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Unread 06-09-2021, 05:34 PM   #1
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Tiling over Hydronic Floor Heat


I'm looking for advice on tiling over hydronic floor heat (new construction)...see picture for what I have to work with. The PEX is half inch with 3/4" OSB spacers separating the runs, about 6-1/2 inches apart. This is on top of a 3/4" OSB subfloor, on engineered I-joists 16 inches OC.

My contractor tells me that past jobs have screwed cement backer board to the spacers and then tiled on top. Hardie says their boards should be set in a mortar bed and screwed in place. Will this work? I've seen discussion about 1-1/4" of mud on top of hydronic floor heat. I hope to keep the tiled floor thickness to 3/4". Suggestions....recommendations? I want this to last and only want to do it once.

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Unread 06-10-2021, 08:51 AM   #2
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Welcome back, Jeff,

The "spacers", are they individual pieces that are laid on top of the 3/4" OSB, or are the channels for the PEX routed into the the top layer of 3/4" OSB?

Does the manufacturer of that hydronic system offer any instructions on how to install a floor covering?

Of course you could ignore Hardie's instructions and simply screw down some 1/2" sheets and hope for the best.

An option; screw down a layer of 1/2" ply, then cover that with Ditra, then tile.
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Unread 06-10-2021, 10:57 AM   #3
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Jeff, combining several manufacturer's products can be confusing for sure. Two things come to mind in your case.

The aluminum heat transfer plate mfg. should give specifics on how they want them covered.

All cement board manufacturers would want thinset mortar under their boards...it a kind of accepted industry standard.

My only concern would be interaction of aluminum plates and the alkaline nature of anything containing cement. May be a nothing, but I'd want the blessing of the plate manufacturer.

Most everything else in that assembly is inert enough not to be a concern.

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Unread 06-10-2021, 02:34 PM   #4
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I'd screw down a layer of 1/2" ply on top of what you have, then use Ditra on top of that to keep the buildup lower. Keep in mind that CBU is not structural, and would need full support underneath. Some of those pockets would likely way exceed the max depth of thinset material, and the stuff shrinks when it cures some, especially when applied too thick. Be VERY careful to map out where the tubing is so you don't puncture it.

If I were starting over, I'd probably use Schluter's BEKOTEC and a thin mudbed for the tubing. You can tile directly onto that, or use Ditra.
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Unread 06-15-2021, 06:01 AM   #5
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Thanks for the help.

The spacers are 3/4 OSB, glued and stapled to the 3/4 OSB subfloor.

Understand the possible interaction of mortar and AL plates....I'll have to look into it... and mortar thickness in troughs. Half inch ply on top w/ Ditra sounds like a good approach without adding much to thickness.

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