Asters, day lilies and tall grasses are among the new plantings on the 22,000-square-foot "green" roof installed on Highmark Blue Cross Blue Shield's Fifth Avenue Place office building, Downtown.

The environmentally sustainable roof project, unveiled yesterday and located three stories above street level on a roof terrace at the corner of Liberty and Fifth avenues, is expected to provide insulation that will lower the building's energy use by 12 percent and greatly reduce storm water runoff.

The region's largest health insurer decided to install the green roof, consisting of 180 tons of soil and 25,000 plants on top of a waterproof membrane, as part of its energy conservation program, said Phyllis Barber, Highmark sustainability coordinator. The soil was spread in varying depths from six- to 18-inch mounds to approximate a flowering meadow.

The roof, which is not accessible to the public or Highmark employees but can be viewed from higher floors of Fifth Avenue Place and surrounding office and residential buildings, replaces a 20-year-old roof and is expected to last a minimum of 40 years. It cost approximately $600,000, about twice as much as a conventional roof, but lasts more than twice as long, said Kristin Ash, a Highmark spokeswoman.

"At Highmark, we're committed to environmental initiatives because we know that creating a healthier environment also helps to create healthier communities and healthier people," Ms. Barber said. "We encourage other local businesses to consider green roofs in their building planning so that together we can make a measurable impact on the health of our rivers and our residents."

Storm water runoff carries pollutants and contaminants into the region's rivers and causes untreated sewage overflows on at least half of the days during the summer recreational season.

Planted roofs are designed to absorb most of the storm water rather than shedding it into storm drains.

Rebecca Flora, Green Building Alliance executive director, said Highmark should be commended for pursuing a project that demonstrates solutions to the regional problems of storm water runoff, energy conservation and reduction of urban "heat islands."