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Q: What is Concrete’s Role in Constructing
Green Roofs?
By Steve Skinner, GRP, Technical Service Manager, Greenscapes*
Green roofs are environmentally friendly coverings for buildings
that help minimize many negative aspects of development. First and
foremost, they help to manage storm water. Additionally, they mitigate
rising temperatures of urban areas and maintain native habitat and
green space. They can help save energy, too.
Green roofs have become increasingly common design elements that
can be found from coast to coast in all climates. While green roofs
are suited to all sizes of buildings, they are more likely to be
found in urban settings where property is at a premium. They also
require a bit of investment in the property. As such, they may currently
be more common on larger and more robust buildings. Concrete structures
are particularly suited to use with green roofs and the added weight
that accompanies them.
Structural Roof Deck
 Clearly,
green roofs are beneficial in many ways. But they require an expert
designer familiar with all aspects of proper construction. The structural
deck must be designed to support the weight of the green roof in
addition to other dead and live loads given in building codes. Acceptable
deck types for green roofs include cast-in-place concrete, precast
concrete, metal deck (with cover board in some cases) and plywood.
But several key considerations for selecting the proper structural
assembly are clearly concrete-friendly:
- Establishing or enhancing structural load requirements to accommodate
the green roof assembly and other dead and live loads
- Protection against wind uplift
- Considerations for fire prevention
- Safety during construction and long term maintenance
Benefits of Green Roofs
As cities grow, natural space is covered by buildings and pavements.
With each impervious surface added, summer temperatures in cities
rise and precipitation can no longer infiltrate the soil, but instead
further strains drainage infrastructure. The loss of green space
reduces the diversity of plants and animals within urban areas and
further isolates human inhabitants from the natural environment.
Green roofs offer solutions to these problems while providing many
other benefits, ultimately improving our standard of living.
•Storm Water Management: The capacity of green
roofs to absorb rain water is the most commonly cited reason for
their rapid rise in popularity. In many metropolitan areas, government
entities have become interested in promoting and supporting the
development of green roofs to reduce drainage system overloads
and flooding.
•Mitigation of the Urban Heat Island Effect: As
vast areas of manmade surfaces heat up many cities experience
summer high temperatures that can be as much as 10 degrees Fahrenheit
above surrounding undeveloped areas. As a result of higher temperatures,
more energy is needed to cool buildings, increased smog formation
results in poorer air quality, and the general livability of cities
is reduced.
•Energy Savings: It is difficult to attribute
a specific value that green roofs can provide in the way of energy
savings because all buildings are different. Nonetheless, it is
clear that green roofs provide some energy savings, with specific
buildings reporting from 10% to 25% annual savings after being
retrofitted with a green roof.
•Reduced Noise Levels: Sound attenuation inside
a building fitted with even a very shallow extensive green roof
can mean the difference between being distracted by the noise
from an overhead jet airplane, nearby busy highway or neighboring
loud factory, or merely perceiving the sound as background noise.
•Habitat for Plants and Animals: Green roofs can
provide the vital habitat that encourages birds, butterflies and
other beneficial insects to remain within an urban area or to
provide a way station between natural areas divided by an otherwise
inhospitable urban environment.
LEED and Green Roofs
The US Green Building Council’s Leadership in Energy and Environmental
Design® program recognizes the contribution that green roofs
can make in sustainable building practices. Green roofs can directly
contribute up to 4 points and make a contribution for an additional
12 points toward achieving LEED certification. These points include:
- Up to 2 points for Storm Water Management
- Up to 2 points for Water Efficient Landscaping
- Partial contribution toward 12 points including Recycled Content,
Reduced Site Disturbance, Optimized Energy Performance, and many
others.
Owner Incentives Many of the benefits of green
roofs can be viewed as being for the greater good. Green roofs are
good for the environment and display the owner’s responsibility
and environmental leadership. And even though they have a cost,
they provide more than just feel-good benefits, they also can save
owners on operating costs and reduce stress on infrastructure.
A green roof protects the waterproofing membrane from climate extremes
and physical abuse thereby greatly increasing the life expectancy
of the roof. Converting or designing a normally unused roof area
as a green roof can add a great amenity to a building. Many municipalities,
regional authorities, and utilities offer incentives for buildings
that include a green roof because it’s good for the environment
and for infrastructure. Numerous studies have documented the health
benefits of access to nature for improving the spirit of healthy
people and hospital patients alike.
Types of Green Roofs
There are primarily two distinct types of green roofs, extensive
and intensive, based on the type of plants grown and the level of
maintenance they require. Between these is a third type, semi-intensive,
considered a hybrid type of extensive and intensive.
Extensive green roofs, or as some term them, eco-roofs, are generally
designed to support a select range of highly drought tolerant plants,
that while attractive, are most highly valued for their self-sustaining
qualities. These roofs are not intended for recreational use but
rather as lightweight, relatively inexpensive systems that maximize
the cost/benefit ratio for realizing the many benefits of green
roofs. These systems can weigh as little as 15 lbs/sq. ft. but generally
fall into the range of 18-34 lbs/sq. ft. making them a viable choice
for almost any roofing application.
 Intensive
green roofs are the true rooftop gardens with weights that range
from 30 lbs/sq. ft to whatever the structure will bear. Concrete
decks and structural systems are obviously a material of choice
for intensive green roofs. Intensive green roofs can be as formal
as any garden on the ground with hardscape features such as ponds,
fountains, and gazebos, or as wild as any prairie, meadow or forest
floor. Intensive green roofs are generally designed as recreational
spaces and add a dramatic amenity in urban environments.
No matter what type of deck is used, a green roof also requires
the following elements:
 •
Roofing Membrane: Due to the nearly always moist conditions
present on a green roof, only a high quality, watertight membrane
should be used. Many roofing experts agree that the best waterproofing
technology for a green roof is to utilize a protected membrane
roof (PMR) where the insulation is on top of the membrane.
• Root Barrier/Physical Protection Course: Most
waterproofing membranes must be protected from physical abuse
and puncture pressure of plant roots. These protective courses
can take many forms including combined components that address
both concerns with one product or separate components where the
two functions are independent. Most protective courses consist
of tough materials that can resist abrasion, penetration and general
wear and tear.
• Insulation: This provides the primary energy
efficiency to the building structure. Green roofs provide additional
energy savings but the often moist condition of the growing media
means that traditional thermal resistance, or R-value is not reliable.
• Drainage Course: This component performs the
critical task of draining excess water to allow the growing media
above to remain well aerated. A common approach is to combine
a panel for drainage, water retention, and aeration over the insulation.
This method addresses not only the drainage of excess water but
also provides a much more complete solution to water management
of a green roof.
• System Filter: This component’s primary
function is to retain growing media fines from being washed out
and prevent the fines from clogging the roof drains and conduits.
 •
Engineered Growing Media: From both a plant growth and
drainage perspective this component is the most critical and least
understood aspect of green roofs. A well designed media will greatly
enhance the success of a green roof and a poorly designed one
will almost certainly result in failure. Most manufacturers utilize
lightweight manmade or natural aggregate as the structural component
for their growing media. The media is usually 50% or less in weight
than natural soil. It is designed to optimize the tradeoff of
water retention and drainage, to have chemical and biological
properties that are conducive to plant growth and to maintain
its structure over the several decades of expected life of the
roof.
• Plants: Compatible plants should be drought
and wind resistant, adaptable to shallow rooting conditions, be
non-invasive yet self propagating, tolerant or adaptable to the
sunlight conditions present, long lived and be generally disease
and pest resistant. Plants for extensive green roofs should also
be quickly spreading types such as sedums to ensure good coverage.
More on green roofs at ConcreteThinker.com
Meet the Expert
 *Steve
Skinner is the technical services manager, GreenScapes, for Colloid
Environmental Technologies Company (CETCO)
in Hoffman Estates, Ill. Steve joined the GreenScapes program in
2010, bringing 7 years of experience with green roofs and 20 years
of experience as a soil scientist and environmental consultant for
sustainable development in the U.S. and throughout the world. He
assists designers with technical aspects of green roofing, including
component and plant selection and drainage and storm water issues.
Steve has promoted the green roof industry through participation
in ASTM’s Committee
on Green Roof Standards, the USGBC,
Green Roofs
for Healthy Cities, American
Society of Landscape Architects, the International
Green Roof Association, and the Earth
Pledge Foundation.
steve.skinner@cetco.com
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