Last week, Denver voters received their ballots for the November 7 municipal election. In addition to considering a $937 million bond issuance and a Denver Public Schools Board election that has garnered national attention, Denver voters will decide whether to mandate the construction of “green roofs” on large buildings throughout the city. The proposed ordinance would apply to all new construction and every “roof replacement” on buildings of 25,000 square feet or more beginning in January 2018.

As the name implies, a green roof is a space containing plants and soil on top of a human-made structure. Proponents cite evidence that green roofs reduce energy consumption, more efficiently manage storm water runoff, ameliorate the urban heat island effect—one recent study found that Denver’s is the third worst in the United States—and improve air quality. Modern green roof technology was developed in Germany in the 1970s, and many U.S. cities have launched green roof incentive programs, including Chicago (2006), Washington D.C. (2006), New York City (2008), and Portland, Oregon (2008). Last October, San Francisco became the first U.S. city to mandate the inclusion of green roofs in certain new construction projects.

The Denver initiative is modeled on a similar ordinance enacted in Toronto in 2009. If voters approve the ordinance, buildings with 25,000 square feet of gross floor area would need a green roof to cover 20% of available roof space. For buildings larger than 200,000 square feet, it would need cover at least 60%. In between, the ordinance provides a sliding scale. Green roof space may be occupied by solar panels, and the ordinance allows for cash-in-lieu fees of $25 per square foot for structures receiving a variance or exemption. The Community Planning and Development Department would evaluate applications for compliance with construction standards contained in the ordinance and issue permits.

Opposition to this initiative is gaining traction. The Editorial Board of the Denver Post took a position against the initiative back in March, and Mayor Michael Hancock, Denver Metro Chamber of Commerce, Downtown Denver Partnership, and several trade and professional associations have followed suit. Their primary concern is cost, which studies have estimated at $10 to $25 per square foot for initial construction, and around $1 per square foot for annual maintenance. Perhaps the most controversial aspect of the proposed ordinance is its application to every “roof replacement,” which goes beyond the mandate of both the San Francisco and Toronto ordinances. In a statement issued last week, Mayor Hancock said that the initiative, “goes too far, too fast and provides no flexibility or opportunity for carrots instead of sticks.”

Turnout in these off-year local elections has historically been low—between about 20% and 40% in Denver—which means that as few as 60,000 votes could decide the fate of this initiative.