Would you lay tile over plywood. I have 7 foot ceilings and want to have tile in my kitchen/dining area but hate the idea of building up the floor too high. Also I don't want the tile floor to be a 1/2 inch higher than the wood floors that join from the living room. The floor in the kitchen has a plank subfloor and my remodeler could put down plywood and then the tile. He said it has been done like that forever with great results but for the best job he should put cement board in between the plywood. Any thoughts?

I think they have a membrane type underlay that adds very little to the buildup.

Si think it is called schluter Ditra or something like that. Not sure on the spelling....I used this on about 500 sq ft about 10 years ago.  Wasn't exactly cheap but well worth it.


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One thing you do not want to do is put a new tile floor over a subfloor that is not strong enough. Your tile and grout will crack and break or pop loose creating a big hassle. You won't notice a half inch less of ceiling height and you will learn to step up or down the little bit from one room to the next.
These sound like big things but compared to tearing out a new floor and doing it again they are small potatoes.

Ideally there should be either cement board or Ditra (my preference) between the tile and the plywood. However, depending on the thickness of the plywood you might be able to get away with it. Will the installer give you a written warranty in case it cracks?

If you do it, I would use a brush on membrane like RedGard on the plywood before the tile. Give it a couple days to cure. That will help isolate the tile from the wood.

If your subfloor has too much flex then nothing I'm recommending will work though. Since you have a tile installer doing the work, I assume he's checked the subfloor flex already.

Look at NobleSeal CIS. I've used it over plywood (subfloor thickness of 1 1/8") and there is no real increase in height as the membrane itself is only .030" thick.
Make sure you meet their minimum recommended subfloor thickness.

Not sure why he wouldn't cut out the 1x subfloor and put in a subfloor suitable for Ditra

The floor over the joists is 3/4 inch thick by 11.5 inch wide planks. He could take it up but those planks seem pretty sturdy to me.

It's all about flex of the floor.  How wide are the joists apart? How tall are the joists  Are you doing natural stone or ceramic or what?  I can see not wanting to add Height but sometimes it's necessary evil.

Floor joists are 7 inches high and 14 inches apart. Doing 12x24 porcelain tile.

I'd get some hardie backer. I did my whole ranch hallway and living and kitchen. Roughly over 500 sqft. I had 3/4 osb sub floor. 1/2" hardie pro grid, then tile.  I did not want ANY cracking, or issues.  I'm glad I did. My joices are 16" on center. I reinforced the side door entrance into my kitchen to take the hard bang from banging off shoes and boots from snow and mud.

You can get 1/4 hardie board.   Don't buy that shit cement mesh board at HD. Get the hardie board brand. Way easier and better quality. Don't skimp. You will wish you did it right once you have to take out a piece of tile in the middle of the floor and replace it.

Look at a product made by custom called easymat. We use all the time and we have great success.

Ditra is what you need. Adds 1/8" to the height. Almost guarantees no cracks if installed properly.  Way more skookum than cement board

The thinset bust be rated for plywood. Laticrete 242 platinum wood work.

Never do it on OSB

there are three different problems to solve when putting tile down:
1) joist deflection (this is the overall
2) subfloor stiffness
3) subfloor adhesion/decoupling

1) depending on a couple of factors, your joist setup may or may not meet basic requirements for the tile type (e.g. porcelain easiest to meet, marble hardest).  use the calculator here:
http://www.johnbridge.com/vbulletin/deflecto.pl

2) subfloor stiffness is a complicated topic (including a lot of old wives tales) but the best references you can find on it is the john bridge tilesetting book ($) OR the schluter installation manual (free):
https://sccpublic.s3-external-1.amazonaws.com/sys-master/images/hcb/h67/8815748186142/DITRA%20Installation%20Handbook.pdf
in the above, find the situation that best matches what you have, and follow the directions.  for example, see "D-SP-TS-16", "Floors, Interior - Structural Plank Subfloor", on page 7.  
this is a common case, and in general means putting 1/2" to 3/4" exterior glue plywood (type BCX underlay is what you want) over the plank subfloor.
 
3) some subfloor materials are not the easiest/best to tile directly to, which is why cement board and Ditra in particular exist.  

ps
cement board of any type does not increase floor stiffness; it simply makes a nice surface for thinset to adhere to.

ar-jedi







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Good stuff.

I used noble ts in my bathroom

And my favorite 'trick' for many years is to add a strap of steel or wood to the bottom edge  (or even side) of joists.

It works very well in unfinished basements.

With steel you have t make sure you pay attention to things like screw loading, hole size to screw shank matching, and per-screw loading.

Wood is more forgiving but adds far less stiffness.

Even a layer of 1x4 flat on the bottom of a joist with enough fastening is often more than adequate and only steals 3/4 inch of lower level headspace.