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Old 09-14-2010, 05:07 PM   #1
MainDish
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thinset over plywood with old glue

I started attacking a really ugly entryway today.
Plan:
modified thinset, heating mat, Ditra, unmodified thinset, porcelain tile

Fair enough?

The floor is currently constructed of two 1/2" sheets of plywood over 16"o.c. 2x10 joists spanning only about 8'. The two sheets is weird, but okay. I assumed that small area should be okay with Ditra.

After pulling up the peel&stick vinyl tiles, it's clear that there used to be some other resilient floor product glued to the plywood. There is residue all over.

Any suggestions on prepping this? Will a modified thinset stick ok?

Worst case, I'll cut out the top layer and add a 3/4" T&G layer.

Thoughts?

P.S. I may start another thread, but another question: Do any of you use something around a heating mat (on unheated areas) to keep your mortar bed level (I'm thinking some kind of cheap mesh)?
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Old 09-14-2010, 07:22 PM   #2
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Welcome JR - You want all that loose or removable glue to come off that plywood. A good razor scraper will do wonders with a sharp blade.

For me I'd see
1) a clean ply surface
2) heat mat installed (who makes this one you are using?)
3) self leveler - usually I will use 1/4" cbu cut to about an 1" to creat3 a perimeter to hold the slc in. Make sure you use a primer.
4) unmodified thin set
5) Ditra
6) unmodified thin set
7) tile
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Old 09-14-2010, 07:46 PM   #3
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What John said plus;

You can decide for yourself which is more valuable: the time and effort to scrape off the residue or install new plywood. The alluring smell of fresh pine plywood has its pleasures, but only you can calculate the work involved. I can't see your house from here.

Optimal: new plywood
Very close second: well cleaned plywood
Acceptable 3rd place: none I can think of.
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Old 09-15-2010, 09:39 AM   #4
MainDish
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Thanks for the replies.
I was kind of hoping someone would say "sure, go ahead and stick thinset to the old linoleum residue." My instinct says otherwise, and I will be replacing one or both layers of plywood.

For the sake of argument,
The self-leveler used as you suggest is primarily to level (duh) and embed the heating mat, correct? As such, with Ditra above it as a decoupling layer, does the self-leveler have to perfectly bond with the plywood or can the old glue patches be buried under it?

Being that this is an entry, it is somewhat sensitive to build height (doors) - which is why I pursed Ditra and porcelain in the first place, otherwise I would've planned cbu and natural slate. Assuming I have new plywood and that moisture is impeded by Ditra, I'm still hesitant to add 1/4" of self leveler.
To answer your question, the heat mat is from warmingsystems.com and I wanted to try and just thinset right over it. This is approved by most heat mat manufacturers as far as I can tell; is it a bad idea?

I used the cheaper heating cable once, and it was awful to try to get it laying flat and required a full 1/4" pour of self leveler. I hoped the mat could actually be installed under the thinset bed.
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Old 09-15-2010, 07:51 PM   #5
jadnashua
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If you want to minimize subflooring buildup, take all of the subflooring off, install ledgers on the sides of the joists, install 3/4" ply on the ledgers to fill in so it is level with the top of the joists (running the normal way you would with the grain), then put at least 1/2" layer of ply on top of that.

SLC can be poured thin, but over plywood, many manufacturers want lath and a miniumum of 1/2" over the top of the highest layer. It is hard to get the slc to flow if it is enmeshed with the heat mat...it needs some height over it. Think pancake batter, not syrup. The slc will be brittle, if it isn't bonded well, it will crack and debond, then it'll sound like stepping on Rice Krispies...not good!
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