Green Roofs Grow More than Grass
Communities and Building Owners Benefit from Green Roofs
By Alex Dunbar, Editorial Associate for The Green Economy
“Green roofs” are built to let grasses and low lying plants grow in a thin layer of soil on building rooftops. With growing use in the United States, along with proven technology, green roofs are becoming increasingly popular and beneficial to businesses and building owners.
Benefits include lowered energy costs, lowered long term maintenance costs, less storm water runoff, lower water management costs.
Environmental benefits
Many groups, like city governments and policy makers, are promoting green roofs because of the improvements they provide towards quality of life. According to the EPA, there are numerous positive benefits that can improve the surrounding community, and not just the building itself.
- Plants absorbs pollutants and improves air quality, which helps improve respiratory health.
- The vegetation keeps buildings cooler, helping to reduce “heat islands” in cities (dry, exposed urban surfaces, such as roofs and pavement can rise to temperatures 50–90°F hotter than the air).
- Rooftop plantings absorb rainfall which lessens the amount of stormwater runoff that goes into a city’s water management system or storm drains. The more water coming through the cities’ pipes, the more money cities spend on treatment and processing facilities.
Green roofs have been incorporated into many buildings as a way of demonstrating a structure’s reduced burden on the environment. But even beyond the environmental benefits, building owners and businesses can see financial incentives and other improvements, especially in urban areas.
“You’re using a biological system to address urban problems that we’re used to addressing with mechanical systems,” says Ed Snodgrass, owner of Maryland-based Emory Knoll Farms and Green Roof Plants. “I tell developers the green roof is performing a service for the building; and it can be done with a minimum amount of cost.”
Cost benefits
Benefits that lower costs associated with green roofs:
- Green roofs last approximately twice as long as a conventional roof.
- Absorbing heat and acting as a building insulator, green roofs reduce energy use and lower heating and cooling costs.
- Water absorption by vegetation reduces the wear on roof waterproofing, which reduces maintenance costs and frequency of repairs.
- On hot summer days the surface of a conventional rooftop can be up to 90°F above air temperature, compared to green roofs which can be cooler than the air temperature. This reduces building heating costs.
Although initially more expensive than a traditional asphalt or shingle roofs, green roofs offer major environmental and economic advantages, from slashing storm water runoff and energy costs, to cooling overheated cities and cleaning their air.
Case Study: US Postal Service
One example can be found in the heart of Manhattan: the US Postal Services’ Morgan Processing and Distribution Center. In 2009 the facility replaced its roof tar and rock roof into a 2.5-acre park for the 3,000 employees that work there, featuring benches, native grasses and trees. Built in 1933, and now a designated historical landmark, the green rooftop provides a retreat for employees, and serves significant benefits to owners. Although project costs totaled $4.4 million dollars, twice the cost of a traditional roof, it is expected to at least last twice as long, making lifetime costs comparable, even before accompanying benefits and savings.
- During the summer months, pollution in runoff emptying into city sewers are reduced by 75 percent, and reduced by 35 percent during the winter. Additionally, the amount of storm water itself going into the New York municipal water system has been reduced by 75–40 percent.
- The new roof has life expectancy of 50 years, where a conventional roof would need to be replaced in just 15 years, according to the installation company URS Corporation.
- More energy efficient than a traditional roof, and the project is projected to save the Postal Service $30,000 yearly on heating and cooling costs
- Without being watered, weeded, or fertilized, the vegetation has flourished through freezes, thaws, summer rooftop temperatures up to 150 degrees, weeks of drought, and torrential summer storms.
Case Study: Saint Polycarp Village
Saint Polycarp Village Apartments is a mixed-use affordable housing complex built in Somerville, MA. The roof features a typical extensive green roof with solar panels and solar thermal surrounding it. The plants cover 2000 square feet of roof space, relatively small compared to more extensive commercial green roof installations. Even with the building’s small green roof footprint, the complex was able to achieve LEED Silver rating, reflecting the aggressive goals of 50% energy savings, 40% water savings, and cooling and filtration of interior air to remove pollutants. Along with other energy savings features, this shows that green roof’s on smaller commercial buildings can be effective at reducing energy costs.
Case Study: Chicago City Hall
As part of an EPA study and initiative to combat the urban heat island effect and to improve urban air quality, Mayor Richard M. Daley and the City of Chicago built a green roof on the 38,800 square foot City hall roof in April 2000. It was completed in 2001 at a cost $2.5 million, funded by a settlement with ComEd electric utility. Serving as a demonstration project and test greenroof, it was monitored for plant survival as well as other environmental features. Though the technology was not as well developed at the time of its 2001 completion, Chicago City Hall’s green roof saves $5,000 a year on utility bills.
“No one else has looked at rooftop gardens to mitigate urban heat island effect, which is a process whereby highly urbanized areas with hard surfaces tend to be degrees hotter than green areas,” said Richard Price of the Virginia-based William McDonough & Partners, the architectural firm that designed the garden.
Looking Forward
The EPA is funding, detailed, full life-cycle analyses (which looks at the roof over its lifetime) to determine the net benefits of green roofs. A University of Michigan study compared the expected costs of conventional roofs with the cost of a 21,000-square-foot (about the size of a standard commercial building) green roof. The green roof would cost $464,000 to install versus $335,000 for a conventional roof in 2006 dollars. However, over its lifetime, the green roof would save about $200,000. Nearly two-thirds of these savings would come from reduced energy needs for the building with the green roof. This means that generally, it is more cost effective to invest in a green roof for business or office building.
Brad Rowe, a professor of horticulture with Michigan State University’s Green Roof Research Program explains “We tend not to look at the long term. The technology is proven. The economics are clear. It’s not just a feel good effort.”
A Yale report on the growth of green roofs in American cities notes:
- Chicago and Washington DC boast a relatively high, expanding percentage of buildings with green roofs.
- Portland city government is offering a five dollar per square foot of “eco roof” grant, with no size limit, as a financial incentive to building owners.
- Toronto is beginning to require a certain percentage of roofing to be covered by vegetation on larger buildings.
These programs and requirements in cities are being coupled with environmental and economic incentives which is encouraging the spread of green roofs. After the higher initial investment, the positives can outweigh negatives for a substantial portion of urban construction in the US, meaning that green roof construction is here to stay.
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