That's stone ballast on top of the porch roof. The load bearing capacity of that area is less than the main structure, so there aren't any planting structures there. However, to offset the lifting/peeling action of wind, the membrane has an added layer of rock on top. Not necessary except when it is necessary. And we get a lot of wind so it is necessary. Hard to imagine the membrane could release after feeling the strength of the adhesive that holds it down, but there is a lot of worst-case-scenario prevention built into the house.
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