Another pic to give reference

Something is shifting and making the seam pop.

Floor not rigid?
Cabinets not sturdy enough to support the weight?

The root problem is always the support under the counter, not the counter itself. Was this new construction that didn't get dried out good before the trimwork and cabinetry was done? Or a remodel where the joists weren't paid attention to? Something is shifting. And you need to find out why. And if it's normal. Or not.

I wouldn't assume that the floor joists are sufficient. They usually are merely sufficient to meet code minimums. Code minimums are actually pretty pitiful when it comes to being able to support anything real. Most will need to actually be reinforced to be stiff enough for tile or other hard surface floor coverings. You need to research the length, depth, and span of the joists underneath your kitchen. Enter the information into the Deflectometer (Google it) and you will probably find that they need sistering or cross bracing. Or both.

>> I'm assuming it is rigid enough.

Dangerous assumption. Our house had a fully permitted, inspected addition put on not long before your place was built and floor joists were both notched in order to sit on a ledger board, and overspanned even without alterations.

kmg67:

Place a level over the seam, front to back, left to right, and in an "X". It should not rock, nor should there be any dips. Use non-compressive (composite) shims if required.

The good news is that your seam is wide enough allow the removal of the failed adhesive. I'd slide a Japanese pull saw into the seam until it was all gone, then remake the seam with color matched methelmethacrylate and Gorilla Grips.

I'd bet your top is moving from dehumidification. The third time should be the charm.

Where do you live? It could be plate tectonics. ;-)

schicksal - floor joists look pretty standard to me...2x12's on 16" centers. They span about 16' from the foundation wall to the steel support beam. Kitchen is L shaped and sits on an outside corner of the house.

Trebruchet - I tried your suggestion with a 24" level and did not see any issues with rocking or dips. I will mention to the installers to use some composite shims next time.

I'm not worried about getting the failed adhesive removed. They were able to do that the last time. The seam looked great...until it failed a few weeks later.

I wanted rolled edges on my kitchen counter and the builder said it would have to be in two pieces. He said if/when the house settled it would break the seam. A regular counter top would come in one piece. I am glad I did as he suggested because now there is a crack where my bar has pulled away from the wall.

This post was edited by EmmaR on Wed, Nov 26, 14 at 9:05