Ditra-Heat Mat Installed Over Plywood

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miket333

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Hello, I just realized I screwed up and installed the Ditra-Heat mat over a plywood underlayment using Tec Full Set unmodified thinset. The options as I see it are 1) rip out mats and reinstall with latex modified, 2) leave as-is and hope the bonding remains intact, 3)??? If the bonding between the mat and the plywood loosens slightly over time, is that really going to cause problems? The tile is bonded to the top of the mat and the whole system is designed for it to "float" so that tiles don't crack due to substrate expansion and contraction. So, if the whole mat is "floating" and the uncoupling membrane is not 100% bonded, what will happen? My feeling is that #2 would be a reasonable risk due to the time and cost of ripping out and redoing...curious to hear your thoughts. Thanks. (P.S. I'm not concerned about potential warranty issues).
 

jadnashua

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You will not be able to reuse the mat as tearing it up will pull off the fleece, meaning there's nothing to attach it back down. Thinset does not stick to the membrane on either side...it is held in place because the mortar flows around the fleece, then cures on the bottom, and by the shape of the pockets (dovetail) on the top, encapsulating it, not like what happens with thinset to a tile.

The heating will cause more expansion and contraction than you'd get with a normal install, so the bond to the floor probably won't last. Schluter follows the industry standards that requires a modified thinset when trying to bond anything to plywood, so it's not just them. The mat gets the 'floating' effect because it's bonded to the floor, and the grid, being open, can flex. The movement caused by expansion will be putting more stress on the mat to floor bond, and that bond won't be as strong, so it's more likely to break. The whole thing could continue to float, but I don't know enough to know exactly the end result if that bond breaks.

Personally, I'd probably bite the bullet and buy new mat and replace it, but it's your house, your choice. Most thinset companies sell an add-in modifier that would let you use the same powder, but use the add-in to make it modified, so you don't have to buy two different types. Be careful to measure accurately if you go that route.
 

miket333

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Thanks for your opinion. I am going to rip it out and redo it with the correct thin set. I spoke to a Schluter rep who said I may be able to reuse the Ditra mats if they are cleaned and if the membrane is not overly shredded and damaged...so, will see how that goes.
 

jadnashua

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If you got the fleece well embedded, it will tear most of it off when removing. You'll know. The longer you wait, the worse it gets, though. If it had been the next day, you'd have a better chance.
 

miket333

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You're right. The old mats cannot be salvaged due to the thin set hardening and not able to break it away from the fleece without ripping the fleece off the mat. So, I've scrapped that idea and bit the bullet and purchased new mats. Now its just a matter of time and effort pulling up the old and cleaning off the plywood underlayment in preparation for the new.
 

miket333

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New problem/question...As I'm ripping out the Ditra mats, it pulled off the membrane that was attached to the foam shower pan. Now I have exposed Styrofoam for my shower base. I'm not finding any information on whether thinset adheres to Styrofoam or not. So, does anyone know if thinset bonds to foam? If not, should I glue a new membrane to the foam base and if so, any suggestions on what glue to use?
 

jadnashua

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Thinset will bond to the foam, but you will want to treat that void as if it was a penetration. While the foam itself is waterproof )think foam coffee cup, but denser), moisture could get behind the remaining layer, and you don't want that.
 

miket333

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The Ditra-heat mat is a waterproof layer above the shower pan and I will be adding a waterproof membrane on top of the mat to keep the heating cable dry so there should be no water coming into contact with the foam base, at least that's the plan.
 

jadnashua

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That's good and what Schluter calls for in their instructions when used in a shower!
 
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