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Old 09-03-2008, 11:55 AM   #1
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radiant floor heat installation issue


Hi folks,

A contractor installed Nexans heating cable over the plywood subfloor in a bathroom I plan to tile. Problem is that he embedded the cable in a layer of regular mortar (i.e., not thinset), and it doesn't appear to be well bonded to the subfloor (and has some cracks in it).

So my question is: is this adequate to lay tile over with a layer of Ditra, or should the mortar be removed and reworked? So far three out of four tile contractors seem to think it should be ok - the fourth is a stickler for detail and thinks the mortar should be chipped out and replaced.

Any thoughts would be much appreciated.

Thanks,
Paul N.

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Old 09-03-2008, 04:22 PM   #2
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radiant floor heat installation issue


It's kind of hard to see what you are describing from here but if you know some of the masonry is loose maybe you should consider removal.

The product requires a minimum of a 1/2" masonry mass. Some cracking may occur as the masonry product dries. They may only be "shrink cracks" from using a too-wet masonry product.

A dry-pack on the other hand shouldn't crack at all if mixed and installed properly. A dry-pack is not pourable and must be placed and then tamped into compaction and screeded off smooth.

Self Levelling Compound is the best way to embed the heating product in my opinion and it is very difficult to make a mistake with SLC. A primer is however required.

A few photos of this project would be nice.

If for sure there is loose or crumbling masonry this will compromise the remainder of the installation.

Why are you using DITRA over all of this? It isn't at all necessary under normal circumstances. You should already have all the substrate you require with the masonry mass you already have.


Last edited by Bud Cline; 09-03-2008 at 04:25 PM.
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Old 09-03-2008, 04:43 PM   #3
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radiant floor heat installation issue


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Originally Posted by Bud Cline View Post

A few photos of this project would be nice.

If for sure there is loose or crumbling masonry this will compromise the remainder of the installation.

Why are you using DITRA over all of this? It isn't at all necessary under normal circumstances. You should already have all the substrate you require with the masonry mass you already have.
It actually looks ok, except for some narrow cracks. I'll post some photos as soon as I can.

As for using DITRA, it was a suggestion of a tile contractor. I spoke to four of them - all seemed quite competent, but two didn't recommend any remedial action, one recommended a Ditra mat over the mortar, the fourth recommended replacing the mortar with Tek 390. I suppose the Ditra makes sense to me as a general protection against movement in the horizontal plane as may result from seasonal expansion and contract of the subfloor transmitted upward through the mortar.

Kind of hard to decide what to do when there's no consensus among the experts...

Thanks,
Paul
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Old 09-03-2008, 04:55 PM   #4
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radiant floor heat installation issue


DITRA offers NO protection from any upward movement, none what-so-ever. Read their Data Sheet. I hope the seller didn't tell you that!!!

DITRA is a fine product but isn't at all necessary in this case. If you want to spend money on the DITRA for piece of mind then that's your call.

Any lateral forces the floor may experience (which DITRA does provide protection against) is already handled by installing the masonry mass. That's another function of the masonry is to isolate the tile installation from the structure.

To use DITRA would be a duplication of concepts and a drain on the ole bank account.
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Old 09-03-2008, 05:12 PM   #5
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radiant floor heat installation issue


Quote:
Originally Posted by Bud Cline View Post
DITRA offers NO protection from any upward movement, none what-so-ever. Read their Data Sheet. I hope the seller didn't tell you that!!!

DITRA is a fine product but isn't at all necessary in this case. If you want to spend money on the DITRA for piece of mind then that's your call.

Any lateral forces the floor may experience (which DITRA does provide protection against) is already handled by installing the masonry mass. That's another function of the masonry is to isolate the tile installation from the structure.

To use DITRA would be a duplication of concepts and a drain on the ole bank account.
I think we're saying the same thing about Ditra - I used the term 'horizontal' to describe what you seem to be calling 'lateral' motion. I realize it doesn't protect against other forms of substrate distortion.

As for the masonry mass protecting against lateral forces... does it? If the masonry is bonded to the subfloor, this implies it will expand and contract with the subfloor (if it can), does it not?. If it isn't bonded to the subfloor, it may crack and transmit that movement to the tile layer unless there's an additional elastic component in the path... ?


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Old 09-03-2008, 06:10 PM   #6
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radiant floor heat installation issue


Quote:
If the masonry is bonded to the subfloor, this implies it will expand and contract with the subfloor (if it can), does it not?.

No it doesn't!
The masonry won't stay bonded to a wooden subfloor for long. The first time something changes it will likely sever and thereby slip a little when things begin to move. It can't stay bonded because the wood and the masonry each have different expansion and contraction rates. That's a good thing.

The masonry is like the tile installation and they will stay bonded and become one, that's what you want.

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If it isn't bonded to the subfloor, it may crack and transmit that movement to the tile layer unless there's an additional elastic component in the path... ?
Nope!
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Old 09-03-2008, 07:43 PM   #7
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radiant floor heat installation issue


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Originally Posted by Bud Cline View Post
No it doesn't!
The masonry won't stay bonded to a wooden subfloor for long. The first time something changes it will likely sever and thereby slip a little when things begin to move. It can't stay bonded because the wood and the masonry each have different expansion and contraction rates. That's a good thing.

The masonry is like the tile installation and they will stay bonded and become one, that's what you want.
Ok, that makes sense... but does thinset stay bonded to the subfloor? The guy who suggested that the masonry layer should be replaced with Tek 390 argued that it should be bonded to the subfloor, otherwise the floor will fail.

BTW, an additional complication is that the heating cable is tied to the plywood subfloor and embedded in the mortar.

Paul
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Old 09-03-2008, 11:27 PM   #8
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Quote:
OK, that makes sense... but does thinset stay bonded to the subfloor?
The Tile Council of North America's Handbook For Ceramic Tile Installations recommends that unmodified thinset be used to bed cement backer boards to substrates. This is because they know the thinset will de-bond from the substrate and produce the desirable conditions mentioned above.

If that is the case then it is reasonable to assume that "unmodified thinset" used to bury heat cables would do the same. I don't have any idea what Tek 390 is. Is that supposed to be TEC 390? I don't find either when I do a search.

On the other hand some forms of "modified thinset" can be used to install tile on plywood and the bonding is sufficient to produce a long lasting tile installation. That being the case if modified thinset were used on your project this would preclude the desired debonding condition.

Then there is the "regular mortar" you mentioned in your first post. I don't know what you mean by that but if it is simply Portland and sand then that is a product we also recognize as "deck mud" maybe your ("dry pack") and should also be suitable for your application. IF, it was mixed properly. A "proper mixture" would be a ratio of 4:1 (5:1) sand to Portland. The mixture is then tossed into place and hard-packed by tamping it with a trowel or float. It is mixed very dry and once tamped you can walk on it immediately. Walking on it that soon isn't recommended but I tell you that to illustrate how the deck mud should be packed/tamped into place. That type of installation shouldn't crack unless it is somehow abused.

Then there is the SLC I mentioned. SLC requires a "bonding primer" so that it WILL stick to the substrate and not curl under it's rapid drying/setting process.

Yow I know, confusing ain't it?

A picture would be nice right about now.

I'm guessing your OK but maybe need a skim coat of modified thinset on top of what's there just to tie it all together on the surface. But I can't be sure that's the thing to recommend without seeing it.

The quandary I'm having is: How the heck you gonna take it out without damaging the heat system?

I swear!!!! I have never had some of the problems that people seem to be able to come up with on these forums. It amazes me the things that can go to hell without warning and variables in advice and experience that exists. There has been a rule book for more than forty years, I guess no one wants to read it.

Maybe tomorrow you could call Nexans at 866-558-3369 and talk to a guy named Scot Cook there. He may be able to advise you. I talked with him just today but before I saw your post. I could have queried him.

Last edited by Bud Cline; 09-03-2008 at 11:36 PM.
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Old 09-04-2008, 05:29 AM   #9
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radiant floor heat installation issue


Quote:
Originally Posted by Bud Cline View Post
The Tile Council of North America's Handbook For Ceramic Tile Installations recommends that unmodified thinset be used to bed cement backer boards to substrates. This is because they know the thinset will de-bond from the substrate and produce the desirable conditions mentioned above.

If that is the case then it is reasonable to assume that "unmodified thinset" used to bury heat cables would do the same. I don't have any idea what Tek 390 is. Is that supposed to be TEC 390? I don't find either when I do a search.
Maybe this is the stuff he was referring to -
http://www.amazon.com/Specialty-TA-3...523107&sr=1-24

Quote:
Originally Posted by Bud Cline View Post
Yow I know, confusing ain't it?

A picture would be nice right about now.
Yes, it is confusing. I'll post a photo this weekend.


Quote:
Originally Posted by Bud Cline View Post
I'm guessing your OK but maybe need a skim coat of modified thinset on top of what's there just to tie it all together on the surface. But I can't be sure that's the thing to recommend without seeing it.

The quandary I'm having is: How the heck you gonna take it out without damaging the heat system?

Presumably it would have to be chipped off carefully. Not a process I'd be keen on embarking on unless it's really necessary. Seems likely that replacing the cable would be easier.

Quote:
Originally Posted by Bud Cline View Post
I swear!!!! I have never had some of the problems that people seem to be able to come up with on these forums. It amazes me the things that can go to hell without warning and variables in advice and experience that exists.

Yes, I've also been amazed at the lack of consistency in advice given by contractors, not just in this area, but throughout the building industry. Sorting out the conflicting opinions can be incredibly difficult and time-consuming.


Quote:
Originally Posted by Bud Cline View Post
Maybe tomorrow you could call Nexans at 866-558-3369 and talk to a guy named Scot Cook there. He may be able to advise you. I talked with him just today but before I saw your post. I could have queried him.
Much obliged, I'll talk to him.

Thanks,
Paul
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Old 09-04-2008, 12:19 PM   #10
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radiant floor heat installation issue


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Originally Posted by pauln View Post
Maybe this is the stuff he was referring to -
http://www.amazon.com/Specialty-TA-3...523107&sr=1-24
Well, I just looked at the materials order list at the time this was done, and it turns out he did actually use the above-mentioned thinset as was recommended by the tile contractor, rather than regular motar.

But now I feel like I need to understand this...

If this thinset bonds to the plywood with attached cable, presumably it would expand and contract with the plywood, and some kind of decoupling membrane would then be desirable, would it not?

Paul

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