Restrictor turns existing urban flat roofs into “Passive Blue Roofs.” It is designed to be used with existing roof drains and will fit around existing drains building on a flat roof without modification. The vertical weirs feature variable flows based on the storm's rainfall intensity. It is made from 1/4-inch thick carbon steel with up to 90 percent recycled content. A black, non-toxic coating prevents corrosion and discourages nesting.