Brief concentrated rain storms can easily overwhelm stormwater entry points and cause severe damage and flooding.  These instantaneous heavy rainfalls can cause flooding in subway systems and underground structures.

In the same way that water is delayed from draining when adding water to a potted flower, a Green Roof acts like a massive sponge and provides a delay mechanism to relieve pressure on stormwater collection/entry points during flash rainfalls.  On very deep soil systems (20"+) a dry Sky Garden Green Roof can stop the outflow of flash rainfalls completely.