Here’s a nice shot of three debris clumps in morning light.  The far clump is in its second year.  Lost island is in the middle and the greenery is starting to take hold.  Debris clumps are light, portable groups of plants that can go into dormancy.  They are light enough where they can occupy middle areas of the roof.  The roots don’t bond to anything so they can be easily moved.  Theoretically an entire roof could consist of these.  They would need some watering regime.  I’ll find out this summer if one of them puts a hole in my rubber roof.  A problem I see is that an elm seed starts to grow and that would mean tree roots.  Tree roots can go through concrete.  IMHO, green roofs should be inspected frequently.