From one side of our apartment we have a view on a roof of a shop in the same building. The view is not very enticing, as you may see from the photos. As there are some plants growing here and there, I was wondering if it would work (apart from gathering the garbage...) to simply seed some mixtures of seeds of flowers there? Do you think they would grow without any help from my side? If yes what mixture/which flowers you would recommend and when should I seed them (we are living in Villach, Austria)? I am not sure how much the roof can carry so I do not want to add any additional soil or walk there if not necessary.
How deep is the growing medium on the roof? What is the growing medium? It's important to note that green roofs in the US are engineered, with specific layers of different media to ensure proper drainage. I assume that that is also the case in the EU. The fact that you have plants already growing on the roof is very encouraging - it looks as if someone has already engineered the soil. ..
In the northern US, we use sedums and sempervivums on green roofs because they require far less water and can withstand drought. You could also look at "rock garden plants" like Talinum and Erodium (really, any fleshy-leaved plant) for variation on the roof. For example, Talinum calycinum (with the excellent common name of Jewels of Opar) flowers in late summer in Wisconsin. Erodium can be both a spring and summer bloomer, depending on species.