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Old 04-01-2007, 10:43 AM   #1
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suitability of subfloor for Ditra in renovation

Hi All,

Just a quick question about best practices vs. minimum requirements when trying to match the height of a new tile installation in a 1960's house to adjacent areas of existing parquet maple flooring.

My background: I'm a flooring newbie, I've spent past few days reading through posts on this forum, I've looked at TTMAC specs, researched articles in Fine Homebuilding, visited the Schluter website, and have attended some big box store seminars on floor tiling.

My situation: our entrance hall and kitchen floors were signicantly higher than adjacent rooms (connected by openings, not doorways), because layers of flooring were applied over each other. I have just removed these layers (clay tile over 3/16" luan plywood stapled 6" o.c. and screwed 3" o.c. ((yes, I said 3" o.c.)), over layers of vinyl tile over 1/4" underlayment screwed 6" o.c., over layers of vinyl tile over 3/8" particle-board-like-stuff nailed at 6" o.c.) down to the original subfloor. The original subfloor is 5/8" plywood, nailed to the joists with ring nails, and seems to be in decent condition, apart from the occasional daub of mud left by the original drywallers. I was able to unscrew all fasteners from the subfloor (even those with shanked heads) instead of tearing them out.
The floor structure is 2x10's in good condition bearing on steel beams and foundation walls, but the joists are at varying spacings- apparently a result of coordination with the ductwork. The widest joist span is 13', and the weakest section has two single 2X10's at 16" o.c., while the rest of the floor is doubled 2X10's at spacings varying from 16" down to 8" o.c. The deflecto tells me that the weakest section is still suitable for ceramic tile.

My concern: the new tile will abut existing maple parquet flooring which is about 5/16" thick (applied directly over the plywood subfloor). Having removed about 1-1/2" worth of cumulative flooring at this point, I would prefer to have the new tile installation come as close as possible to the surface of the existing parquet, but clearly need to construct the new floor properly. Both Schluter and TTMAC say that Ditra can be applied directly over 5/8" plywood, which would allow me to bring the new tile flush with the existing parquet.

My questions:
1) Is applying Ditra over the existing 5/8" plywood subfloor advisable? If so, should i be scrubbing it clean of all traces of compound, screwing it to the joists and removing existing nails?
2) If i should add another layer of plywood before the Ditra, what is the least thickness advisable, and should it be fastened to the subfloor and not the joists, or to both floor and joists?
3) If i have to apply another layer of plywood and thus raise the new tile above the existing parquet, am I better off (from a cost balanced against performance perspective) removing Ditra from the assembly entirely and simply applying a thicker second plywood layer?

Any advice you can offer would be greatly appreciated.

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Old 04-01-2007, 03:58 PM   #2
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That 5/8" minimum is for new material...yours has all sorts of nails and screw holes and who knows what. I'd put another layer of ply on top of it, thicker is better, but a minimum of 3/8". To keep the levels minimum, Ditra (it's much easier to cut and install) and tile. Some floor tile is quite thin. I was surprised at the thickness of the floor tile my mother chose for her shower...barely 3/16" thick, but really tough stuff. So, unless yo utear off the 5/8" and replace with 3/4" new stuff (or thicker), I'd be somewhat leary about tiling directly to it.

When installing the second layer, you try to avoid the joists. there is an article in the liberry that describes the procedure quite well.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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Old 04-03-2007, 06:22 AM   #3
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Thanks Jim,
Just one more question- I searched the archives (using Google this time, so actually getting results) and came across the fastener placement/sheet overlap article you mentioned, but I also came across two conflicting suggestions concerning the type of glue to use.
I should first say that as I'm using Ditra to uncouple the tile, I gather from what i've read that gluing and screwing the new 3/8" to the existing subfloor is the way to go.
In many different threads though (sometimes even by the same person) i see the glue joining plywood plies (not plywood to joists) described as construction adhesive on one hand, and as wood glue on the other.
I'm now in the process of scraping and brushing the existing subfloor as clean as I can. Wood glue rolled on with a paint roller just before screwing down the 3/8" seems to make more sense to me than gunning beads of thick pcl500 over it. Is using wood glue over an existing subfloor a reasonable thing to do?
Thanks again for your time,

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Old 04-03-2007, 08:37 AM   #4
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Richard: Yes, a full spread liquid wood glue is what you want to use. Titebond II is a good example. Any adhesive like PL 400 gunned from a tube will leave voids. Not a good thing to have under a tiled floor.

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Old 04-06-2007, 10:00 AM   #5
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Just a quick note for any fellow tile noob who might come across this thread while searching the archives for glue related posts....
May sound obvious, but then again I didn't think about it before I started....
Yesterday I began rolling glue over my existing subfloor and screwing down my pre-cut pieces of overlay, carefully following the recommended fastening schedule. After a few hours I went to the basement to swap my dead drill battery for a freshly charged one.
Walking across the basement floor, I wondered... "Hey, what are all those pea sized yellow balls on the floor?"
Previous screw holes+ glue+ pressure from above= intermittent periods of glue precipitation.
Sun is shining today tho...
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