The grass is greener up on the roof

The Hennepin County Board of Commissioners has approved plans for a green roof demonstration project at City Hall.

The 5,800-square-foot green roof will top City Hall's elevated courtyard. Green roofs are designed to trap storm-water runoff and reduce the urban heat-island effect.

The Municipal Building Commission, which manages City Hall at 350 S. 5th St., secured $160,000 from private donors for the $468,000 green roof, including funding from the Xcel Energy Foundation, the Natural Resource Group Inc. (NRG) and the Mississippi Watershed Management Organization. The county and the city plan to split the rest of the project's cost since they jointly own the building.

The sponsors plan to use the City Hall green roof as a model for other projects in the Twin Cities.

The New Central Library, set to open in May at 300 Nicollet Mall, also features green roofs with low-growing succulents and bedrock bluff prairie plants.

Many Downtown condo developers have also proposed green rooftops for their projects. While green roofs are more expensive than traditional roofs, they typically last twice as long.