Thank you. I see.

Nevertheless, will it be safe enough to stay in place for the near future?

The top cannot be supported by the waterfall ends only. It will sag and when it does, the waterfall miters will self-destruct. They have to; it's math.

Thank you. I was afraid that something like that would happen eventually, if left as is. However, the installation guys assure that it is safe. We somehow hesitate on demanding to redo all.

I found thin metal shims in Amazon and thought to make a tight contact between the marble and installed metal brackets (12'' long, 3/8 thick, 8'' in overhang portion of bartop).

Will this prevent sagging?

Alternatively, I could attach wooden corbels in the mid, though we did not want them initially.

There is nothing wrong with waterfall countertops per se, but they are a trend that many installers can’t execute. These need to be pulled and re cut. Hopefully you haven’t paid yet. Don’t pay a penny until it’s done.

" However, the installation guys assure that it is safe. We somehow hesitate on demanding to redo all."

OK, then he should have no issue with your posting the pictures of his work on social media. Ask him that. It is a crap install and it wiull fail. Asking for a 3 year warranty does not mean he will ever fix it when it fails, especially if he thinks it is a good now.

"I found thin metal shims in Amazon and thought to make a tight contact between the marble and installed metal brackets (12'' long, 3/8 thick, 8'' in overhang portion of bartop).

Will this prevent sagging?"

Maybe for a short time, but you just voided your warranty with him and give him the out he is looking for to repair it when (not if) it fails.

At a minimum, use thin black plastic shims on top of the metal brackets to ensure they support the counter top. They are easy to stack on top of each other. Lowes should have them.

100% redo. Skipping the waterfall would better fit their skill set.

This must be the 20th post this month, and the thousandth this year on this same topic of waterfall counters being installed badly.. When will this fad end so this forum can get back to discussing kitchen design?

I paid a 60%. On the internet, I did not find any 'standards' for the waterfalls. Is it more like a craft? Thus, some say that it is subjective to evaluation and complains.

As for now, (if we close gaps by trim...) everything would look quite good.

Would it be reasonable to request an extended warranty (in case bartop will indeed sag and miters will break) and may be some discount for quality issues (e.g., 10% of project cost) that would require my input? Meanwhile, I can do shims as a possible way to prevent sagging?

" As for now, (if we close gaps by trim...) everything would look quite good. "

Does not fix the problems.

" Would it be reasonable to request an extended warranty "

AGAIN, not if, but when. He thinks the work is good what makes you think he will fix it when it fails?

" Meanwhile, I can do shims as a possible way to prevent sagging? "

AND AGAIN, not fixing the issues and giving another out by your installing shims.

It's your countertop, your home and your dollars. Do what you want but that is in no way, shape or form , acceptable workmanship.

Once their check clears, you're never going to see these guys again, I don't care if the warranty is signed in the blood of their first born male child.

Just got off the phone with a guy in a $2,000,000.00 home with failing miter joints. No, I'm not coming to fix them. Take out the trash. Fortunately, he's sitting on a lot of the GC's money.

There are always acceptable variances in standards for all things remodeling. This is outside of what would be typically acceptable.
If the substrate was not level, they should have shimmed it or refused to install.
The gapping shouldn’t be consistent across the entire underside and the support brackets should be supporting.
I would ask for a manager to come out to look at it with you and review your concerns.