underlayment

[rreddy]

Greetings!

We had a leak in our tub surround that we ignored for several months. When the patch on the ceiling below finally turned the right shade of brown, we decided to fix the problem by re-modeling the bath. I bought JB's first book, which was very good, and Michael Byrne's book which, dare I say it, was even better, at least for those of us who don't want to be mud people.. I now know just enough to be dangerous. To cut to the chase, I was hoping for some advice on an underlayment question.

The sub-floor is 1/2" plywood. The original floor tile had an additional underlayment of 5/8" plywood, bringing the total thickness to the recommended 1 1/8". (The joists are 2x10, 16" o.c, 12' span).

I removed the floor tile along with the 5/8" plywood layer (which fortunately was nailed down rather than screwed). The original 1/2" sub-floor is in good shape for the most part, though a small portion is rotted at one end. I will be replacing the rotted portion and shoring it up with a 2x4 cleat underneath. I'm now considering two options for the underlayment:

(1) use a layer of 5/8" plywood as before

(2) use 1/2" plywood with 1/4" Hardibacker on top.

Any recommendations ? Will the second option (1" of plywood plus 1/4" Hardibacker) offer enough structural support ? I've been reading that CBU doesn't add structural strength.

Also, I'll be using 1/2" CBU on the walls, and was wondering whether to go with Durock or Hardibacker. Is there anything to recommend one over the other, or is it just a matter of personal preference ?

Thanks in advance.

Raghav

[Mike2]

Welcome to the Forum Raghav.

Both of your plywood sub-floor options will meet/exceed TCA guidelines which call for 19/32" minimum plywood when using CBU. So your choice but my personal preference would be to add the 5/8".

As far as Durock vs Hardibacker on the wall, again your choice. I personally prefer Durock as I find it easier to set the screws flush to the surface using Durock.

[John Bridge]

Hi Rag, Welcome.

Michael Byrne and I are friends, and I'm always ready to recommend his book. I have a couple links to it over on my real web site.

What about adding the 5/8 and topping it off with Schluter "Ditra"? Ditra add and eighth of an inch.

http://www.schluter.com

[rreddy]

Mike/John,

Thanks for the responses. I'll go with the 5/8" plywood as advised. I was at Home Depot and Lowe's this evening looking for exterior grade plywood. I didn't find anything rated Exterior. There were a couple of brands rated Exposure 1 (Georgia Pacific Plytanium and some other brand marked U/L D Fir). Is it OK to use these ?

Regarding the Ditra, I'd be happy to use anything that makes the job go easier. Judging by the forum input, Ditra's popular with the pros, but it's still a bit of a mystery to me. There's no store nearby that stocks it, so I can't get a good look at it before I order it.

[John Bridge]

All the construction grade plywoods use exterior glue. Buy BC, CC Plugged or Sturd-I-Floor. If they have it, Sturd-I-Floor is the best. It's tongue and groove. The other stuff works, though.

[rreddy]

Can I cheat a bit and just add 1/2" plywood instead of 5/8"? This would give me a total of 1", but in two layers. My reason for wanting to do this is that I already have a couple of 1/2" plywood sheets lying around the house.

[bbcamp]

Yes, the 1/2" will work, but it has to be the right stuff. Exposure 1, BC or CC plugged.

[John Bridge]

da Injineer have spoken.

I think Exposure One means exterior glue, and all of the plywoods he mentioned have exterior glue.

Did I do good, Bob?