Earlier this month, Denver joined San Francisco as one of the first cities in the United States to mandate green roofs on new buildings. Beginning next year, 20 percent of rooftop space for newly constructed buildings must be covered by gardens or solar panels. The mandate applies to all new structures over 25,000 square feet, as well as roof replacements on existing buildings that meet the size threshold. Residential buildings with fewer than four stories are exempt.
“Green roofs improve the livability of cities by bringing nature and green space back into the urban environment,” says Jennifer Bousselot, a faculty member in the Department of Horticulture and Landscape Architecture at Colorado State University. “Green roofs function as ecosystems on top of buildings ... Their benefits are many, but are primarily related to managing and filtering stormwater, moderating urban temperatures, acting as filters to improve air quality, and providing habitat for pollinators and beauty for anyone that overlooks them."
Why Green Roofs?
One of the most pressing environmental issues for Denver is a phenomenon known as the urban heat island effect, which means that a city is much hotter than the surrounding rural areas. In 2014, Climate Central identified Denver as being the third worst urban heat island in the United States. Not only do hot urban environments have serious health effects for humans, they also require more energy to cool indoor spaces.
By absorbing heat and insulating buildings, green roofs help mitigate urban heat islands. “If we can cool a city to ambient temperatures, as in nearby rural areas, then we reduce this cycle of energy used to combat heat,” Bousselot says. “Plants act like evaporative coolers because they need to be cool to stay alive.”
A focus on green architecture is also important in Denver, and its surrounding communities, as one of the fastest growing metropolitan areas in the United States. “Denver needs to include more sustainability and resiliency into the historical rise in development happening in the city," says Andy Creath, who founded his company, Green Roofs of Colorado, in 2006. "Sustainability has taken a backseat in these developments and the Green Roof Initiative is a way for the city to show leadership in sustainability and resiliency."