Rooftop detention/retention can be achieved through the use of vegetated roof covers or
“green roofs”. Green roofs consist of a waterproof membrane, root barrier, drainage system,
filter cloth, lightweight growing medium and vegetation. Green roofs increase the
time of concentration, provide stormwater retention and detention, decrease total runoff by 50-
90%, and delay runoff by as much as 4 hours (Perry, 2008). Other benefits include extended roof life and reduced heating and cooling costs. Vegetated roof covers can often be added to existing roofs without additional structural design (Perry, 2008). Costs can be very high, but
lifetime maintenance can be minimized through plant selection and proper installation.