HOW GREEN ROOFS WORK
Green roofs reduce the amount of run-off from storms, as well as the rate of run-off flow, and retain water.
Rainfall on the rooftop can be absorbed into a porous substrate and then taken up by the plants. The water is then stored in plant leaves and roots, and undergoes transpiration. Moisture on the leaves of plants evaporates back into the atmosphere.
As it is, roofs represent approximately 40-50% of the impermeable surfaces in an urban environment. By adding greenery to roofs, a greater quantity of water can be absorbed into the soil, rather than become run-off.