# Best option for underlayment under a tile floor.



## Bud Cline (Mar 12, 2006)

Typically the subfloor should be a minimum of 1-1/8" thick. Tile-backers offer no structural improvements whereas plywood will. Use plywood.

But then installing tile directly to plywood is also risky business under some conditions.

Your real "best-bet" is to use a product called DITRA made by Schluter Systems. DITRA offers a warranty when used over 5/8" ply but keep in mind this is still a minimum.


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## Nestor_Kelebay (Jun 17, 2008)

There's two things you have to keep in mind:

The subfloor has to be strong enough not to flex when you walk over it, otherwise the grout joints will crack.

However, it's a lousy idea to tile directly over wood. The reason why is that wood isn't dimensionally stable. It swells and shrinks with changes in it's moisture content. So, if you tile over plywood, and you get water seeping through the grout somewhere, then the wood under that spot will swell, potentially causing more grout cracking. And, if that happens in an area that's likely to get water splashed on it periodically, you end up with a snowball rolling down hill; more splashing means more swelling means more cracking means more swelling means more cracking...

It's this dimensional instability in wood that's the reason why they recommend tiling over a cement backer board. Those CBU's don't swell and shrink when they get wet, so there's no stress on the grout lines, and they don't have an excuse to crack. Any moisture that does get into your tiling then still causes the wood below to swell and move, but that ends up causing stresses in the cement board underlayment, not on the grout lines. The situation is very similar to standing on a fault line. There may be tremendous stress in the rock you're standing on the day before the earthquake, but you feel absolutely nothing. It's the same for the grout joints. There may be stress between the wood and the cement board underlayment cuz the wood is wet and moving, but the tiles and grout joint are completely unaware there's anything wrong cuz the stress is on the CBU, not on the tiling.

As was mentioned by the previous poster, the cement board will give you no additional strength. You need plywood to get strength so that the floor doesn't flex when you walk on it. But, you still need that cement board layer between the tile and the plywood to isolate the dimensional instability of the wood from the grout joints. And, lots of times that requires that the ceramic tiled floor be higher than that in the rest of the house.

In my opinion, ceramic tile simply isn't that much better a flooring option to have a floor height difference you're potentially gonna trip over. REAL linoleum is much stronger than the sheet vinyl being offered. I have vinyl composition tiles in my building, and they stand up well in a kitchen. If the floor height difference is a concern, I'd look at other flooring options. The world doesn't revolve around ceramic tile.


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## Weathermaker01 (Jun 20, 2008)

Bud & Nestor thanks for your responses.

So if I (my wife) wants tile then I take it my best bet would be too:

1/2 plywood
1/8 Ditra
1/8 mortar
5/16 tile
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1 1/16 approximate total height

OR

3/8 plywood
1/8 Ditra
1/8 mortar
5/16 tile
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15/16 approximate total height


Then I'd probably need to get a custom cut oak threshold as the tile would be 3/16 or 5/16 higher than the hardwood. I guess this would be okay, as it is the opposite of the way it is now, the hardwood is about 3/8 higher than the current floor.

Does my logic and calculations sound reasonable?


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## angus242 (May 1, 2008)

weathermaker

your floor deflection rating is .432" which is acceptable for ceramic installations but you're on the low end. I'd go with the 1/2" ply and deal with the height difference with a transition strip. Much better to have a solid installation than a 100% even transition.

It's important to follow Schluter's recommendations when install Ditra. Laying over plywood, you'll need both modified and unmodified thinset.

A properly laid tile floor will last a long, long time!

good luck


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## JazMan (Feb 17, 2007)

That rating is the approximate deflection for the joists, (which meets standards), BUT it has nothing to do with the subfloor deflection between joists. In other words you need more plywood to stiffen between joists, BUT that will NOT change the joists' deflection rating. 

I recommend you install as thick an underlayment as possible, 1/2" or thicker. Deal with the transition with a ramped threshold or? You need to leave a gap between the tiles and hardwood anyways. I assume the hardwood is already in place?

Jaz


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## angus242 (May 1, 2008)

ok, let me be more specific. Your deflection rating for your JOISTS is .432" which is acceptable for ceramic tile installations. 
I, too, am completely on board with the suggestions of more is better with additional plywood. 1/2" is the MINIMUM I'd use.


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## Weathermaker01 (Jun 20, 2008)

Angus & JazMan I appreciate your replys.

I've noticed that the 5/8 subfloor deflects between the joists and also that the overall floor has a bit of bounce to it when someone heavy walks on it. Should I consider laminating plywood to the sides of my joists to beef them up?

Would I be better to go with 5/8 T&G and no Ditra or 1/2 plywood and Ditra?


Thanks again!


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## angus242 (May 1, 2008)

Should I consider laminating plywood to the sides of my joists to beef them up?
*If possible, sure! Your deflection rating of your joists is approx. L/444 which is on the lower end of what it should be. By going with only 1/2" ply, yes, it would be a smart choice!*

Would I be better to go with 5/8 T&G and no Ditra or 1/2 plywood and Ditra?
*Go with as much ply as possible AND the Ditra. You definitely want the Ditra!*


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