Oh boy. My gut reaction is your shower curb has been made from wood and is swelling. I suspect you have a traditional shower design and the installer screwed a backer board into the shower curb or the holes drilled in the shower curb by the glass guys is letting water in. Then after using the shower it is slowly getting wet. This is causing the tile to heave and the grout to fail. What happens when you tap on the tile? Does it sound hollow or solid?

Resetting the floor tile can be tricky and not knowing how the floor was prepared can create issues during the repair. What style water proofing did your guys use? A picture of the drain might give us some clues as to what floor assembly you have.

Do you know how the shower curb was made?

Did you flood test your shower?

Did you soak test your tile?

With out a soak test we do not know if the glue holding the tile in place is water based. If it is I suspect that you will become and expert in time at resetting the small tile in place. Can you place any spare tile you have in some water and let that sit a day. What happens when you lift out the tile? is the back side sticky and white? Do the tiles fall off easy?

Thanks for the response John.

And...crap, I bet you're right about the curb. Looking at it from outside the shower, there's a similar crack running on the outside from one end of the curb to the other. The previous owner had all the remodeling done so unfortunately I don't have the details on how it was done, but it does sound hollow when I tap on it.

Should we stop using the shower until we get this fixed?

As for the floor tiles, again I don't know how they were installed but I've included a pic of the drain, and another of the back of the tile. When the tiles came out, they just popped right out. It was as if nothing was holding them in at that point other than the grout. No sticky residue on the back, just smooth and glossy. Six have come up so far.

Also, the tiles started coming up before I noticed the crack in the curb. Now I'm wondering if the two problems are related. Maybe the floor tiles were leaking as they loosened and that created the humidity that caused the wood in the curb to swell?

Back side of one of the tiles that came out.

I agree with John, this shower needs to be gutted to the studs and rebuilt. Sorry about that. This time use a surface applied membrane in place of the usual vinyl liner.

"...I suggest you contact a local professional tile setter and get their evaluation...." - Creative Tile Eastern CT

That is a step in the right direction. I would hire them for a day. Get them to remove the glass panel and the tile on top of the curb. Then we can check thigs out.

You might be able to repair the curb and save the shower. It will take some skill to do so but if the only penatrations are from the glass install it might be saveable. I would demo the curb carefully and inspect how the curb was built. If you find that it was covered with Denshield or CBU and screwed through the liner then the chances are that was done from the side as well.

If you find that the curb was done with diamond expand lath and mortar you might just save this after all.

The drain shows that it is a typicall shower build using a liner and then mortar bed. This means you have hopefully between an inch and quarter to two inchs of mortar beteween the tile and the liner. So you can chisel out a little of the thinset residue and the reset the tile with new glass tile thin-set. Laticrete makes a good one.

With any leak you need to worry about the extend of the damage. If you have access from below - open up the drywall and inspect for more issues.

In Australia they have very strict waterproofing codes and a bathroom like yours would see the entire shower floor waterproofed. This so in case of a leak water can not run away from the shower and do damage elsewhere.

I do not like penatrations in my shower curbs at all.

It's a single-story home on a slab foundation so there's nothing below. Regardless, it sounds like the next step is to have a pro come out and take a look. Thanks for all the comments thus far, it's very helpful!

or a foam curb

Foam works.....concrete or a "brick build" would also work.

Update: I had two tile professionals come out and look at this. The consensus was that while there's clearly some movement, it's not an urgent problem. As I mentioned our house is a single story on a concrete slab foundation so even if there is a leak, there's nothing below that it could damage. If there is moisture it may even be coming up from the slab rather than the shower. The shower pan is hot-mopped and that would go over the curb, so that also makes it less likely that a penetration from above would be allowing water through to the curb.

So for now I've caulked the cracks and we'll see how it goes from there.

good luck on it.....