extensive green roof

So I don't think that green roofs are all that hard to make. For an extensive green roof, you need:

1. decking material
2. rubber/PVC membrane
3. something such as PVC to stop the roots from penetrating layer #2
4. drainage layer, pumice,gravel, Mira-Drain fabric are all possibilities.
5. 2-6 inches of gravel/soil with those water-absorbing crystals mixed in. You want something that will keep its volume over time and of course allow the plants to grow and thrive. or at least survive.
6. shallow-rooted xeric plants like sedum.

Oh yeah, and probably hook it up for drip irrigation too. That could be under layer 5 or in it.

Basically, you get someone to put on a standard rubber membrane roof. Then tack on the other stuff, dump in some growing medium and plant. Easy as 1 2 3. ;-)