Chicago is the green roof capital of the U.S. In support of the City’s broad interest in green roofs, the CDA has installed 445,590 square feet (over 10 acres!) of vegetated green roofs at O'Hare, including the first vegetated roof ever installed on an airport traffic control tower administration building. The sedum plant species used by the CDA are tolerant to drought and do not attract wildlife. The CDA has also installed 16,166 square feet of vegetated green roofs at Midway on two different buildings.
Vegetated roofs at airports are both practical and cost effective. Operational and maintenance cost savings result from increased roof life, decreased energy use through reduction in the airport’s overall heat island effect, and reduced stormwater runoff, as well as acoustical buffering and air quality protection.
By protecting the roofing membrane from exposure to ultraviolet rays, temperature extremes and precipitation, a vegetated roof prolongs the roof’s lifespan from 15-20 years to 40-50 years. Although a conventional PVC roof has a lower installation cost, it can cost around $9.00 per square foot for tear off and re-roofing every 15-20 years. At this rate, for example, the 174,442 square-foot FedEx Main Sort Building vegetated roof's increased longevity is anticipated to save the CDA approximately $1.5 million in avoided re-roofing costs by doubling the roof's life span.
Vegetated roofs improve thermal insulation and counteract the urban heat island effect, leading to reductions in heating and cooling demand and costs. The heat island effect causes overheating of cities and urban areas during the summer, leading to excessive energy consumption for air conditioning and pollution from electric power generation. Energy savings are estimated at around $0.20 per square foot each year. Since the vegetated roof was installed at the FedEx Main Sort Building, a 30 percent reduction in annual heating and cooling costs has been reported.
Vegetated roofs increase stormwater retention, filtration, and evaporation in a predominantly impervious urban area. It is estimated that vegetated roofs at the CDA's airports retain 70-90 percent of the precipitation that falls on them during the summer and 25-40 percent in the winter. For example, the vegetated roof at the FedEx Main Sort Building is estimated to retain approximately two million gallons of storm water annually.
The layers of a vegetated roof include soil, plants, and a trapped layer of air. These layers are effective at absorbing sound waves produced at an airport by construction equipment, vehicles, and aircraft. It is estimated that a four-inch vegetated roof substrate layer can reduce the level of sound penetrating the inside of a building by up to 40 decibels.
Roof foliage absorbs dust and greenhouse gases and filters airborne particles. It is estimated that one square meter of vegetated roof can remove up to four pounds of airborne particulate matter per year.
Millions of visitors observe O'Hare's vegetated roofs while traveling on the Airport Transit System, driving to the airport along Mount Prospect Road, or when flying overhead, making the CDA's commitment to the environment very visible. O'Hare, the second busiest airport in the U.S., was visited by nearly 67 million passengers in 2012. The sedum plant species used by the CDA have white or yellow flowers and green, light green, or maroon foliage that catch the eye of passing visitors yet are tolerant to drought and do not attract wildlife. The vegetated roof installations also support a broader city-wide interest in green roofs – Chicago is the green roof capital of the U.S.
There are some unique elements and regulatory factors in an airport environment that present challenges to expand vegetated roof space including wildlife attractants, protection of mission critical equipment, height limitations during construction, secure access restrictions and prevention of foreign object debris (FOD).
Damage to aircraft from FOD can be very costly. Therefore, the CDA and its tenants have designed green roofs in compliance with wind speed requirements to prevent the plants or bedding from blowing off the roof and onto aircraft movement areas. The CDA's vegetated roof plantings must also be a species unattractive to wildlife to minimize the potential for aircraft bird strikes. All the species are perennials naturalized to North America and can be found as ground cover in many states. The table below offers more information on these approved species.