Marty,
This system actually sits on top of the deck joists. The bottom of the system is actually visible from below the deck. The part that actually sits on the joists is finished aluminum that acts as the ceiling as well as support for the decking above. The product is structural so you don't need nearly as many joists. Since I can't put a url to link to, because I don't have 15 posts,

just search WahooDecks and the product is DryJoist. They have a diagram on the site. Looks to be a pretty neat system though I'm sure it is pricey.