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Old 01-01-2009, 01:39 PM   #1
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confusion over thinsets for installing Ditra over heated flooring


I am preparing to install Ditra over thin-wire heated floor heating, which in turn will be installed on a plywood substrate.

My confusion is whether I need to set the heted flooring in modified thinset as per the instructions, and let it set before I lay down the Ditra. If so how long would it be necessary to allow for setting before putting down the Ditra. If this scenario is the case, then would I now need to use unmodified thinset to adhere the Ditra to the previously set modified thinset?

If there is no need to effectively have the two layers, then is it safe to assume that the modified over the wire heating on the plywood is correct to apply to the Ditra?

Thanks for any help, that may relieve some of the confusion.

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Old 01-01-2009, 06:27 PM   #2
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confusion over thinsets for installing Ditra over heated flooring


Regardless of what you read here, ALWAYS follow manufacturers instructions. I am sure they cover your situation there somewhere. But since you are using a product designed to cover breaks, I would use modified thinset.

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Old 01-01-2009, 06:58 PM   #3
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confusion over thinsets for installing Ditra over heated flooring


Quote:
Originally Posted by fhknox View Post
My confusion is whether I need to set the heted flooring in modified thinset as per the instructions, and let it set before I lay down the Ditra. If so how long would it be necessary to allow for setting before putting down the Ditra. If this scenario is the case, then would I now need to use unmodified thinset to adhere the Ditra to the previously set modified thinset?
Yes, set the heated mat or wires in modified thinset and let cure as per the instructions on the bag. Then install Ditra using non-modified thinset.
That's the instructions per Schluter. You can technically use modified for the entire process BUT you void the Schluter warranty and dramtically increase the curing time for the thinset under the Ditra.
I'd stick to the instructions as per the manufacturer!
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Old 01-02-2009, 12:33 AM   #4
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confusion over thinsets for installing Ditra over heated flooring


The main question is; are you spreading modified thin set on the entire plywood floor in order to install the heat wire? If so, you should use un-mod to install Ditra because its substrate is thin set, not plywood.

If part of the plywood remains bare, or if the wires and Ditra get installed in one process (unlikely), then you'd have to use modified in order to get anything to bond to plywood.

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Old 01-02-2009, 11:34 AM   #5
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confusion over thinsets for installing Ditra over heated flooring


You'd be better off using a self leveler for the heat instead of thinset. Then use unmodified for the ditra.
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Old 01-04-2009, 08:52 AM   #6
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confusion over thinsets for installing Ditra over heated flooring


Since he is installing this on plywood, won't he have a problem with the self-leveling cement unless he uses lath?
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Old 01-04-2009, 09:03 AM   #7
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confusion over thinsets for installing Ditra over heated flooring


Do not use self leveling cement. Not really level either. Jazman states to use modified to bond to plywood. BUT... non-modified is recommended by the TCA because it will not bond to the plywood. This is what you want. The plywood can shrink and move. The non-bonded area above (wire, thinset, thinset tile) will not move. Thus not crack. So to be clear... non-modified first. Modified for flexibility next to set tile.
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Old 01-04-2009, 10:53 AM   #8
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confusion over thinsets for installing Ditra over heated flooring


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Do not use self leveling cement. Not really level either. Jazman states to use modified to bond to plywood. BUT... non-modified is recommended by the TCA because it will not bond to the plywood. This is what you want. The plywood can shrink and move. The non-bonded area above (wire, thinset, thinset tile) will not move. Thus not crack. So to be clear... non-modified first. Modified for flexibility next to set tile.
Would you expect getting a level surface over the wire with non-modified thinset to be very difficult?
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Old 01-04-2009, 11:30 AM   #9
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confusion over thinsets for installing Ditra over heated flooring


Bob,

You've got it backwards and maybe you're not familiar with what Ditra is? You must think Ditra is a CBU and not a membrane?

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Old 01-04-2009, 11:39 AM   #10
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confusion over thinsets for installing Ditra over heated flooring


yep. that is what I was thinking.. oops.
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Old 01-04-2009, 12:05 PM   #11
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Do not use self leveling cement. Not really level either.
HOGWASH!

Exactly how many heated floors have you personally done Bob? Really: How many?
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Old 01-04-2009, 12:46 PM   #12
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confusion over thinsets for installing Ditra over heated flooring


There are several self levelers that work just fine over plywood. Some require a thicker pour than others. You just have to be careful which one you choose.
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Old 01-19-2009, 07:21 PM   #13
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confusion over thinsets for installing Ditra over heated flooring


My master-bath plywood substrate is flat but has a bad slope: more than 12mm (1/2") over 8 feet. One corner is 18mm (3/4") lower than the highpoint.

I used WEDI on the walls and WEDI Fundo as shower base (shower was on a perfectly level section of an adjacent alcove that was made part of the room) and I am trying to decide what to do about the floor.

It cannot be leveled with SLC only because the threshold is on the low end of the room and the plywood is already 1" higher than the bedroom's floorboards. The SLC pour plus mesh plus tile would make it 2" higher. But I can forget about leveling and hide the slop behind the vanity.

Also, I want to use something like Ardex Feather Finish to smooth out 1-2mm differences in height between adjacent plywood panels on the substrate.

What type of uncoupling membrane should I put on top of this plywood + Ardex Feather Finish?
(there are two layers of plywood, each 3/4" thick; 10" joists 16" o.c.; unsupported span 10')

Wedi 1/4" panels?
DITRA?
Something else?

And what kind of thinset if there will be bare plywood and also patches of Ardex Feather Finish at the joints?

Thanks

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Last edited by diy888; 01-19-2009 at 07:33 PM.
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