City of Maryland Heights // May 6, 2016
What is a Green Roof?
An aerial view of most urban areas shows swathes of
asphalt, black tar, and gravel-ballasted rooftops. Heat
radiates off of the dark roofs, and water rushes over the
hard, hopefully impermeable surfaces. Yet, there is a
new trend that breaks up the monotony of common
roofs: green rooftops. Long popular in Europe, green
rooftops have begun to appeal to homeowners,
businesses and even cities as an attractive way to
promote environmentalism while solving the problems
of conventional roofs. Green roofs supplement
traditional vegetation without disrupting urban
infrastructure -- they take a neglected space and make it
useful.
Green roofs last longer than conventional roofs, reduce energy costs with natural insulation, create peaceful retreats for
people and animals, and absorb storm water, potentially lessening the need for complex and expensive drainage systems.
On a wider scale, green roofs improve air quality and help reduce the Urban Heat Island Effect, a condition in which city
and suburban developments absorb and trap heat. Anyone who has walked across a scalding parking lot on a hot, summer
day has felt one effect of an Urban Heat Island.
The layers of a green roof must, like any roof, accommodate drainage and protect the building from the elements with a
waterproof membrane. But they also must create a growing area and potentially provide support, irrigation and root
protection barriers while staying as light as possible.
Two types of green roof exist: intensive and extensive. Intensive green roofs are essentially elevated parks. They can
sustain shrubs, trees, walkways and benches with their complex structural support, irrigation, drainage and root
protection layers. The foot or more of growing medium needed for an intensive green roof creates a load of 80-150 pounds
(36-68 kilograms) per square foot. Extensive green roofs are relatively light at 15-50 pounds (7-23 kilograms) per square
foot. They support hearty native ground cover that requires little maintenance. Extensive green roofs usually exist solely
for their environmental benefits and don't function as accessible rooftop gardens.
One of the most famous American green rooftops, Chicago's City Hall, combines extensive, intensive, and the intermediary
semi-intensive systems on one retrofitted roof. Under the Mayor's direction, the City of Chicago's Department of
Environment City Hall pilot program kicked off a citywide push to support green rooftop systems with incentives and
grants.
What Does a Green Roof Do?
The initial expense of green rooftops often turns away prospective clients.
Because green roofs require professional design, careful structural analysis
and multiple layers and systems, even extensive green roofs usually start at $8
per square foot, significantly more expensive than the $1.25 per square foot
for built-up roofs (BURs) [Source: EPA]. But benefits and incentives, like those
laid down by the City of Chicago, are prompting new green-rooftop projects.
And, as the American green-roof industry grows, prices will drop.
In the meantime, long-term economic benefits already outweigh the start-up
costs. Because green roofs protect the roof membrane from harsh weather
and ultraviolet (UV) radiation, they can last twice as long traditional roofs.
Green roofs also have a fairly stable surface temperature, remaining at air
temperature or cooler while traditional rooftops can soar up to 90º F (32º C)
above air temperature [Source: EPA]. The extra growing medium and
vegetation insulates the building from intense temperatures and minimizes
heat gain. According to a Canadian study, even a six-inch extensive green roof
can reduce summer energy demands by 75 percent [Source: Professional
Roofing].
CLASSIC GREEN
Green roofs and roof-top gardens
have existed for thousands of years.
The Hanging Gardens of Babylon,
one of the Seven Wonders of the
World, used an elaborate irrigation
system to create a terraced garden
paradise outside of modern-day
Baghdad. Northern Europeans once
chose traditional sod roofs as a
means of insulating houses. Today,
green roofs are prevalent, or even
required, in some parts of Europe. In
Germany, 14 percent of all roof area
is green.
[Source: EnviroZine]
These benefits are encouraging eco-minded homeowners, businesses, and cities to build green rooftops. Green roofs
mitigate water runoff and sewer overflows. Vegetation and soil act as a sponge, absorbing and filtering water that would
normally plunge down gutters, wash through polluted streets and over-tax sewer systems. A green roof's plants remove
air particulates, produce oxygen, and provide shade. They use heat energy during evapotranspiration, a natural process
that cools the air as water evaporates from plant leaves.
Evapotranspiration and the shading provided by plants help counter the Urban Heat Island Effect brought about by an
excess of reflective and impermeable surfaces in cities and suburbs. Because Urban Heat Islands increase temperatures in
urban and suburban areas, they amplify the demand for air conditioning and launch a cycle of energy consumption that
contributes to global warming. If green roofs become a common building initiative, cities can reduce the uncomfortable
effects of Urban Heat Islands.
Green roofs replace a hard infrastructure with one that's not only more efficient, but also beautiful and useful. Green
rooftops offer office workers a rooftop retreat and apartment residents a place to plant gardens or relax. Even nonaccessible green roofs create stunning aerial views for surrounding neighbors and provide wildlife with a secluded, safe
space.
Sarah Dowdey "What is a Green Roof?" 11 July 2007.
HowStuffWorks.com. http://science.howstuffworks.com/environmental/green-science/green-rooftop.htm 5 May 2016