Craig, I have never designed or been involved in one until recently. And my involvement is as the owners rep on the project. The project is out for bids now.
I questioned the designers extensively. And I'm not totally sure they know what they are doing. But I'm told the following is critical or at least beneficial:
Use an engineered lightweight soil mix, with a high drainage value that precludes ponding. (Still seems odd to buy dirt but there are places that sell it for this purpose.)
Slope the deck for positive drainage.
You may need an irregation system since the soil mix will not hold moisture for days on end.
You may need to research vendors for a adequate waterproof membrane and protection system. My project is using a system by furnished by "Hydrotech USA" and includes a mambrane, root barrier protection, non-woven prefabricated crush proof air layer filter fabric, mositure retention mat, and a drainage/retention panel.
The system vendor's should have lots of information specific to your project.
