Subfloor under Ditra question


Old 06-28-09, 11:10 AM
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Subfloor under Ditra question

Hi there-
I have a few questions about subfloor prep for ditra installation.
Subfloor is 5/8" plywood with 1/4" osb over top, previous owner had linoleum floors. I am removing the paper backing and glue that stuck to the floor after pulling up old linoleum (nightmare, tedious job!). Using a steamer and putty knife to scrape up. I still have a bunch of places that I'm working on that have old glue, trying to decide how important it is that I get ALL of it up. I'll be using a modified thinset over subfloor to adhere DITRA.

From scraping (as gently as I could), there are many areas that the OSB has fine "hairs" of wood sticking up.

1. Am I ok with the 5/8" plywood and 1/4" OSB for ditra install?
2. Will those wood hairs cause any problems?
3. Will the modified thinset/ditra adhere properly to any areas that still have glue?
4. If I'm going to use a self-leveling compound in a few areas, will the thinset/Ditra adhere properly?

Charlotte, NC
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Old 06-28-09, 03:39 PM
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1. Nope. You need to pull up the 1/4" underlayment too.

2, 3. My answer to #1 should fix that.

4. No problems.

Can I ask what your floor joists sizes (2x?), joist spacing (center 2 center), and the unsupported span of those joists?
Old 06-29-09, 04:04 PM
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Remove the 1/4" underlayment.

Ditra over 5/8" plywood is a bare bones minimum installation. Consider adding more plywood, minimum 3/8", thicker would be better. Then install ditra.

Is the subfloor square edge plywood or t&g? If its square edge and you decide to install ditra over it, you'll need to add blocking under the plywood edges to fully support the seams between the joists.

Any leveling/flattening of the floor should be done under the ditra, not over it.

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