first of all, you have to ensure that the load-bearing design of the roof is human-bearing, and the thickness of the grass base also depends on the load-bearing of the roof.

I don't know whether you are in the south or the north. If you are in the north, you can use Poa pratensis. If you are beautiful, you can use Oxalis, and you can still bloom. There will be no odor, as long as you don't use pesticides. Water storage board+non-woven fabric+washed perlite particles or ceramsite+sterilized planting soil is enough for basic treatment. Because of the small area, watering can be directly connected with indoor drinking water. This kind of grass-roots treatment has very good water retention and permeability.

If it's in the south, Manila or Taiwan Province grass will be suitable. Sedum lineatum is easier to survive, but it doesn't look as neat as the first two.

personal opinions for your reference.