My overloaded frameless cabinets develped a sag in only one place: the 36+ inch wide cabinet over the fridge. That is where booze bottles go to die. I don't even know what some of the stuff is, eg. bitters?!?

There is a solid oak light rail running below the other cabinets. Maybe that is helping the others be stable?

The frameless cabinets in our kitchen are over 15 years old and hold a lot of heavy dinnerware in the upper ones. There is no sagging.

My frameless cabinets are 10 years old. Overloaded to be sure, but no sagging.

My frameless cabinets came with supports for the wide cabinets. They are hard plastic supports that you put into the back of the cabinet wall under the center of the shelf. My cabinets are loaded and the shelves do not sag. The holes were predrilled at the factory.

The only cabinet I've ever had that sagged was a framed, builders grade POS, it had a support bar across the bottom that actually broke. It was a corner cabinet with a lazy susan. My frameless cabinets are much better made, I'm not worried. Now, if you get builders grade frameless cabinets, that might be a different story.

Good luck, and get what YOU want.

Hmm, its taking a lot of effort to say something pg rated. If I talk about age, weight, stretchiness, uppers, sag, and bottoms, the temptation is irresistible!

Sag is mostly about the cabinet bottom being too thin. Some people have the same thing happen with particle board shelves and turn them over every once in a while.

If sag happens on a 3/4" shelf, it'll certainly happen on a 1/4" or 3/8" cabinet bottom. Adding edge supports only gets a cabinet so far before the reality of too many pasta jars, giant cans of olive oil and gallon jugs of cheap brandy make the cabinet succumb to gravity.

You need bitters to make a Manhattan.
Frameless can rack, but only if not correctly installed.
The bottom shelf of a frameless cabinet longer than 33" could sag under load. The bottom panel should probably be made from sturdier stuff than the sides.
Casey

Thanks, everybody, and LOL bmorepanic!

I believe the custom builder that I am currently considering uses high density particle board (or something like that) as a default, but will upgrade to plywood. Would plywood be less likely to sag?

It's all about quality. High quality frameless trumps poor quality framed. High quality particle board trumps poor quality plywood.

Make sure you're getting the best quality they'll give you, but inspect anything that's an upcharge to make sure that it's really worth the money.