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DENVER — Voters in Denver will decide if new large buildings should be required to have rooftop gardens or solar panels.

Supporters of the green roof initiative said the proposed mandate is an effective way to reduce energy costs and air pollution.

Volunteers said they collected 45 more valid signatures than what was required to get the measure on the November ballot.

The proposal, modeled after a law that’s been on the books for the past decade in Toronto, aims to fight Denver’s “urban heat island effect.”

Cities such as Chicago, Washington, San Francisco and Seattle have taken similar requirements, according to green rooftop designer Andy Creath.

The ballot measure requires developers of most new buildings — 25,000 square feet or larger — to install rooftop gardens, solar panels or a combination of the two.

The force behind the effort started with one man who has no political background.

“I really just wanted to do something for the environment,” initiative founder Brandon Rietheimer said.

Rietheimer, a manager at a Red Robin restaurant, started by organizing volunteers. Rietheimer and the volunteers collected thousands of signatures.

NAIOP — Colorado’s Commercial Real Estate Development Association — argued an incentive program for developers would work better than a government mandate.

Association leaders also said Colorado’s dry climate would require increased water usage on rooftop gardens, and that could be an unintended consequence.

“We’re not too concerned about water usage,” Rietheimer said. “It doesn’t take a whole lot of water.”

If approved by voters, gardens or solar panels would be required to cover at least 20 percent of a rooftop.

It’s a requirement that will increase the initial cost of a building, but it’s something Rietheimer views as an investment.

“These green roofs last four times as long as a traditional roof,” he said.

The proposal, as is, doesn’t impact existing large buildings until those buildings need a roof replacement.