Green Roofs Reduce Storm Runoff

Last updated on April 28th, 2022 at 12:35 am

The boldest feature of a completely “green” building is a “green” roof. Milwaukee city officials are encouraging building owners to add green roofs, because the grass and plants absorb water, rather than conventional roofs, where stormwater immediately runs off and flows into the sewer system.


Green roofs help reduce the burden on Milwaukee’s sewage system, which has had some problems with overflows during heavy rainstorms.


“Green roofs, in a way, are an urban comparable to stormwater ponds in the suburbs,” said Robert Monnat, chief operating officer for Mandel Group. “It doesn’t make sense to use highly valuable urban land for stormwater ponds.”


Green roofs are not cheap. A conventional roof costs about $5 per square foot, while a green roof can cost $14 to $18 per square foot, according to Norm Ammermann, senior sales representative for Milwaukee-based FJA Christiansen Roofing Co. Inc. To create a green roof, a waterproof membrane must be installed under four inches of soil.


Perhaps the biggest benefit of green roof, for the owner of the building, is that green roofs tend to last twice as long as conventional roofs, Ammermann said. A conventional roof lasts about 15 to 20 years, while a green roof lasts about 30 to 40, he said.


Another benefit is that green roofing systems reduce the roof temperature. On a hot summer day, a typical black roof could be 140 to 160 degrees, while a green roof on the same day would be 95 to 100 degrees, Kenneth A. Pientka chief operating officer of Madison-based Planning Design Build Inc. said.


As a result, green roofs help reduce the air conditioning costs for a building.


“It amounts to about a 75 percent savings on your air conditioning costs in the summer,” Ammermann said.


Ted McNamara, vice president with the Milwaukee office of FJA Christiansen, said green roofs do not absorb as much summer heat, resulting in cooling equipment operating less frequently and more efficiently, both reducing energy usage and extending the lifespan of the cooling system.


“Your air conditioning equipment is not put through the stresses of the surging of the on and off cycles,” McNamara said. “And your building will maintain itself at a much more constant temperature.”


However, green roofs are heavier than conventional roofs, so the building structure must be able to support it. One square foot of a green roof weighs about 24 pounds, Ammermann said. Installing a green roof on an existing building may require structural improvements that add even more to the cost, he said.


“Not every building is a candidate for a green roof,” Ammermann said. “It does add a significant amount of weight.”


FJA Christiansen recently completed installation of its first green roof in the Milwaukee area at its own offices at 2101 W. Purdue St. The new roof, which required more support beams to be put in place to support the increased weight, was completed after about six days of work.


Many of the businesses that are putting green roofs on their buildings are doing so for aesthetic reasons, which provide less tangible benefits.


Mandel Group put a green roof on top of the parking garage for its Gaslight Lofts development in Milwaukee’s Historic Third Ward. The green roof provides a park-like grassy area, with some small plants, creating an appealing view for residents and a relaxing place that they can enjoy the outdoors. The back doors of several of the apartments open up right to the green roof, creating a large backyard feel.


“The environment at Gaslight is pretty cool,” said Monnat. “Some homes have direct access to the green roof. It basically amounts to an outdoor living room for these people. It’s phenomenal.”


It cost about $175,000 more to put in the green roof on the Gaslight Lofts parking garage, Monnat said, but it provides an added amenity that makes the apartments more valuable.


Mandel Group also will put a 17,000-square-foot green roof on top of the parking garage for the University Club Tower, the luxury condominium high rise that the company is building near the lakefront in downtown Milwaukee.


That roof will include a dog run area.


The University Club Tower parking garage green roof will cost an additional $600,000 to $650,000, said Phillip Aiello, senior development manager for Mandel Group.


Big Bend Development will incorporate green roofs in a condominium development the company will build at the former site of the Milwaukee Center for Independence at 1339 N. Milwaukee St. in downtown Milwaukee. The development will feature an eight-story, 40-unit building; two 13-story, 43-unit buildings; and eight three-story, side-by-side townhouses. Construction should begin in late spring and be completed in 2007.


“They’re little parks on top of structures,” said Randy Scoville, a partner with Big Bend Development. “It makes the site more appealing. Instead of having all paved surfaces that are kind of unattractive, you provide an amenity. It doesn’t come free. There is a cost to it, but it does provide an added amenity.”

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