The earth in my neck of the woods is also very much all clay. Our beds are 12" deep. I did not till beneath the beds and added only good soil in the bed. The drainage is fine and I suspect yours will be too.

I wouldn't add gravel. Have you thought about putting newspaper down before you put the bed down? I don't know if it is necessary with such a deep bed or not, maybe someone else knows. But that would keep any weeds from germinating from below. I did my beds a little differently since I had an existing garden. I still think you will be fine. I am sure that didn't help much, but I wanted to try alteast since I recognize you from the home decoration forum :)

Your drainage will be fine, gravity will suck any excess out of the bed for you. I am going to ditto the suggestion to line the bottom with newspaper or cardboard as a weed smothering trick.

Fill of 6" is often enough over fine soil with an open bottom, but over a wire mesh and clay, I'd go at least 12" and the full 16" would not be too much.

You guys are awesome.Thanks for the responses! I'm glad I don't have to add gravel. Newspaper is much cheaper! :)

I agree with everybody else here. With a good mix of soil even on top of clay (mine is too) you should have no drainage problems and therefore no need for gravel.