Why did voters pass the Green Roof Initiative in Denver?

In November 2017, voters passed the Green Roof Initiative that requires large buildings to install green roofs or solar. It is important to understand the motivation behind this passing. Although, we do not know that exact reason everyone voted for this, we know the main ideas behind the messaging of the Green Roof campaign.  

Reduce Urban Heat Island and improve Air quality

The Urban Heat Island effect is when urban areas become hotter than the surrounding rural areas due to the large amount of impermeable surfaces, such as concrete parking lots and black rooftops.  According to the EPA, during the summer months pavement and roofs can be 50–90°F warmer than shaded or moist surfaces (1). It is estimated that anywhere from 5%-10% of energy used by a community in the summertime is compensation for the Urban Heat Island effect (2). Denver is the 3rd highest Urban Heat Island in the US.

More green spaces reduce the Urban Heat Island and the overall ambient temperature in our community. Increasing vegetation in urban areas can also improve storm water management and air quality. More green spaces in Denver could mean more vegetation to filter pollutants from the air. A study in Toronto found that 58 metric tones of air pollutants could be removed if all the roofs in the city were converted to green roofs (1). The level of reduction in pollution depends on the type of vegetation and roof design, with extensive green roofs having a lower impact than intensive ones.

Why is this important? First off, almost 50% of Denver residents do not have any kind of air conditioning unit or system in their home. As summers in Denver reach record high temperatures each year, so does the risk of heat-related illness and deaths. Large areas of Denver contain what are known as “vulnerable populations,” such as the elderly or those with pre-existing conditions that affect the bodies ability to regulate temperature, such as diabetes. Keeping our city cool protects those most vulnerable in our community while improving the quality of life for everyone else.

Storm water management

Urban development in major cities has created more surfaces laden with concrete and steel instead of soil and trees. This not only increases the Urban Heat Island but also causes stormwater issues. Green roofs reduce the amount of stormwater a building has to treat because the soil and vegetation soak it all up (and in many cases can reduce pollution runoff). So not only are you reducing the impact your building has on the natural environment, but you’re saving money in the long term on stormwater fees. Many cities even have programs that give a reduced rate to buildings with vegetative roofs or permeable surfaces like brick parking lots.

In many cities, developments often require a retention pond for stormwater runoff, requiring the purchase of additional land. With a green roof, such purchases can be reduced or mitigated entirely. For example, a 4″ extensive sedum roof could potentially capture 60-75% of annual rainfall. A deeper growing medium could retain even more water, reducing stress on sewage systems and instances of flooding. If the runoff from large storm events is delayed, our sewer systems and bodies of water do not overflow.

This was the main messaging that resulted in Denver voters passing this by 8.5%. There was also messaging around energy efficiency and reducing greenhouse gases and resonated with voters. These became important criteria in creating the revised Green Building Initiative, which we will discuss more next week.

Sources:

1.     CURRIE, B.A. & BASS, B. (2008) Estimates of air pollution mitigation with green plants and green roofs using the UFORE model. Urban Ecosystems 11: 409 - 422.

2.     YANG, J., YU, Q. & GONG, P. (2008) Quantifying air pollution removal by green roofs in Chicago. Atmospheric Environment 42: 7266 -7273.