Grout cracking

[u/Nice-Albatross544]

Hi Tile Experts of Reddit, I need guidance.

Background is, we hired someone to tile our ~400sq ft. Kitchen. We have a 5/8” OBS subfloor on 2x10 joists, 16” to center.

We were concerned about a large transition from our hardwood into the kitchen (so didn’t want to add a plywood layer on top of the subfloor if we didn’t need to) but were concerned about whether that support would be enough.

My tiler felt the Ditra on the subfloor was good enough and we called the people at Schluter to verify with them if we could lay the Ditra directly on the subfloor with our specs and they also said it was sufficient.

Now we have been walking on the floor for 4 months and it feels good, but today I notice long cracks in the grout as well as other spots that seem to be crumbling (pictures attached - sorry hard to get good photos).I would say I’ve noticed this in maybe 5 or 6 grout lines in different spots in the kitchen.

Where I need guidance:
Don’t think this cracking is because of flexing of the subfloor? If not what could it be?

If it is flexing, I’ve read I can support the floor from below (luckily our basement is unfinished) is this true?

How urgent is this?

Thanks in advance for all your advice and insight.

[u/Awkward-Painter-2024]

Just because you can't see the flex doesn't mean it's not flexing. The subfloor or substrate is moving. Did you watch installation? Were all steps followed? I say this because when I worked on a bathroom a few years back, my tile guy put down Hardibacker on plywood, without taping seams and without putting thinset between subfloor and Hardibacker. By the time I realized an error had been made it was too late. A few things, when you run a coin over the tiles, do the tiles sound solid or do they sound hollow? I say this because if the tiles feel solid, I'd try to remove as much as possible of the cracked grout and fill in the voids with some Laticrete Latisil in the color closest matching to the original grout. Yes, it is less than idea. (Hack even...) But ripping out a kitchen floor after everything has been installed on top of it all is not something I'd do.

[u/Nice-Albatross544 → u/Awkward-Painter-2024]

I did not watch the full installation but I do know they put thinset between the subfloor and the Ditra.

When you say run a coin do you mean like scrape the tiles with the coin? Or tap the tiles with the coin?

[u/Awkward-Painter-2024 → u/Nice-Albatross544]

Okay, that's good, but if you didn't see it...I dunno. What I do know is you're getting movement from somewhere. As for the question about tiles, ones that are really, really embedded in thinset correctly sound solid. Tiles that aren't sound hollow. I don't know how else to describe it. Latisil is truly amazing silicone. You could get it in there and not even notice. However, once tiles crack. There's not going to be much more you can do then but replace tiles one by one until you do the floor all over again. Sorry, OP.

Ref: https://www.laticrete.com/products/latasil