Both Temple University and the
Philadelphia Water Department have displays of green roof technology
at this year's Philadelphia Flower Show. Landscape Architect Edgar
David directed a group of students working on Temple's extensive demonstration.
He says one of the biggest environmental problems in urban areas is
the number of impermeable surfaces such as roads, driveways, and rooftops,
which don't allow rainwater to slowly filter into the ground.
"By literally replacing the land that the building displaces,"
says David, "we can help mitigate a lot of those problems that
development causes." David says green roof materials can absorb
up to 40 percent of the water from a regular rain event, and then
slowly release it.
the Flower Show, Temple's exhibit features a small wetland area
created below one end of the roof, which can treat water with natural
process, and an infiltration trench on the other side, where water
can slowly filter into the ground.
David says preventing stormwater runoff and sewer overflows is
just one of the benefits of green roofs. They can also improve air
quality, provide natural habitat for wildlife, and even wind up
saving money in the long run, although they are more expensive to
install than traditional roofs.
"Basically it's the ultraviolet radiation of the sun that
breaks down conventional roof systems," says David. "And
with this vegetative roof layering, you're basically absorbing all
the sun's energy through that and protecting the underlying drainage
membrane. So it's not uncommon that a green roof will last decades
longer than a conventional roof system."
A green roof actually consists of a special growing medium, made
of organic and inorganic materials, but especially formulated for
roofing. It is not the same thing as soil. A proper system also
requires a strong structural base to support the weight of the roof.
But once in place, a green roof requires very little maintenance.
Regular rainwater is all that's needed to keep an established roof
growing medium can actually absorb up to 40 percent of its volume
in rainwater," says David, "so the smaller, more regular,
frequent events which contribute to the large volume of precipitation
will actually be held in the roof system. And it's only during some
of the larger storm events that you actually get some of the water
draining through and running off."
Because of the roof's ability to reduce stormwater overflow, the
Philadelphia Water Department has also taken an interest in the
technology. Water Department Spokesperson Ed Grusheski says runoff
can have dramatic effects on Philadelphia's drinking water, supplied
by the Schuylkill and Delaware Rivers.
"Some of the sewers go directly into a local stream,"
he says, "and 80 to 90 percent of the pollutants in the rivers
today are from stormwater runoff. So we're making great efforts
throughout the city to control that, and gardeners can be a great
help in that respect."
Grusheski says when water falls off a typical rooftop or paved
surface, it can rush along the ground, picking up pollutants as
it goes, before entering the sewer system or nearby streams. Green
roofs provide an attractive way to slow the water down.