I have used sedum on 2 buildings now with great success.
You can get it in the the form of plugs, a blanket roll, or ready planted in interlocking modular trays with reservoirs in the bottom, to store excess water, which the plants can then use in a dry period.
The last is the most expensive system, but seems to work really well for me in the UK.
For sedum, the substrate should be very low in nutrients...it needs very little to survive, as it is a mountain plant that grows naturally on barren rocks and in crevasses. A mix of perlite and very little soil is best. I weed it once a year, and spray it with a hose pipe only after about 2 weeks of drought. Lovely flowers all summer.
As other viewers have said.... you need to check that the roof structure can bear the weight! I'm strong, but can barely lift a tray when it's throughly soaked.