No reinforcement necessary. It'll be fine.

Run it by your fabricator before they come in with it. They’ll have the best answer.

I'm the fabricator! 😬

Fwiw I stood on the edges while painting and nothing buckeled or groaned.

Umm, well… ok. You’re braver than I. That’s a job I turned over to pros. I know you can order kits, but I was glad to finally find my soapstone locally. Good luck! It’s a really dense stone, so it’s really heavy. That said, I don’t think we had any more support than you do.

Take another look mama, please. Those are old-school 3/4" thick plywood cabinets. Much more stout than the bushes-glued-into-sheets composition of today's stuff.

Per Joseph's comments, these are old school, very sturdy and dense. Mostly concerned about sink and dishwasher, particularly where the seams will be

Happily the back plate is fastened to studs. Planning to run screws into studs and a couple 2x2 in a few places. I don't disagree that they are dated in many ways, but upon closer look I'm confident they are up to the task of holding my counters secure. Drawer glides, layout, etc are valid critiques for another day. Wish me luck!

I would not be standing on those. Nor would I put stone on them. They are not nearly as strong as some seem to think. There aren’t enough verticals in that run under the window, and that is where you get your support for anything topping them that creates compression forces. That is one reason that new cabinets, that are individual boxes, will support far greater weight. More vertical cabinet sides support more weight, without deflecting and collapsing. The faceframe in those will keep them square, but isn’t doing much at all to resist the compression forces of weight.

You know, they do have a point. I’m no engineer, but they do look to be lacking the usual mass of modern cabinets. I understand budget constraints, but I also question putting nice stone on those. I had better cabinets, but still waited until I could get new ones before installing my soapstone. At minimum, I think it would be prudent to add more cross support. Better if you could replace - even if it’s box stuff from Lowe’s.

I already gave my professional opinion in your other thread, where you wanted to just put stone on top of the existing laminate counters. Do not do this. But what do I know, I’ve only been building furniture and cabinets for 31 years. I obviously have a lot to learn from a sink repair guy.

Thanks everyone for you advice. We did reinforce a few places. Final results are SOLID and beautiful. Not easy but no miscuts or mistakes. Very happy with results!

"I obviously have a lot to learn from a sink repair guy."

Apparently:

No legs either.