From an article by Don Behm of the Milwaukee Journal Sentinel:
The roof of Milwaukee’s Central Library sprang to life, and went to work, in Friday’s steady rain.
Thousands of sedum, a ground-covering plant, and clumps of chive and ornamental grasses – all perennials – were planted Friday in a six-inch layer of small gravel and soil spread across 30,000 square feet – nearly seven-tenths of an acre – to create a green roof atop the historic building, said Taj Schoening, business operations manager for the Milwaukee Public Library.
Its job is to mimic nature. The living roof will collect and store thousands of gallons of rainwater during a downpour, rather than allowing the clean water to drain immediately to a street sewer, Schoening said.
Each gallon of fresh water kept out of the pipes decreases the risk of sewer overflows, according to Kevin Shafer, executive director of the Milwaukee Metropolitan Sewerage District.
Combined sanitary and storm sewers on Wisconsin Ave. in front of the library quickly fill with rain in a deluge and begin spilling into the district’s deep tunnels. Pipes draining street sewers into the tunnels are closed as the underground caverns fill, causing street sewers to overflow to local rivers and Lake Michigan.
Green roofs can become saturated after hours of heavy rainfall, and additional rain would slowly begin to drain to a street sewer. But that delay in draining to a sewer buys time for the district’s system of tunnels and sewage plants to treat earlier flows.
An added benefit for taxpayers is the durability of the library’s green roof, Schoening said.
“This will double the life expectancy of our flat roof,” she said. “We won’t have to do this again for 40 years.”