Green roofs on flat roofs - does that make sense?

I'd say it's not as simple as "makes sense or doesn't" - it really depends on how you want to use your roof and how much effort you're willing to put in! I thought about it for a long time before I actually did it myself. Sure, the advantages are pretty impressive: better thermal insulation in summer, fewer temperature fluctuations, rainwater gets buffered instead of rushing straight into the sewers, and visually it just looks nicer than a gray concrete box. I love having my herbs and succulents up there! But honestly, the structural integrity has to be right - not every flat roof can handle the weight, and if you later find out your supporting structure isn't sufficient, it gets expensive.

What surprised me: the maintenance is more work than I thought. You need proper drainage, the plants need watering during extreme drought periods (and I'm not doing that constantly by hand, no way!), and if you're not careful, unwanted visitors can move in. The initial investment isn't cheap either - installation with protective layer, drainage system, substrate - it all adds up! For me personally it was worth it because I like tinkering with plants anyway and didn't need my roof for other things. But if you just want to cut your heating costs by 10-15% and don't have much interest in gardening otherwise, it might not be worth the effort for you.