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Unread 08-31-2021, 05:39 PM   #1
jeffnc
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Mosaic tile adherence on floors

I've had troubles on a few occasions when using the smaller (and notably, thinner) mosaics over Ditra (actually over a Kerdi shower pan once as well). Some tiles are just not sticking.

I'm talking about 2" square or hex mosaics. One problem I think is contributing is the mesh on the mosaic, which is a relatively large part of the surface area of such a small tile. Since that mesh is not stuck very hard to the tile (or once wet, maybe not at all), the tile gets less coverage. And of course we're going to the limit of what Ditra can do, because now we're just adhering to the waffle "pylons" really - not getting much stick to the plastic itself, which at this size is probably near 50% of the tile surface that isn't going to adhere well in the best of circumstances.

But the problem is exacerbated with some very thin tiles. Now your thinset layer has to be thinner, because if you press those tiles down, it's going to ooze up between every grout joint and become a major headache.

I once installed a penny size mosaic over a Noble sheet. It was also a thin tile and I pressed the tile in well to get good adherence. I spent the next couple days grinding out the excess thinset with a tiny diamond tip with a Dremel. Never again.

This last time, some tiles were literally coming up as I was vacuuming with my shop vac. Laying them individually I was able to get just the right amount of thinset on each tile and make sure it was wetly adhered, but what am I, an ancient Roman tile mosaic artist? I can't do that for a whole floor.

Normally I use a 1/4" trowel and then flatten it out carefully so the ridges don't ooze up the grout joints. Then I press the sheets down carefully with a grout float. This usually works well but not always.

Has anyone got any tips for this problem?
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Unread 08-31-2021, 09:31 PM   #2
Tool Guy - Kg
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Mosaics are challenging. Thinner mosaics are more challenging. Thinner mosaics over Ditra is most challenging.

Ditra isn’t as friendly with 2” mosaics as continuously flat substrates, so choose another substrate as needed.

Choose exactly the right sized trowel. Flattening a combed bed like you mentioned is a great technique. But if it’s still oozing through the joints, switch to a smaller trowel. I use V-notch trowels for mosaics most of the time. Use the absolute smallest possible trowel that will give you proper coverage.

If tiles aren’t sticking, pay additional attention to your prep or embedding technique. Maybe switch to a stickier mortar. Some mortars transfer to the mating surface easier than others and some need assistance. Nobody wants to burn mortar onto the backs of individual 2” tiles to get them to stick because it takes forever and it’s too easy to make a mess, but in some rare cases, you might have to do that.

Reject mosaics with ridiculously thick mesh backing that covers almost all the tile’s back side. Of course it’s easier said than done when a customer absolutely loves the tile and nothing else will suffice. But if the tiles aren’t going to stick...and your name is being attached to this job, why allow that pain to exist in your life?

Avoid extremely thin mosaics if possible.

Okay, all those tips are meant to avoid the ooze in the first place. But if you can’t avoid the ooze:
Use the oldest, most worn out wimpy sponge (for square mosaics), or a damp sash paintbrush (for either square, demonic penny rounds, hex, or whatever else) to gently clean out wet mortar from the joints before it dries. If you use a paintbrush, get one of those paintbrush cleaning spinners to help keep the brush from getting crusty. Charge appropriately to cover the additional time.
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Unread 08-31-2021, 10:58 PM   #3
jeffnc
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Thanks Tool Guy for the tips. Can you give me an example of a sticky mortar? That never occurred to me. And I think I might have to just go with a cementitious board for these next time.
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Unread 08-31-2021, 11:28 PM   #4
Tool Guy - Kg
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Look for ‘full contact’ in the name or description. It’s not a miracle mortar...but it transfers easily to the mating surface, hence the ‘sticky’ description.

Mapei Ultracontact mortar.
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Unread 08-31-2021, 11:48 PM   #5
jeffnc
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Great, thank you.
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Unread 09-01-2021, 06:07 AM   #6
MesaTileworks
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I’d say ditch the Ditra and go with a CBU fir your substrate. That seems like a big factor?
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Unread 09-01-2021, 07:20 AM   #7
jeffnc
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I'm really speculating, and it's just one of those things that is better to experience than explain, but I think it was a question of several issues combining together that really should all be addressed: the substrate, the amount of thinset, and the stickiness of the thinset are the variables that can be changed. The mesh backing and thickness of the tile really can't (assuming it's the tile the customer wants and not my own project.)
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Unread 09-15-2021, 09:58 PM   #8
MesaTileworks
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I know that Schluter specifies a minimum tile size. Can’t remember if it’s 2” or what, but I’ve done a couple of installs with 2” or smaller tile where we went with a CBU underlayment for exactly this reason. I checked in with Schluter and they were like, “yeah, that’s too small.”
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Unread 09-16-2021, 01:32 AM   #9
jeffnc
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Yeah, 2" is the minimum size. I installed a smaller mosaic early on before I knew any better (well, the mosaic were various sizes in the pattern, but the smallest one was only 1/2"). Ironically, I had no problem with that one. Ignorance is bliss I guess.
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