How to tile over tile - should I use a membrane between tiles?

6 years ago

We're updating our basement bathroom. The existing tile is 8x8, good shape, laid over concrete subfloor. We are not taking up this tile - we've done that enough in this house :-) My question is: should I use a Schluter tile underlay membrane between the tile layers? I tried contacting Schluter but no answer yet - they don't have this question on their website that I can find, either. I just want to ensure the new tiles will never crack. There are a couple hairline cracks on the existing tile from the concrete shifting - we've lived in this house five years and they were there when we moved in. No new cracks since, but I to ensure new tiles don't crack. I've read lots about applying tile over tile using modified thinset, but nothing to be found about putting a membrane down, then the new tile. Anyone done this?

Featured Answer

MongoCT

6 years ago

TCNA has guidance...I think TR712.

Laticrete offers a spec sheet for direct tile-over-tile using their 254 Platinum thinset, as well as a few other thinset combinations.

I'd first clean the tile surface (TSP is what I favor), then when dry do a bond test to see how well the thinset adheres to the tile. If not well, then the tile can be abraded with a carborundum disc or equivalent, or a primer can be used to improve adhesion. A couple of primers I've used in the past are Laticrete's Prime-N-Bond and Mapei's ECO Prim Grip.

Tip of the Day: A primer is much easier than abrasion. lol

While you can tile over tile, I personally prefer a membrane. I like Nobel CIS for situations like this.

Clean the tile

Prime the tile

Adhere the CIS to the tile

Then tile over the CIS

Grout

Cocktails for all involved

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Comments (6)

Corinne
Original Author
6 years ago
I heard back from Schluter. Here's my question and their answer:

Question:
- Can the uncoupling membrane be installed directly over existing ceramic tile? This is a basement - the tile is installed over the concrete subfloor. Tile is in good shape - just ugly. We don't want to remove it. So if the membrane can be installed over top, what type of thinset should we use to bond the membrane to the existing tile?
Answer:

If the following requirements are met, we expect this type of installation to be successful. However, given the wide range of potential existing conditions, this application is not covered by a specific detail in the Schluter-DITRA Installation Handbook and is therefore not covered under warranty.

In principle, DITRA can be installed over existing tile. In general, the following minimum requirements must be met for a successful installation.
1. The substructure must be adequate, per the DITRA Installation Handbook (e.g., joist spacing, plywood thickness, etc).
2. The existing tile must be well bonded; any loose tile must be removed.
3. The tile must be cleaned before the DITRA installation to remove any bond breakers.
The DITRA can be installed in a similar fashion to what is called out in detail D-V-T in the DITRA Installation Handbook. (I looked this up, and it's referring to installing over vinyl.)

Option 1. Use a rapid-setting modified thin-set mortar suitable for bonding to existing tile to install the DITRA
Option 2. Skim coat the tile with a modified thin-set mortar or suitable cement-based embossing leveler, allow it to dry, and install the DITRA with unmodified thin-set mortar

For further details please see attached Ditra installation Handbook
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Corinne
Original Author
6 years ago
The only thing I'm not sure about now is what modified thinset mortar to use. I asked Schulter if Versabond Modified Thinset Mortar would work (which I already have) and they only replied that they recommend their own fast-set mortar. Of course :-)

Actual answer: Even though we do not warranty this type of application, we recommend using a premium rapid set for the installation of the Ditra membrane over tile. Schluter has a premium rapid set mortar called the FAST SET.
PRO
Creative Ceramic & Marble/ Bill Vincent
6 years ago
last modified: 6 years ago
I would not bother with using Ditra. If you need Ditra, you shouldn't be setting over this tile. All you should need is to scarify the existing tile (scratch up the face for a better bond) and then use a premium thinset (versabond is NOT a premium thinset). My personal choice would be one of two-- either Laticrete's 254, or Mapei's kerabond mixed with Keralastic liquid latex.
Corinne thanked Creative Ceramic & Marble/ Bill Vincent
Corinne
Original Author
6 years ago
Bill - we have considered tiling right over the tile without the Ditra, as it would certainly be easier. The existing tile is in good shape, solid, nothing loose. However, there are a few hairline cracks in three of the tiles (which were set right on the concrete subfloor), so I'm worried the new tiles would eventually crack too. That's why we were thinking Ditra, so if the floor shifts again, the new tiles won't crack.
PRO
Creative Ceramic & Marble/ Bill Vincent
6 years ago
If there are any cracks present then you shouldn't be going over the tile even WITH a membrane. No way to determine exactly what's causing those cracks without taking the tiles up.