Aside from rooftop gardens providing resistance to thermal radiation, rooftop gardens are also beneficial in reducing rain run off.
A roof garden can delay run off; reduce the rate and volume of run off. “As cities grow, permeable substrates are replaced by impervious structures such as buildings and paved roads.
Storm water run-off and combined sewage overflow events are now major problems for many cities in North America. A key solution is to reduce peak flow by delaying (e.g., control flow drain on roofs) or retaining run-off (e.g., rain detention basins).
Rooftop gardens can delay peak flow and retain the run-off for later use by the plants.”