Green Roofs are they worth it?
GS1200 Natural History of Illinois
September 29, 2008
Dr. Linda Vick
Green Roofs, Why Bother?
When we think of a roof we think of something that protects and covers the top of a building or structure. But now days, there is more possibilities for the use and function of a roof top. It is called a Green Roof. A green roof is creating a rooftop that is covered with plants, grasses or small shrubs, trees, and soil. Green roofs help save energy, retain rain water, and can add beauty to any building. There are other possibilities such as producing food on rooftops. This could be the next step in organic food production. The use of green roofs can improve urban environments by providing benefits that may last for years to come. Here in Chicago, Mayor Daley, after visiting Europe was inspired by all the green roofs. He had been investigating urban heat islands, which is the way temperatures in urban areas stay consistently higher than the surrounding rural areas. Several dozen elderly Chicago residents had died during a heat wave and Mayor Daley wanted to do something to prevent this from happening again. In collaboration with the U.S. Environmental Protection Agency and Mayor Daley, Chicago became one of five cities to begin the urban heat island pilot project. The EPA evaluated and chose Chicago because of the ozone problems and by how much the city would benefit from a reduction in heat islands, the availability of data, and how much the community and local government were willing to help the project (EPA, 2008)
According to The World Green Roof association, green roofs may provide such benefits as: - Protecting local watersheds through storm water management - Reducing the urban heat island effect
- Promoting biodiversity within built up areas
- Improving air quality by trapping and removing particulate matter - Increasing the local agricultural capacity of built up areas - Reducing the energy requirements of a building’s environmental systems - Making urban spaces more beautiful and inviting for humans that live in them - reduction in peak load (lower energy costs) for the building with the green roof - longer roof life
- considerable insulation from noise pollution
Part of having a green roof is just the beauty of it. A green roof can be aesthetically pleasing; adding color and variety to an otherwise black and gray view from above. Rooftops can become gardens that provide an environment of relaxation and leisure activities. Plants provide bio-physical benefits by absorbing greenhouse gases, retaining storm water, and creating fire retardant protection. According to the EPA, “Green roofs also filter pollution from rainwater. This is achieved by the root systems' bacteria and fungi, which utilize the natural filtering processes of bioremediation and phytoremediation [pic]. As a result, the non-point source pollutants, [pic]nitrogen and phosphorus, are broken down and detoxified. This beneficial process increases over time as rooftop plants and root systems mature, (EPA, 2008).” During the flooding that we had a few weeks ago, green roofs could have helped soak up some of the rain water by acting like a sponge, soaking up moisture as it falls. It may not have been enough, but every little bit helps. The EPA suggests that three to five inches of soil or growing medium absorbs 75% of rain events that are one-half inch or less (EPA, 2008). When rain falls a green roof captures and evaporates it, which reduces the amount of storm water runoff. This helps protect rivers and streams and prevents sewer overflowing. In areas like Chicago that is mainly concrete and asphalt rainwater has few places to soak into soil during storms, therefore flooding can occur. Green roofs would help give the rainwater a place to go. Here is a comparison from the EPA website that shows the difference in a green roof versus a traditional or common rooftop construction (EPA, 2008). [pic]
The costs to build a green roof can be double what it costs to build a traditional roof. The benefits can be great especially in an urban area like Chicago. The best time to build a green roof is when a building is being constructed. In this case it will double the life of the roof membrane and reduce energy costs for the building owner. The other time is when a roof needs to be repaired or replaced. The cost also depends on the type of green roof you will have, the climate and choice of plants and paving you use. There are two types of green roofs:
• Extensive: Lighter than intensive green roofs. Soil is 1-6 inches deep and weighs 15-50 pads per sq. foot. • Intensive: Heavier than extensive green roofs. Soil is 6-24 inches deep and can weigh 80-150 pads per sq. foot. It depends on the structure of the building, how much weight it can hold, how the existing roof membrane is, and whether the roof has adequate accessibility for maintenance and care (Great Lakes Water Institute, 2008).
Green roofs have to have an irrigation system. There is either a drip system which is permanently installed or a sprinkler and drainage system. Green roofs also need maintenance, but an extensive roof only needs maintenance the first 6-12 months as the plants are being established then only seasonally as necessary. Other costs include the growing medium, plants, and fertilizers.
Green roof planting requires considerations such as what the space is being used for, budget, maintenance costs, how long the green roof will last, location, exposure, humidity, dryness, climate, what type of medium will be used, and irrigation system. Green roof plants need to be tough and less nutrient-reliant compared to those found in ground gardens. It can take many months to construct a green roof because of the complex layers that have to be installed. These layers copy the conditions found in nature. As seen in the picture above, the layers have to be placed on top of each other starting with the insulation layer, moisture barrier to protect the building from leaks, next a drainage layer that would be made of gravel, clay or plastic. The drainage layer is what keeps the soil aerated and helps retain moisture. The top layer is the growing media and plants. Depending on the size of the plants used, plants may be planted in pots to establish size and health then planted in the media. (Snodgrass, E. & Snodgrass, L., 2006 pg. 43). The US Environmental Agency gives some examples of green roofs in the United States: • The Gap Headquarters in San Bruno, CA installed a 69,000 square foot extensive green roof in 1997. • Ford Motor Company has installed green roofs on its corporate headquarters. • The Church of Jesus Christ of Latter-Day Saints Conference Center in Salt Lake City, Utah installed a 348,480 square foot extensive and intensive green roof in 2000. • Contractors recently completed a 30,000 square foot extensive green roof project on the Montgomery Park Business Center in Baltimore, Maryland. • Ducks Unlimited, a nonprofit organization dedicated to the conservation of waterfowl, included two green roofs totaling 28,190 square feet on its national headquarters. • Private and public interests in the City of Chicago and the City of Portland have installed or are planning to install over 43 and 42 green roof projects, respectively. The urban heat island pilot project helped bring about the green roof over City Hall. The green roof was designed to test its cooling effects. Results show that the roof surface temperature has been 7 degrees cooler on average than surrounding rooftops. Summer temperature has been shown to have dropped 30 degrees cooler than surrounding building. Chicago offered 20 residents and small business owners the City of Chicago Green Roof Grant, of $5000 towards a green roof project. The True Nature Foods company took advantage of this project and converted a former automotive shop into a “Victory Garden”. They have taken it a step further and have made it a vegetated roof. The roof will serve an additional purpose of supplimenting their current local farms and vendors with their own vegetables and herbs grown overhead (Pilotin, ¶3). Chicago is one of the leading “green” cities in America and is paving the way by example. Besides City Hall, Chicago now has green roofs on Chicago Center for Green Technology, Millennium Park and the Peggy Notebaert Nature Museum, not to mention some 200 residents. Green roofs have many benefits but for Chicago the most important ones are the reduction of higher temperatures in the city, called urban heat islands, noise reduction, energy savings from reducing heat loss in the buildings, and reduction of air pollution.
U.S. Environmental Protection Agency, (2008, April 8). Urban Heat Island Pilot
Program. Retrieved September 28, 2008.
University of Wisconsin (2008) Great Lakes Water Institute Green Roof Project.
Retrieved September 27, 2008.
http://www.glwi.uwm.edu/research/genomics/ecoli/greenroof/index.php Snodgrass, E.C. & Snodgrass, L.L., (2006) Green Roof Plants: a resource and
planting guide. New York: Timber Press, Inc.
Pilotin, E. (2006, Aug 1). Inhabitat: Chicago green roof project. Retrieved
September 27, 2008. http://www.inhabitat.com/2006/08/01/chicago-