Green roofs are an old invention; traditionally
they were sod roofs laid on top of log roofs to insulate the houses. Today, at
their simplest they involve planting a garden on the roof of a building. They
often involve roofs that use other green technology, including solar panels and
reflective surfaces. Green roofs are often seen to be a “novelty,” used only for aesthetic value, but they
have increased in popularity. Green roofs are often used to restore greenery to
urban environments, influence the microclimate of the building, reduce air
pollution, and manage storm water runoff.
There are
two types of green roofs: extensive roofs have a media depth of less than six
inches, and intensive roofs, which are deeper, and can accommodate trees and
plants with deeper roof systems. Green roofs are made up of several layers to
allow for proper drainage, plant growth, and roof waterproofing.
Storm water management is a
relevant urban issue: when rain or snow fall runs off into city water systems, bringing
with it pollutants, including phosphorus, animal waste, automobile oil,
construction sediment, and pesticides, from around the city. This decreases the
quality of drinking water, causes soil erosion, and contaminates natural bodies
of water. The issue is seriously exacerbated wherever combined sewer systems exist. Ways
of managing storm water include directing the run off to soil, rather than
letting it sit on impervious concrete and roads. Drainage systems and cisterns
can also be built to manage water flow, and prevent pollution by collecting
water in a clean place. This water can be re-purposed for watering plants, or
even redirected into a building’s water main. Roof surfaces make up a large
part of the impervious cover. Conventional roofs can be sources of heavy metals
like lead that contribute to runoff pollution.
Currently, many areas in
Philadelphia experience flooding due to sewage overflow when there are heavy
rains. Building more sewers and pipes would be a very expensive and labor
intensive endeavor, and the city has directed funding to greener options
involving rain gardens, green roofs, and porous cement, which allows water to
dissipate into the earth underneath. The
plan, “Green City, Clean Waters” is an eight hundred million dollar investment
in developing green rainwater systems. Since the beginning of the program in
2006, the program has reduced runoff by more than 1.5 billion gallons annually. Another benefit of utilizing green roofs for collecting
storm water is that the water is filtered through natural processes, removing
all traces of chemicals and pollutants. Any materials suspended in the water
tend to stick to the soil, and are absorbed by the plants, leaving clean water
to be collected by the cistern or aquifer. Green roofs are a way of managing
water and filtering it naturally, even bringing it to potable standards. They are
particularly effective in neutralizing acid rain.
Many consider storm water runoff mitigation to
be the primary benefit of green roofs due to the prevalence of impervious
surfaces in urban and commercial areas and a failing storm water management
infrastructure. Case studies of greens roofs done by engineering
group Magnusson Klemencic Associates to quantify the costs of green roofs and
the amount of runoff they create. Optimal soil cover was found to be four
inches deep—a depth that would allow plant to take root, but also provide a
volume through which it is easy for water to evaporate through. The studied
roofs mitigated runoff from 65 to 94%, meaning that in best conditions, only 6%
of rainfall became runoff, and cost of adding a green roof to a building costs
about seven dollars per square foot. Another study shows that average
percentage of roof rainfall retention ranged from 48.7% (gravel) to 82.8%
(vegetated), highlighting that the significant element in retaining the storm
water was the goring medium (soil), rather than the plants.
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