DENVER (AP) -- Denver became the latest city to require rooftop gardens or solar panels on big new buildings, which backers say will keep the outdoor air cooler, make storm water easier to manage and reduce the amount of energy burned by air conditioners.
A ballot initiative mandating environment-friendly roofs in Denver had 54 percent approval in final but unofficial returns Thursday.
Denver joins San Francisco, New York, Paris, London and other global cities that require or encourage builders to put "green roofs" on large new buildings.
Rooftop gardens and solar panels absorb some of the sun's heat or put it to work generating electricity. Greenery absorbs rainwater and releases it more slowly, so storm sewer systems aren't overwhelmed, advocates say.
"The world-class cities are realizing that roof space is an asset for the city's residents and for the building owners, so they're either requiring, or incentivizing or both the use of that roof space," said Steven Peck, founder and president of Green Roofs for Healthy Cities in Toronto.
Denver's measure goes further than most, requiring many existing buildings to be retrofitted with green roofs whenever the old roof wears out and is replaced. Older buildings that can't support the heavier weight of a green roof can get an exemption.
Opponents worry the mandate will be costly and hurt nonprofits and affordable housing programs. They also argued the city should have tried incentives to encourage green roofs, not require them.
Advocates concede green roofs cost more, but they argue that they pay for themselves in about six years by keeping buildings cooler, resulting in lower utility bills, and they protect underlying roof materials from wear, so they last longer.
Proponents also say green roofs can help cool off urban "heat islands," which occur when dark, exposed city surfaces bake in the sun all day and release the heat into the air at night.
Peck and others claim a host of other benefits: Rooftop gardens are amenities for building residents and office workers, they raise the value of the property and they make buildings more attractive to tenants.
Opponents said it can cost up to $27 a square foot (0.1 square meter) more just to make a new building strong enough to support the weight of a green roof. That doesn't include the price of the soil and plants or the cost of watering the plants in a dry climate like Denver's, said Kathie Barstnar, co-chairwoman of Citizens for a Responsible Denver, a coalition that campaigned against the measure.
Barstnar also said the wording of the measure means owners of older buildings that are exempt from the green roof mandate must pay a fee every time they replace the old roof with a new conventional one.
Barstnar said her group has not decided what to do next but has not ruled out some type of legal challenge.
Brandon Rietheimer, who managed the Denver Green Roof Initiative campaign, acknowledged the wording is confusing. He said backers never intended to make owners of existing buildings pay for an exemption, and they will ask city officials to clarify that.
He defended use of a mandate, saying incentives are too slow.
"Incentives will only do a building here and there," he said.
The mandate will also create a more certain market for green roof builders, he said.
"They can really allocate funds and really bring that industry here to the city, which really drives cost down," he said.