# proper subfloor preparation for tile



## AustinDB (Sep 11, 2006)

I'm putting down 12" tile on a 3/4" plywood over 16"OC joists and would like to know if using Ditra means the additional layer of 1/2" is not needed. 

I use to use an additional 1/2" plywood topped with a layer of Kerdi, but that was a number of years ago before Ditra hit our market-I've been out of the loop for a few years. 

Looking for a solid install-thanks!


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## PrecisionFloors (Jan 24, 2006)

No. Additional plywood not required if I remember correctly.


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## Robinson_Cnst (Jan 5, 2012)

Whats the joist size and span? Figure your deflection and go from there. Its all in the instructions.


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## MIKE ANTONETTI (Jul 17, 2012)

Schluters website says you can, ceramic , not stone. The home we were on the other day had 3/4", I don't know I felt vibrations while moving equipment on the floor, I personally would put the 3/8" plywood staggered and nailed like crazy, then the schluter as recommended for stone. It's too late once it's done.


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## Stoneyard (Mar 30, 2014)

Personally I wouldn't risk it. I would check the sub-floor seams and stiffen it up if needed. I would put down 3/8-1/2" AC plywood running opposite of the sub-floor then staple 3-4" on center with a few dabs of construction adhesive below. Then your modified thin set then the Ditra. Having a tile job go bad and having to tear it up later and redoing it is huge compared to a few hours of labor laying down the underlayment. Who knows if Aunt Betty is coming over for Christmas dinner and she just happens to weigh 400 lbs.


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## MIKE ANTONETTI (Jul 17, 2012)

Water got under the tile on this job, I think it was the shower water flowing over the edge of the tub, don't bother building them to pitch back into the tub, no that makes no sense! Could've been the bowl seal as well though no odor or bacterial / mold growth.


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## angus242 (Oct 20, 2007)

Guys, you cannot use 3/8" ply UNLESS it's a 7 ply board. Most are not. As a matter of fact, I only know of 1 manufacturer that produces 7 ply 3/8".

If your deflection is > L/360, you don't need to add anything extra. Ditra adds some height. NobleSeal doesn't. Peel n stick like Green Skin and Mapeguard 2 give superior crack isolation (per manu's specs).


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## angus242 (Oct 20, 2007)

Stoneyard said:


> Personally I wouldn't risk it. I would check the sub-floor seams and stiffen it up if needed. I would put down 3/8-1/2" AC plywood running opposite of the sub-floor then staple 3-4" on center with a few dabs of construction adhesive below. Then your modified thin set then the Ditra. Having a tile job go bad and having to tear it up later and redoing it is huge compared to a few hours of labor laying down the underlayment. Who knows if Aunt Betty is coming over for Christmas dinner and she just happens to weigh 400 lbs.


If you use a plywood underlayment it must:
..be installed perpendicular to the joists.
..be a minimum of 7-ply (regardless of thickness).
..free of any construction adhesives.


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## B.D.R. (May 22, 2007)

Why not wire or plastic mesh and self leveler?
That adds to 1/2


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## TNTRenovate (Aug 19, 2010)

Stoneyard said:


> Personally I wouldn't risk it. I would check the sub-floor seams and stiffen it up if needed. I would put down 3/8-1/2" AC plywood running opposite of the sub-floor then staple 3-4" on center with a few dabs of construction adhesive below. Then your modified thin set then the Ditra. Having a tile job go bad and having to tear it up later and redoing it is huge compared to a few hours of labor laying down the underlayment. Who knows if Aunt Betty is coming over for Christmas dinner and she just happens to weigh 400 lbs.


If you are going to lay another layer of plywood you would lay it perpendicular to the joists, screw to subfloor but not to joists.

And while the plywood will stiffen up a floor, I would first make sure that the deflection rating is met and then address subfloor.


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## Tech Dawg (Dec 13, 2010)

B.D.R. said:


> Why not wire or plastic mesh and self leveler?
> That adds to 1/2


That's ok if you live in Jersey :laughing:


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## angus242 (Oct 20, 2007)

Tech Dawg said:


> That's ok if you live in Jersey :laughing:


It is April Fools Day.


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## Stoneyard (Mar 30, 2014)

angus242 said:


> If you use a plywood underlayment it must:
> ..be installed perpendicular to the joists. *I meant joists, brain fart*
> ..be a minimum of 7-ply (regardless of thickness). *Agreed, but keeps the height down for the transition at the door
> *..free of any construction adhesives.


 *I disagree, the underlayment and subfloor move separately from the Ditra and tile.*


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## GO Remodeling (Apr 5, 2005)

What are you disagreeing to?

Years ago, manufacturers said to lay underlayment 90* to subfloor. But now it should go perpendicular to joists. Just don't line up the seams or screw to the joists. 
Schluter's handbook has a good explanation of how to add wood underlayment.


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## angus242 (Oct 20, 2007)

Stoneyard said:


> *I disagree, the underlayment and subfloor move separately from the Ditra and tile.*


Not sure what you are disagreeing about. All of my comments are based off industry minimum standards. To disagree with them is to disagree with an entire united body of manufacturers and testing facilities that create these standards.

Since you didn't reply to the adhesive comment, perhaps that's what you're talking about? It has nothing to do with movement of substrates but creating tiny voids under the ply underlayment that can cause horizontal movement.


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## CompleteW&D (May 28, 2011)

Man.... it's GREAT to come to the site and see Angus posting again. It's good to see you again! Your wisdom and "_cantankerous-ness_" is a welcome sight indeed.

We'll all learn more with you around. Don't be away so long next time. :thumbsup:


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## MIKE ANTONETTI (Jul 17, 2012)

I don't know about the 7 ply issue, nor turn it this way or that, nor the L360, there are many factors and you can always go beyond these minimum standards. What about trapping moisture underneath and plywood swelling, certainly there's enough force to pop a tile up, and combine with vibrations in the floor. Evaluation of each situation is critical for a long lasting installation.


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## AustinDB (Sep 11, 2006)

I've never heard of a 7 layer plywood, but googled Halex. There are no distributors in my area and my flooring co hasn't heard of it (General Flooring). 

After reading Schluter's Ditrahandbook, it states I can go over the existing 3/4" w/ Ditra and as an added option, I could leave the vinyl down (which is in good condition) and install the Ditra overtop. That feels like a "dirty" option, I'm use to overdoing things. Have you guys had/seen callbacks/failures with this method?


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## MIKE ANTONETTI (Jul 17, 2012)

I think he's BS'n on April Fools day, looks like I have to Amp up my reading on install instructions , been coasting awhile.


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## tileman2000 (Feb 14, 2011)

If the subfloor isn't done properly, Ditra will not keep the tiles from cracking. I saw this first hand 2 weeks ago. The original installer used Ditra and 20x20 porcelain tiles in a kitchen and connecting dining room. I was there doing the upstairs bathrooms, but ended up replacing a few cracked tiles from the kitchen. From what i could tell, the Ditra was stuck well to the floor and there was plenty of coverage on the back of the tile, but since the subfloor wasn't done properly, I'll probably be making a few more trips to replace tile.


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