Recently I toured the Illinois Department of Agriculture's green roof on their administration building at the State Fairgrounds in Springfield. It is an impressive sea of green sedum growing atop a structure of concrete and steel.
Green roofs are becoming an important part of sustainable urban development. These living roofs provide many benefits, including reducing surface temperature, absorbing rainfall to reduce runoff, and even reducing indoor sound.
The Illinois Department of Agriculture's green roof was installed in 2010. Here are some statistics they provided about the roof during our tour.
- It is 28,800 square feet (2/3 of a football field) and covers about 20 percent of building's roof.
- It weighs 28 pound per square foot when fully wet
- It absorbs 480,000 gallons of rainwater annually (equal to 1 ½ football fields one foot deep with water)
- The roof contains 15 different species of sedum, a drought tolerant groundcover that grows 1/4 to six inches tall.
- Plants grow in four inches of soil in trays installed over a watertight membrane layer
This is the second green roof I've toured in recent years. The National Great Rivers Research & Education Center in Alton, IL is an impressive example of construction using green technologies in accordance with LEED (Leadership in Energy and Environmental Design) certification standards. In addition to its many other impressive features, the buildings green living roof collects and stores 100% of all storm water. This roof is comprised of 12 inches of growing media and native prairie grass.
Roof gardens are not for everyone or for every roof. According to the Smart Energy Design Assistance Center at the University of Illinois (www.sedac.org), the first question to ask when considering a green roof is if the roof can accommodate the additional weight. The roof may be designed to hold a heavy snow load, but not a water logged garden.
A factsheet by SEDAC indicates that although green roofs have many benefits, they have long investment payback periods. In other words, it takes a long time to recoup the additional costs of installing this type of roof.
My house will likely need a new roof in the next few years, but I doubt a green roof will work for us. Although I love the idea and energy efficiency it brings, it will not work on my sloped roof.