I have a retaining-wall bed with only 16" of soil over gravel. Roses do well but need a little extra irrigation, as water can't move up from below. If you have a rock layer below, that reduces internal drainage. I would say at least 16" above rock, preferably more, and at least 8" above grade to provide drainage for the feeder roots. Don't discard your clay but mix it with organic matter and other soil. The best soils include clay. I'd get a soil test or at least check the pH of existing soil before proceeding. Aim at a pH of 6-6.5. You may or may not need to add phosphate, depending on the soil test.

Don't dig or work clay soil when it's wet.

I try to go two feet high minimum. With an aging back, I'd go even higher if it's realistic. Also give yourself a decent width if the beds will be linear.

What's the shape of the beds? I prefer non-linear beds myself, altho it's purely personal taste. I've found that plants look outstanding propped up in front of a rock wall. That kind of look, somewhat informal, looks more natural if the wall isn't straight. I generally plant in mounds, retaining edges with stacked stone where necessary. Because I have about 18 inches of topsoil, my mounds tend to be about 18 to 24 inches high at the summit.

No advice on the bed depth, just wanted to second what Michael said about not working in the clay when it's wet. If you do, it ruins the soil texture for the year. Our clay is actually very good soil, with lots of nutrients. When the soil is dry enough to walk in without sinking, try picking some up. If it's like modeling clay, wait a little longer. When it breaks apart easily, it's time. I've found mixing in a LOT of aged horse manure does wonders. Even with raised beds, I'd still try to improve the soil to help with your drainage. It will also be eaiser for the roots to get down in the soil and give them some good fertilizer.

Berndoodle- Your beds sound amazing. Any pictures?

I think I have similar soil to yours; lots of clay, but no rocks. I've played with heights from 6" to 20" and I'd have to say about 8' to 10' is about as most you need. I try to amend with a lot of compost (mostly bags from the big lots).

Lav, I don't have any really decent pix. I can show you the effect in my messy, informal wild garden. I'm sorry that the shots of newly- planted gardens look flat, a bad shot + the effect of mulch. This site gently slopes from right to left. The river cobble in the foreground is a drainage swale right to left, planted with native carex. For good drainage during wet winters, the soil has been graded into gentle pillows that run parallel to the swale. You can see one mound where the bench sits. The soil is retained with a short wall about a foot high, but the mound height is 18 to 20 inches at the center. Beyond the bench is another, larger mound, with the end near the stepping stones retained with stacked stone. Across the stepping stones is another small retaining wall for another small mound.

Grown out, this garden looks very different. None of the grading is apparent any more. All the eye perceives is a bit of texture in the plantings.

Here is a link that might be useful: {{gwi:221100}}