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Unread 08-06-2020, 05:11 PM   #1
Toots
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What Configuration with Ditra in this situation?

Hello all. I started typing the backstory for this install, but you know what....it doesn't matter. What I have is: a main floor laundry room that currently has brand new 5/8" tongue and groove Sturd-I-floor T&G plywood over 16" OC joists. The span is 10' of 40 yr old fir 2x10s, so we're good on structural deflection, but the in between joists is another matter.

I am not a straight tile professional, but I have done a LOT of it. This is also not my first rodeo with DITRA by any means. However, I'm at a bit of a question as to how to proceed with the ceramic installation in this case.

I know DITRA can technically go over 5/8" ply, but man, this room is going to have a heavy, large washing machine in it, and its feet will not fall on joists. I am a bit skiddish about going with the absolute minimum DITRA is approved for with.

Okay, add to this, my local lumber supplier has gone out of business due to COVID. I suddenly do not have access to any kind of local high quality underlayment in stock. If I want to add strength to the floor, my option is basically what is in stock at Menards, and I have never used their plywood products other than roof sheathing. I can not wait to order stuff in. I don't have enough height to give to the bottom of an exterior door to do another 5/8" of any product + DITRA.

So my options are:

#1.) Trust DITRA's warranty and just go over the 5/8" plywood and hope that washing machine isn't too heavy or crazy on spin cycle(front loader so it really hits the floor hard).

#2.) Add 1/2" unbranded span rated OSB from Menards.

#3.) Add unbranded 3/8" or 1/2" BCX sanded plywood from Menards, that they claim is approved for underlayment.

I'm leaning toward the OSB. Even though plywood is "better" usually, at least I know the OSB will not have voids, even though it won't add as much strength.

Oh I should maybe mention that I've also never gone over only 5/8" with DITRA before, so again, makes me skiddish to push anything to its approved limit. Anyone actually done it?
Any opinions?
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Last edited by Toots; 08-06-2020 at 05:26 PM.
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Unread 08-06-2020, 06:34 PM   #2
jadnashua
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Not all brands of OSB are created equal, and some may be incompatible with some thinset material, so that's a consideration. Advantec seems to work pretty well from my limited experience. Some have a waxy coating that doesn't.

A ply with better than 'D' faces and either exposure 1 or exterior rated glue should work for you as long as it's at least 3/8" thick, and finding some that thickness that's any good can be tough, leading to more likely 1/2" stuff.

Ditra gets different class ratings, depending on the subflooring and joists used. You probably want one of the heavier ratings with a washing machine. Now, some of them vibrate more than others with an imbalanced load. My front loader seems almost immune from vibration, but it has probably 200# plus of ballast weights in it! Think the shipping weight was close to 300#.

There's a chart in the Ditra Installation handbook that describes what rating each approved configuration provides. Light residential probably wouldn't be wise, but heavy industrial is overkill.
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Unread 08-06-2020, 09:19 PM   #3
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Welcome, Andrew.

To be clear, you are not looking to add an underlayment, you're needing to add a second layer of structural subflooring. The Ditra you want to use will be your tile underlayment.

There is no BCX grade plywood, regardless what your home center might put on the label. There is a BC grade plywood and that's usually an exterior glue plywood, which is what you're looking for. The BC grade will not have open voids in the faces or inner plies. Just what you want for your application.

Keep in mind also when you consider (or hopefully discard) the idea of tiling over your existing subfloor that Schluter's recommendation is based upon a test (ASTM C-627) conducted with all new material in perfect condition, near perfectly installed over joists with zero deflection, none of which you have, and the test needs to pass only once.

Schluter would also point you to the customary caution in all the methods in the TCNA Handbook that tell you that for known point loads the "engineer and/or specifier (that's you) "shall specify a substrate to accommodate the concentrated loads." You have a failure of your tile installation in the application your propose and claim it was a failure of the Ditra, the very best you might get from Schluter is a smile.

My opinion; worth price charged.
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Unread 08-06-2020, 09:57 PM   #4
Toots
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Thanks for the replies!

Yes, the 5/8 actually is brand new in perfect condition. About half the house was ripped back to floor joists and new subfloor put in due to well....not being done well the first time lets just say. However I 100% understand and agree with your point that you can't really count on perfect numbers on anything that is a reno.

I guess I was looking for opinions on just how close to the envelope I was pushing it.

As I said, adding 5/8" is out of the question, I'll hit exterior door bottoms. Do you think 1/2" will be enough? It seems like short of hiring an engineer, you kind of have to take your pick of what kind of point loads you think the floor will experience.

Normally, even on a modern T&G 3/4" subfloor, I'd pile up another 1/2". I just don't like taking chances. And yes I've pretty much abandoned the idea of tiling straight on the 5/8.

Also, thanks for the advice on the "BCX." Hence my hesitation, I've never heard of such a thing either. Lets just say....Menards is not a place I willingly shop for lumber regularly.
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Unread 08-07-2020, 07:10 AM   #5
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Two options spring to mind. Remove the existing 5/8", install cleats along side the joists, and install 3/4" ply between them and then 3/4" ply on top.

If that's not possible, and if you have the space for 1/2", I'd give serious consideration to installing the new 1/2" B/C exposure 1 ply over the existing 5/8" using a full spread of wood glue and suitable screws.
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Unread 08-07-2020, 08:29 AM   #6
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Andrew, here's a good article from our Liberry showing what I think is the best way to install that second layer of plywood.

And while I'm a very large fan of gluing everything to everything else in a wood framed subfloor package, one of the authors of that article has assured me that they can come very close to the same deflection test data with their closer fastener schedule as they might with a glued second layer. I still think the glue is better, but you hafta decide if it's worth the effort.

1/2" plywood over 5/8ths" plywood or OSB over 16" joist centers is a very good subfloor. I would always prefer plywood for the second layer if I intended to bond anything to it.

I see no problem at all buying plywood from the home centers so long as you can determine what you're buying. Seems they've all gone to that BCX or ACX or similar description and I suppose we're supposed to think the X means exterior glue. I prefer to see a grade stamp, but......

My opinion; worth price charged.
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Unread 08-07-2020, 08:43 AM   #7
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5/8" passes the minimum performance values using a perfect world test. If you want minimum performance and have an otherwise perfect floor system which is unlikely, it should work. By the way, the plywood tested was Doug Fir.
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Unread 08-07-2020, 11:07 AM   #8
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Yeah that article is the same table in the Schluter handbook, not surprising given the article's author.

I ordered my plywood this morning, 1/2" B/C. I'll go with their screw schedule of 6" OC with coated #8 deck screws. I think I'll also just put some 2x6 perpendicular blocking screwed and glued every 8-10" in the joist cavity where the feet of the washer and dryer will fall. That should add an enormous amount of localized stiffness for extra insurance.
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