By Melissa Garcia

DENVER (CBS4) – A group of activists is pushing for a new building code that would require rooftop gardens on large structures across the city and county of Denver.

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Specialized gardens known as green roofs are designed to improve urban air quality and cool temperatures. Similar initiatives are already in place in cities like San Francisco and Toronto.

“We need to act on climate change now,” said Brandon Rietheimer of the Denver Green Roof Initiative.

The group is collecting signatures to put a measure on the ballot that would create a new building code requiring buildings greater than 25,000 square feet to turn a portion of their roof into a green roof or a combination green roof and solar roof.

Research ranks Denver as the 11th worst city across the United States when it comes to ozone pollution. It is also ranked as the 3rd worst city in a heating trend known as the urban heat island effect.

“That’s when the city heats up from all the concrete, all the buildings,” Rietheimer told CBS4’s Melissa Garcia.

In addition to improving urban air quality and cooling down temperatures, green roofs help to manage storm water and to increase biodiversity.

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If voters pass the measure, it will affect existing buildings and new construction, although there are some exceptions including nonprofits, schools and structures that can’t support the weight.

The up-front cost to install green roofs is higher than a traditional roof but advocates argue that they last four to five times longer. They also end up saving money on hail damage repair and other costs in the long-run, according to Rietheimer.

“(They) reduce the building’s energy consumption too. 70-90 percent of heat energy is transferred through the roof,” he added.

So far, initiative volunteers have collected roughly 4,800 signatures. They’ll need to collect about 5,000 more in order to get the initiative on November’s ballot.

Volunteers are on-hand for community members interested in signing the petition at Sunken Gardens Park from 9:30am to 12noon every Saturday, or most weekday evenings and weekend days at Cheesman Park. Intiative progress and contacts members are on Facebook at facebook.com/denvergreenroof/.

The deadline for the group to turn in the required signatures to the city of Denver is Aug. 20.

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Melissa Garcia has been reporting for CBS4 News since March 2014. Find her bio here, follow her on Twitter @MelissaGarciaTV, or send your story idea to mkgarcia@cbs.com.