Green roofs are a very specialized area of gardening, starting with the construction requirements and making sure the structure can support the weight of wet planting mix. Since you are building from scratch, you have the opportunity to decide what kind of green roof you want to design for, and how deep to make the soil matrix. Far more goes into choosing plants than I can go into here, but I can certainly supply you with some resources to get you started.
Here is a very thorough fact sheet: 
Vegetated Roofs: Low-impact development fact sheet
There are many more resources listed at the end.
The two magazine articles I have attached have excellent information on plants for green roofs, and sources.
There are a couple of great books on plants for green roofs. 
Planting Green Roofs and Living Walls by Nigel Dunnett and Noel Kingsbury. 2003, Timber Press.
Green Roof Plants by Edmund C. Snodgrass; Lucie L. Snodgrass. 2006, Timber Press.
Hopefully this will get you off to a good start.

Thank you very kindly!!! I appreciate your helpful response.