An urban superhero

“Look, up in the sky! It’s a bird. It’s a plane … No, it’s a green roof!” 

With all the hype around the new “Man of Steel” blockbuster this summer, it is important to bring attention to the real superheroes bettering society, like green roofs. Locally, these superheroes are cooling our city, handling stormwater and beautifying otherwise blighted spaces.

What is a green roof?

A green roof is a roof of a building or a parking facility that is partially or completely covered with vegetation and a growing medium. Unlike soil, the growing medium is especially configured to remain lightweight while providing nutrition to plants and resistance to wind erosion.  Everything below is kept dry because planting is done on top of a waterproof membrane. The benefits of green roofs are plentiful: they are designed to absorb rainwater, provide insulation, extend a roof’s lifespan, cool surrounding areas, help mechanical equipment operate more efficiently and create a beautiful rooftop garden. 

There are two basic types of green roofs: extensive and intensive. Extensive green roofs include less than 6 inches of growing medium and are generally limited to groundcovers and other shallow rooted plants that require little maintenance. Extensive green roofs exist solely for their environmental and aesthetic benefits and are not designed to support foot traffic.

On the other hand, intensive green roofs require a half-foot or more of growing medium and have the ability to sustain shrubs, trees, walkways, patios and benches due to complex structural support, irrigation, drainage and root protection layers. There are many examples of intensive roofs in the Twin Cities such as the Walker Art Center’s parking garage roof, Coffman Memorial Union Bookstore and parking ramp at the University of Minnesota, and Cancer Survivors Park at Marquette Plaza.

A 21st Century Marvel

The Twin Cities’ titan of green roofs is found atop the massive Target Center in downtown Minneapolis. It is the fifth largest green roof in the world and is just over 2.5 acres at 113,000 square feet. In 2007, the City of Minneapolis commissioned a study of the Target Center with our firm to analyze sustainable building options, including feasibility studies for solar energy, wind turbines and a green roof.  There were various reasons at the time why solar energy and wind turbines were deemed inappropriate for the site, such as shadowing from local buildings and poor wind resources, but an extensive green roof was deemed to be a perfect solution, for several reasons. 

At the time, the Target Center generated a very large volume of stormwater that contributed to overwhelming the city’s stormwater infrastructure during major storm events.  The Target Center was in need of a technology that would help slow and process stormwater before it left the site.  

For a bit of context, back in 2005, we drafted a Stormwater Utility Fee Credit policy with Council Member Lisa Goodman that was adopted by the City Council. The credit is designed to incentivize properties with large stormwater volume runoff (i.e., those properties with large impervious surfaces like parking lots and roofs) to deal with stormwater onsite by investing in green infrastructure. Green infrastructure uses environmentally friendly techniques to manage stormwater onsite with approaches such as rain gardens, pervious pavers, catchment ponds, cisterns, and green roofs. In contrast to green infrastructure, grey infrastructure uses sewers to carry water offsite and downstream.  The problem with this is that water, known as the universal solvent, absorbs almost everything in its path and increases in volume and velocity as it accumulates in sewer systems. This produces large volumes of water that can overwhelm city infrastructure, with costly consequences —including contamination of our drinking water.  To incentivize investment in green infrastructure, the city now credits property owners on their stormwater bill for a quantity or quality reduction in volume or pollution loads.

Naturally the proposed Target Center green roof was a perfect candidate for the credit, which helped to mobilize support for the project.  Projected annual savings from the credit and strong support from city leaders encouraged Target Center’s eventual investment in the project, which was completed in 2009. A 100 percent credit on the Target Center’s stormwater bill saved $123,000 in its first year alone. The green roof also contributes to the City by limiting costly infrastructure downstream, like storm sewer upgrades and water purification.

Additionally, the Target Center’s old roof contributed to heating up the surrounding city.  Its dark membrane roof absorbed heat from the sun and radiated heat to surrounding areas, referred to as the urban heat island effect.  This also caused mechanical equipment located on the roof to operate less efficiently, making air conditioning in the summer unnecessarily more expensive.  The green roof is able to significantly lower the air temperature due to the cooling effects of evapotranspiration, the process where plants transfer water from the earth’s surface to the atmosphere.  These benefits have led to additional cooling savings for the Target Center and surrounding buildings, as green roofs are naturally cooler and can reduce air conditioning costs by an average of 10 percent. 

The Bottom Line

According to a City report produced in 2011, the incremental cost of the Target Center green roof was $2.7 million.  In addition to the stormwater utility fee credit of $123,000, a savings of $211,000 a year was reported due to delaying roof replacement.  Because green roofs protect the roof itself from ultraviolet radiation, they can extend the life of the roof be two to three times.  These benefits added up to a payback of 6.8 years, assuming a 4 percent annual escalator.  The green roof is expected to save the city about $6 million in operating costs over the next 20 years. 

Perhaps most important benefit?  The Target Center green roof is now beautiful.  Surrounding buildings that used to look out onto an unsightly roof, such as the Graves 601 Hotel, now have views of a roof top garden—enabling business owners to command higher rates and rents, contributing to the City’s tax base.  Moreover, the City of Minneapolis has also benefited from marketing itself internationally as a progressive green city, worthy of businesses interested in investing in green cities and living up to its new marketing slogan, “City by Nature.”  Final analysis: the Target Center green roof is an urban hero truly worthy of a superlative. 

Greening Your Bottom Line is a monthly feature brought to you by the managing principals of Sustology, Craig Wilson and Melissa (“Lissie”) Rappaport Schifman.  Founded in 2005, Sustology helps businesses and organizations green their bottom line.  For every sustainability-related topic—from renewable energy to water conservation to waste reduction to building materials—there are pros and cons, costs and benefits, and definitely a lot of confusion over this massive subject that affects us all.   This column hopes to help sort through the fluff so readers can make their own, well-informed decisions.