I've seen green roofs on bus stops and favelas in Rio, Brazil, where their roofs have terrible infrastructure. From what I understood, they basically partnered with a local university to try to reduce the heat island effect in favelas, as most of them had no greenery at all

They came up with a cheap solution involving a plastic liner, similar (perhaps the same) to those used in ponds; a geotextile layer of bidim, which is a lightweight material; and plants that don't need soil to grow, like those found growing on rocks or trunk of trees. Which means it is super light and still provide the same benefits as a green roof with soil in it