DIY Home Improvement Forum banner

1 - 17 of 17 Posts

·
Registered
my truck box
Joined
·
7 Posts
Discussion Starter #1
so this bathroom used to have VCT on it, but it popped off real easy, but the glue used for the VCT is still on the plywood. does anyone know if unmodified thinset can be used to put ditra straight on it?
or should it be fast setting thinset since the plywood seems a bit sealed due to the VCT glue?
anyone have experience with this?

thanks for looking either way!
 

·
Registered
Joined
·
646 Posts
You’re in uncharted waters here. You may get someone who responds that he/she did this and that it worked out fine, but you’ll have no idea if the adhesive in their situation was the same as yours or bonded to the substrate the same as yours.

Schluter says that it is possible to lay DITRA and tile over old VCT, but since you wrote that it came up easily I think that you did the right thing not doing that.

So, is there a thinset that will bond to old VCT adhesive? I’ll guess that you won’t find that as a suitable substrate on the spec sheets of any thinset, but I might be wrong. Considering all the work that is probably still to go into the rest of the project (tile, grout, baseboards, toilet, cabinets, etc.) I would be unwilling to take the chance and discover later on that the DITRA didn’t bond to the old VCT adhesive. Here are the options that I’d explore rather than hope for a good bond to the VCT adhesive:
  • Can you accommodate an increase in the floor height, even if it requires adding a height transition at the door threshold? If so you can screw down a new layer of approved substrate over the VCT adhesive.
  • Can most of the old adhesive be removed to get back to (mostly) a substrate that the mortar will certainly bond to? Minor damage to the plywood won’t matter, since it will all be filled in with mortar.
Since this bathroom did not have tile in it before, are you confident that the structure is suitable for tile? A bit of deflection in the floor that is unnoticeable when walking on it doesn’t matter for VCT, but it does for tile. Here’s a good intro on the subject, but the folks on this forum might be able to help. Guessing that this is for a second floor bathroom, you’ll need to provide the joist spacing, joist span, joist size, subfloor thickness and materials.
https://www.finehomebuilding.com/2016/01/06/prep-a-subfloor-for-tile

Have you investigated the “flatness” of the floor? It’s OK for the floor to be unlevel as long as it is flat. Now is the time to do something about it if it’s not flat, rather than when you’re laying the tile.

Chris
 

·
Registered
my truck box
Joined
·
7 Posts
Discussion Starter #4
chris, thanks for the detailed reply.
there was tile before. it was 1/2" concrete board screwed throught the VCT to the subfloor. with a 6" ceramic thinset'd to the concrete board.
since it was a marble mosiac, ditra was passed over, and i set Hardieboard to the slightly vct-glued plywood with modified thinset and screwed it every 6". came back next day and tiled. glad a spent the time prepping the hardieboard with thinset. besides being a decent decoupler, it gave me a smooth straight substrate to lay the mosaic, which saved me so much time and headache
 

·
Registered
Joined
·
5,702 Posts
Man…when will I learn to check the OP posting date before replying? In this case I’m going to blame huesmann, though :smile:

I think you're confused - the first post in this thread is very recent.


You don't seem confused with your answer though - it was very good.


While you're obviously not going to find this sort of thing covered by any manufacturers warranty (how could they possibly?), almost all the time I think it would turn out fine.
 

·
Registered
Joined
·
5,702 Posts
chris, thanks for the detailed reply.
there was tile before. it was 1/2" concrete board screwed throught the VCT to the subfloor. with a 6" ceramic thinset'd to the concrete board.
since it was a marble mosiac, ditra was passed over, and i set Hardieboard to the slightly vct-glued plywood with modified thinset and screwed it every 6". came back next day and tiled. glad a spent the time prepping the hardieboard with thinset. besides being a decent decoupler, it gave me a smooth straight substrate to lay the mosaic, which saved me so much time and headache

By "hardieboard" I assume you mean HardieBacker.


Thinset underneath backer board is not optional, so I'm glad you did that.

Keep in mind that while Ditra and other products are uncoupling membranes (or decouplers as you say), cement boards such as HardieBacker are definitely not. There's nothing wrong with using it in your application, especially with a small mosaic, but it is the opposite of uncoupling, just FYI.
 

·
Registered
Joined
·
646 Posts
Why me? :confused1:
Hmmm….TorontoGuy, when did you make your initial post, approximately?

After I read your post of yesterday it seemed that you were telling us about quite a bit of work, more than could probably have been accomplished in a day or two. I looked at the dates of the posts and even though it appeared that huesmann’s was from yesterday, your initial post appeared to be from long ago (well, I’m almost positive that it did, but now I’m questioning my sanity). Today your initial post appears to be from just three days ago, 09-12-2020. So, when was it that you made that post?

Chris
 

·
Registered
Joined
·
646 Posts
Re-reading my latest post, I shouldn’t have used the word “yesterday”. I made my post Sept. 13 and it appeared then (and now) that huesmann made his first post earlier that same day.

Chris
 

·
Registered
my truck box
Joined
·
7 Posts
Discussion Starter #11
By "hardieboard" I assume you mean HardieBacker.


Thinset underneath backer board is not optional, so I'm glad you did that.

Keep in mind that while Ditra and other products are uncoupling membranes (or decouplers as you say), cement boards such as HardieBacker are definitely not. There's nothing wrong with using it in your application, especially with a small mosaic, but it is the opposite of uncoupling, just FYI.
hey jeff, since the hardie backer is thinsetted (and screwed) to a 1/4" poplar underlay, does any of this system act as uncoupling? the original (and very old) pine board subfloor is the bottom of this crazy sandwich.
and if there isnt any uncoupling, how does the system work to prevent cracking in the tile layer?

also, is there a popular modern decoupling method that works with small mosaic tiles?

thanks for the reply btw.
 

·
Registered
my truck box
Joined
·
7 Posts
Discussion Starter #12
PS, for the future, is there a decoupling membrane that is suited to small mosaic? perhaps a different brand?
what is everyones recommended method to laying mosaic on old wooden subfloor houses?
 

·
Registered
Joined
·
5,702 Posts
hey jeff, since the hardie backer is thinsetted (and screwed) to a 1/4" poplar underlay, does any of this system act as uncoupling?

Nope.




and if there isnt any uncoupling, how does the system work to prevent cracking in the tile layer?

It's a different approach. They used to use heavy, thick mortar beds set in metal lathe stapled to the subfloor. It was the opposite of uncoupling - make the floor sturdy, then make a massive installation on top of that that couldn't move. Then they introduced the cement boards. That's where people starting cutting corners - the system worked when done absolutely correctly, but it was cheaper and the lower skilled installers made more mistakes or simply didn't understand the physics involved.





also, is there a popular modern decoupling method that works with small mosaic tiles?
The only one I'm aware of is NobleSeal TS.



Now, when the others say they "dont' work", it doesn't mean that exactly. The only one I'm familiar with that doesn't warrant it is Schluter Ditra. The others might or might not warrant it, or recommend it. But they are basically just covering their ass. It might work fine, depending on just how big the mosaic is and how lucky you get.

The reason they don't recommend the mosaics is because of the adhesion method. Thinset does not stick to plastic, and the membranes are made of plastic. Normally thinset would stick to cement board or even bare plywood. But these uncoupling membranes are different. It doesn't form a chemical bond, it forms a mechanical bond. Sort of like a dovetail joint in wood. Those waffle squares are not really squares. They are dovetailed out at the bottom. So the thinset is chemically stuck to the tile, but on the floor side, it's only dovetailed in there which holds it in place. So really the tile is basically supported by a bunch of little pylons. The problem with a tiny mosaic is that each piece is not necessarily bonded to a waffle and sitting on a pylon. Some of them are only sitting on top of a thin layer of plastic with no real support. So if you look at the image below, you can see some little tiles might only be sitting on a little plastic pylon instead of a chunk of thinset, and that might or might not support it.



https://sccpublic.s3-external-1.amazonaws.com/sys-master/images/hc5/he3/8808915828766/Ditra_DitraXL-csd.jpg
 

·
Registered
my truck box
Joined
·
7 Posts
Discussion Starter #14
so nobleseal TS seams like a similar methodology to schluter kerdi?
would you use it on the floor as an uncoupling membrane?
 

·
Registered
Joined
·
5,702 Posts
so nobleseal TS seams like a similar methodology to schluter kerdi?

Yes, but significantly thicker. It is designed for use in showers as well, but I would not like to have to use it in a shower. Kerdi is 8 mil, NobleSeal TS is 30 mil.



would you use it on the floor as an uncoupling membrane?

Yes it is designed for that, whereas Kerdi isn't. They claim crack isolation, whereas Kerdi doesn't


https://noblecompany.com/products/nobleseal-ts/
 

·
Registered
my truck box
Joined
·
7 Posts
Discussion Starter #17
found this, which goes into detail about what you were saying Jeff.
thanks again for your help.
 
1 - 17 of 17 Posts
Top