Shower Curb - Slate Tile

None, other than slate is not a good choice for a shower. No natural stone is, really. The slate is prone to delaminating, with fissures and voids that fill up with soap and crud. I would expect the rest of the shower to show some signs in the future.
I know that’s not helpful to you now, but there’s not much you can do about it at this point.

Can you post some pictures that we can look at to give you a more informed opinion ?

You say that is the only problem area? On the outside of the curb? If so, I would probably attempt to pop that off and patch it. Don’t tell anybody I suggested that, but if there doesn’t seem to be any damage or the waterproofing doesn’t appear to be compromised, that’s what I would do first. It may turn out that a more extensive job is called for, but maybe not.

Yeah that’s the only problem area. I generally don’t trust myself to mess with plumbing but if it’s on the outside of the door i could do that on the outside of the curb. Damnit what a pain!

Keep in mind that the slate may just crumble in your hand while working on this.
Some slates, when left submerged in water for a period of time will actually turn to mud.

Just by looking at that ankle breaking slope, I would say that nothing was done right in that shower build.

Just my 20+ yrs experienced opinion.

We used Wooten’s flooring for a similar situation last year when we had signs of water damage from a bad shower renovation the previous homeowners did.

I wonder if there is a sealant they could use on the slate after they do the fix to buy more time with it?

Have to wonder if the cleaner being used on it dissolved it? I’d be real careful what gets sprayed on that surface moving forward.