While a rooftop garden isn't technically a "cool roof," it achieves many of the same benefits as qualified cool roof products and reduces stormwater runoff to boot.
A green roof, or rooftop garden, is a vegetative layer grown on a rooftop [source: Environmental Protection Agency]. Green roofs shade the buildings below from direct sunlight and reduce both surface temperatures and surrounding air temperatures through evaporation and transpiration, the process by which plants remove moisture from the soil and emit it through their leaves. In fact, rooftop gardens are so effective that the surface of a green rooftop can actually be cooler than the ambient air temperature [source: Environmental Protection Agency].
With installation costs ranging from $10 to $25 per square foot, green roofs are more expensive than other cool roof options, but it's hard to put a price on the improved quality of life they can provide for building occupants, particularly in urban settings where a rooftop garden may be the only patch of green as far as the eye can see.