The beauty of a rooftop garden is hard to resist. Building owners looking to transform unused rooftop space into functional, visually impactful areas can expect added property value and aesthetic appeal from a green roof. As a result, rooftop gardens are being incorporated into virtually every type of commercial structure.
From an environmental and building maintenance standpoint, green roofs serve many functions. They moderate and detain storm water runoff, supplement insulation to reduce heating and cooling costs, and increase urban biodiversity by adding habitats for birds and beneficial insects. Green roofs also mitigate the broader problem of urban heat island effect where high daytime ambient air temperatures remain elevated after sunset due to accumulated heat radiating from dark and non-vegetated roof tops. The protective layer of a green roof can also extend the life of the waterproofing system by shielding it from the degrading effects of sunlight.
A green roof with sedum and a drip irrigation
system ensures water efficiency at the University
of La Verne in California.
Eaton Vance, an investment management firm headquartered in Boston, Massachusetts, was interested in the social and aesthetic appeal of a green roof. When its 310,000 square-foot office building underwent a full retrofit, the company added an outdoor entertaining space. The landscaped roof deck with 200 flowering perennial plants, a 350 square-foot blanket of blooming sedum and numerous potted planters filled with Skyline Honey Locust trees affords employees and visitors colorful foliage in an urban high-rise environment.
Types of Green Roofs
- Intensive – accessible, park-like gardens with conventional trees and plants requiring irrigation and ongoing maintenance
- Extensive – involve a blanket of low maintenance plants like sedum, succulents, ornamental grasses or ground cover and are typically not accessible to the general public
- Semi-intensive – a combination of both intensive and extensive components
While the social and aesthetic benefits of green roofs can be obvious, many are not intended for the general public. Often the purpose is to meet sustainability goals such as reducing energy costs, improving air quality and conserving water.
To reduce urban heat island effect, energy consumption and stormwater runoff, the University of La Verne in California selected GreenGrid©, a modular green roof system manufactured by Weston Solutions for a 700 square-foot green roof at its new campus center near Los Angeles. Pre-grown trays with drought-tolerant sedum varieties were installed along with a drip irrigation system to provide maximum water-efficiency. Protecting the roofing membrane with a green roof has major long term cost benefits — studies show that the lifetime of the waterproofing membrane can be doubled.
If installed on a new building, a green roof should be viewed as a part of the whole roofing system. For some roofing system manufacturers, this integrated approach is a prerequisite to their warranty of the roofing installation. It is critical that the installation and long term growth and maintenance of the plantings do not compromise the waterproofing layer of the roof component.
Green Roof Benefits
- Offer new outdoor amenity space and aesthetic value
- Capture and direct roof top storm water runoff
- Supplement a building’s insulation, reducing heating and cooling costs
- Moderate ambient air temperature beyond the building to reduce urban heat island effect
- Improve air and water quality by filtering and reducing pollutants
- Increase wildlife habitats and biodiversity in urban settings
- Respond to market demand for more green space in urban areas
An example of this is the U.S. Census Bureau headquarters in Suitland, Maryland that includes 53,000 square feet of extensive green roof which was installed using American Hydrotech’s proprietary monolithic system. Adherence to the roofing manufacturer’s details and specifications was a prerequisite to American Hydrotech’s warranty of the green roof system. The roof areas are accessible only for maintenance purposes. This green roof detains a portion of every rain event as the primary source of water for the sedum plants. Excess water flows through the roof drain system and is collected along with other storm run-off into a retention pond which supplies the onsite irrigation system.
Building owners or managers considering a green roof should examine the site and the building as a whole to see where the environmental impact and life-cycle costs are located. Green Roofs for Healthy Cities (www.greenroofs.org) offers an online calculator to help compare roofing alternatives and determine whether energy savings and reduced operating, maintenance, repair and replacement costs justify the initial cost of a green roof. Be sure to consult an experienced landscape contractor or landscape architect who can help identify all of the variables that go into building a green roof.
About the Author
Kelly F. Duke is Vice President, Pre-Construction Services for ValleyCrest Landscape Companies. He oversees the design, installation and maintenance of many of the company’s green roof projects. He can be reached at kduke@valleycrest.com.
