Floor substrate options [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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01-26-2003, 12:36 AM
Thanks for all the recent advice on waterproofing a knee wall. It seems to be going well so far.

I'm getting ready to prepare the substrate for the bathroom floor and am confused about substrate thickness requirements.

I have 3/4 (23/32?) plywood sitting on 2x8's, on 16" centers. The longest span is about 11'.

I've taken a look at some of the previous posts (e.g. "1- 1/8 in. subfloor requirement") but actually ended up more confused than when I began. Would I be better off adding 1/4" or 3/8" plywood (glued & screwed, etc.) and then 1/4" CBU (thin-set and screwed) or could I get by with just laying down some 1/2" CBU (thin set and screwed)? I'll be laying down 6"x12" travertine tiles. The 12" side will be parallel to the joists.

I'm kind of limited in what I can do in terms of height. I have some sliding closet doors that have a track that is currently about 1 1/8" above the curren subfloor. I can't really go any higher than this without interfering with the doors sliding. The tile is 3/8" + 1/8"(?) thinset so I have at most 5/8" to play with.

Thanks in advance for your help,

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01-26-2003, 08:45 AM
Hi Tom, Is there any way you can sister those joists below or break that span with additional support?? The numbers just arent making it for a natural stone. Even with the additional Plywood I think you may be gambling.
Sistering a 2x8 to each joist would be one option Post and beam below the area to break that span would be another. Let us know if you have this option. :)

01-26-2003, 10:54 AM
Todd's right about the joists. They are fine for ceramic tile, but not stone.

You do have another problem. You will need another 5/8" or so of plywood, then an underlayment, for stone. Stone is soft and prone to cracking under the best of circumstances, so it requires "double wood" to be sucessful. Getting this to work given your height restrictions seems not likely.

Ceramic tile can work under your conditions. You'll need to add Ditra, over the existing plywood, then tile. Adds about 5/8" above the existing plywood.


01-26-2003, 01:00 PM
Thanks guys!

After further inspection (and crawling under the house), it looks like the max unsupported span for the joists is ~7'. There is a beam than runs under them so it's not actually 11'.

In addition, I think I can raise the sliding doors a bit to make up for any additional substrate thickness. If I add another 5/8" plywood and then 1/4" CBU would this work?

It also looks like I should be able to sister the joists, although it wouldn't be trivial. Assuming I can do the plywood and CBU as described above and the span is 7's would I still need to sister the existing 2x8's?

Any thoughts on an isolation membrane of some sort?


01-26-2003, 01:33 PM
If you add another layer of ply to the 3/4" I dont see a problem w/ stone if your span is actually 7'. I'm not a CBU guy but to achieve the smallest thickness before tile would be to use Ditra. Don't know if you have it in you area but with the 3/4" already down add another 3/8" ply and then lay the Ditra, which has a height of an 1/8".

Ditra is a iso membrane.

01-26-2003, 02:08 PM
2x8 joists on 16" centers, 7 foot span is good for stone! No sistering needed.

What K_tile said about Ditra and 3/8" plywood is good for stone. That works for your sliding door issue. So would your suggestion of adding 5/8" plywood and 1/4" CBU.

01-26-2003, 02:48 PM
Other then Ditra , Nobleseal is a option. I t offers more compresive strength then ditra since it is a flat membrane, and therefore is fully supported.www.noblecompany.com