I'll stick my neck out here since someone is bound to come along with a different perspective.

There are shallow root systems and then there are shallow root systems - could mean simply that although they'll put out an extensive and fibrous system, they might not travel down to tap the resources of the nearest aquafer. But 12-14" seems doomed; a topple over waiting to happen. Stakes wouldn't hold even in a light breeze at that depth. Perhaps an industrial sized planter/container would provide the depth for the roots to anchor themselves - but more tonnage for the roof to bear.

Any tree that CAN reach a height and width of 20' or more is not one I'd want to be standing under if planted in a foot of soil over a hard surface like a roof.

For planting a Cornus Kousa which you probably already know... dig Hole 1.5 times the diameter of the root system, but only as deep as the root system. (Keep in mind, the rootball of a large specimen might already be deeper than your available space.) Do not plant too deeply, meaning the top of the root crown should be above the soil line.

Most container grown plants of any size are going to have an existing root depth of 12". I'm trying to visualize the depth of the three gallon shrubs we sell at the garden center where I work...I want to say they are at least 14" deep. So you've got an issue right there.

OK---I did a quick google...found the linked PDF. From what I can see, one essentially makes a taller integrated planter for trees and shrubs, to accomodate their greater root depth. I don't have time to research further...but my google string was trees suitable intensive green roof list.

The linked PDF was interesting, and informative. It is for a specific company that sells green roofing systems, but it gave a pretty good overview of the different ways one can create a green roof.

With luck, someone with actual experience will post.
Have fun!

melanie

Here is a link that might be useful: Intensive green roofing system