Denver’s status as one of the nation’s most intense urban heat islands has spurred a group of activists to propose a “green roofs” ballot initiative — a measure that could animate environmentalists while mobilizing developers to fight it.

But the people behind the initiative, which would require gardens atop most new buildings of at least 25,000 square feet, also were motivated by a more recent factor: the election of Donald Trump as president.

“I’m very passionate about climate change, and with our recent election, it’s time for our citizens to take the initiative and battle some of the climate changes we are experiencing,” said Madison Backens, a biology student at the University of Colorado Denver. She has taken on the role of primary sponsor.

“Because our government isn’t really supporting that right now,” she added, referring to Trump’s public skepticism toward the scientific consensus on human-caused climate change.

The group of activists skews young — Backens is 22, and campaign manager Brandon Rietheimer is 30 — but their effort is being taken seriously by city attorneys, an urban horticulture expert, and skeptical developers of apartments and office buildings.

Rules would be among most stringent in U.S.

If approved by voters in the Nov. 7 election, the new local building code requirements sought by the backers would be among the strongest in the United States. The measure would follow the lead of San Francisco, which in December adopted an ordinance that forces developers of certain buildings to incorporate solar panels or roof gardens.

But Denver’s measure isn’t assured of making the ballot yet. The Denver Elections Division has approved the initiative for petition-gathering, and now proponents have nearly six months to collect more than 4,700 valid signatures from registered Denver voters.

Backens said more than 30 people turned out for the Denver Green Roofs Initiative’s kickoff gathering Thursday night.

How green roofs can help

In large cities, heat-radiating roofs and pavement often elevate temperatures several degrees in the summer heat. A 2014 analysis by Climate Central, a scientific advocacy group, found that semi-arid Denver’s average daily summertime temperature during the previous 10 years was 4.9 degrees higher than in nearby rural areas; it recorded the third-largest “heat island” effect among U.S. cities, behind Las Vegas and Albuquerque.

Advocates of green roofs — also called “living roofs” — say the increased vegetation reduces that heat effect, results in less storm-water runoff and also helps fight air pollution.

Rietheimer says he looks to Toronto as a model, since it became the first North American city to approve a green-roof requirement in 2010. He also researched building standards for rooftop gardens and added proposed structural guidelines to the proposed initiative.

What the initiative says

The proposal sizes up a new building based on gross floor area (excluding parking) and sets an increasing percentage of available roof space that must be covered by “green roof” components.

The required coverage proportion ranges from 20 percent for buildings between 25,000 and 50,000 square feet to 60 percent for buildings of more than 200,000 square feet.

Industrial buildings would face different requirements, while multifamily residential buildings of four stories or less would be exempt entirely. Building owners also could opt to incorporate solar panels to fulfill part of the requirement. City officials would be able to make other exceptions or allow smaller roof gardens in certain cases.

“These standards in the bill will make sure that they are built properly and that they are maintained properly, so that the plants thrive,” Rietheimer said.

The green roof requirements also would be triggered for existing large buildings by roof replacements and additions that increase the size to 25,000 square feet or larger — elements that could prove controversial.

Developers see a burdensome mandate

Rocky Sundling, the incoming president of the Colorado Apartment Association, sees many drawbacks to the proposal — starting with the idea of the city issuing what he sees as a costly mandate that could be tricky for some projects to carry out.

Other American cities, including Chicago, Seattle and Portland, Ore., have tended to offer density bonuses or other incentives to encourage the incorporation of green roofs, rather than blanket requirements. Denver offers no building incentives for green roofs.

“We certainly agree that there are some advantages to having green roofs,” said Sundling, who also is a regional vice president of the National Apartment Association. “But mandating it seems counterintuitive. If it is a good solution, the market will make it happen.”

Jennifer Bousselot, the marketing and event coordinator for the Colorado Native Plant Society, teaches a course on green roof culture at Colorado State University. She called the proposed initiative “bold and exciting” and attended the city legal review of the measure to answer technical questions.

“I greatly admire the initiative because it is a positive step towards environmental stewardship for the city of Denver,” she wrote in an e-mail. “It would be a dream come true, as many of us in Colorado have been working towards this kind of policy breakthrough for 10 years.”

Rietheimer, who recently moved to Denver from Lakewood, said he and others first lobbied city planning officials and City Council members to pursue the ordinance legislatively.

But Denver updated its building code last year. The Department of Community Planning and Development was hesitant to pursue a green roof mandate.

Spokeswoman Andrea Burns said the department would prefer to give architects and engineers “the flexibility to design a roofing system that works best for their needs and their budgets” — while leaving it to builders to decide whether to incorporate green or solar components.