fine.

You might get by without the extra layer of ply, but considering you've got the extra height, I would do it anyway just for the peace of mind.

You've checked your floor joists, and they're adequate for a tile installation, right?

Hi Joe,

I feel your frustration with the answers at JB's place. But essentially I think you know a single 3/4" OSB subfloor is adequate as long as it's installed correctly. More plywood is always better, and that's what a few people were telling you. But there are limits.

I feel very confident your project will work fine with either 1/4" CBU or Ditra. I have done hundreds that way including friends and relatives, (inlaws, daughter) and give a lifetime guarantee. No need to worry since it's a recommended installation proven over time.

Whichever way you go, how much space will you have under the door? You realize you'll want space for a rug right? Otherwise raising the door is feasible. I've never done it, but saw it done one time on a job I did over the old ceramic. A carpenter can do it most of the time in half a day.

Jaz

thanks Jaz - very comforting. So - I would be better off removing the 1/2" ply I have started to lay and simply use the 1/4" hardiboard instead - right on the OSB? That was my original plan until I discoverd the tile blogs! You feel that would be better than simply tile on the 1/2" plywood? I like the idea of strengthening the subfloor with the ply. I don't necessarily like the idea of tiling directly to the ply however, I can't do both. Since I originally had 3/8" ply down, I feel that the overall integrity of the floor will be less than it was if I go with the CBU right on the OSB? Though, I guess the added benefits of Ditra's uncoupling / load distribution attributes may off-set the fact that I no longer have the 3/8" underlayment. I was starting to convince myself that since they originally installed the tile right on the 3/8" ply and everything was fine, if I install right on the 1/2" ply I will be even better off than previously. I guess I'm perplexed why everyone seems to be so adverse to tiling on plywood?? Don't understand the contraction / expansion concerns? The floor is the "first floor", above the basement, so the temperatures are eseentially constant year round. I certainly could understand if I was laying the ply over a garage or someting like that - where the temps would vary significantly during the year.

If I remove the 1/2" and instead use Ditra XL right on the OSB, I will now be flush with the adjoining rooms and at my original floor height. Would be nice.

I now know how wonderful it must be to be rich!! You simply write a check and let the installer worry about all this stuff!

If I had to choose between a second layer of ply and CBU/Ditra, I'd choose the CBU/Ditra.

Jaz,

Today I crawled under my wetroom installation and had my grown son bounce up and down on the floor.

What a revelation! Too much loose floor. Many more fasteners later and things are improved, in that there is no more movement over joists. But I now see that 3/4 plywood is just not enough to minimize flexure.

Tomorrow, I'll ring-shank down an additional 1/2 inch piece of plywood, nailing plywood-to-plywood, no glue, just nails.

Am I correct that this is the best procedure?

The floor in question will be the shower pan. I have seen very many different approaches, but I want to get this one right.

If I understand correctly, next layer should be CBU (Durock, 1/4 inch), screwed directly to plywood (no thinset, no tar paper), then comes the sub-pan, then the CPE liner...

Am I correct so far?

Regards,

gnu_B

Screws would be better than nails for that second layer of ply.

For a tile shower, there's no need to have a CBU on the floor.

Put down a piece of plastic (this is to keep the subfloor from drawing the moisture out of the deck mud, so anything will do), then nail or staple down wire lath. Now goes the pre-slope and liner, the CBU on the walls.

Go here for further:

http://www.ontariotile.com/preslope.html

Kman,

Thanks for the link. I knew there should be a page on this blog which detailed that information.

Most helpful.

BTW, the second layer of plywood really helped a lot.

I am curious why not use construction adhesive when laying down the second layer? I did not, I followed procedures outlined on this blog.

Many thanks,

-gnu_B

Some will tell you to use glue. The thinking against it is that lines of glue may not flatten out completely, causing small voids between the subfloor and underlayment. Those voids would translate to movement under foot traffic, which is detrimental to a tile installation.

A full spread of glue, combed out completely with a notched trowel, would be acceptable. I would hate to be the one to demo that floor, though.

Joe,

Thanks to Kevin for answering when I wasn't around.

I'd be interested to know where you found that the CBU didn't need thin set under it, just fasteners. That's only on walls.

Jaz