proper subfloor preparation for tile

No. Additional plywood not required if I remember correctly.

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

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.

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.

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).

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.

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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.

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

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.

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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.

B.D.R. said:

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

Click to expand...

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

Tech Dawg said:

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

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It is April Fools Day.

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.

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

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.

Stoneyard said:

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

Click to expand...

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.

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:

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.

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?

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.

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.