ANDOVER — The Merrimack Valley's first green roof will take root at Phillips Academy next year, and town officials here hope the idea blossoms.

A 2,000-square-foot roof covered with tiny shrubs will cap off the $30 million addition to the Addison Gallery of American Art that is set for completion in winter 2009.

"We're excited about this," said Andover town planner Paul Materazzo. "We've been trying to push this for a while now. Now if a developer is a little unsure about the system, we'll have something to point to."

Green roofs are used to soak up rain and prevent contaminated runoff. They absorb heat and cold — acting as insulation — making them more energy efficient. And they're also more attractive than a flat stretch of black roof.

Once the roof is complete, it will take a few years for the shrubs, called sedum, to thrive.

"It functions more like undeveloped land than a building," said Phillips' director of facilities Michael Williams. "We have buildings that will look down on the roof, so students will see plants instead of a flat expanse of roof."

This will be the first expansion of the Addison Gallery of American Art since it was opened in 1931. Plans call for adding about 13,000 square feet onto the 22,000-square-foot building. Work should begin in July and last for 18 months.

The school has raised about $20 million for its expansion — all through fundraising. Williams could not provide a price tag for the green roof.

"It's more expansive definitely, but there are energy savings long-term," he said.

These "living roofs" are already popular in Europe..Locally, they can be seen at Boston City Hall, the World Trade Center in South Boston, and the Ikea in Stoughton..

Ipswich was the first town north of Boston to get a green roof in 2006, on top the Whipple Annex housing project.

Williams doesn't know of any others.

"There's not a lot in the immediate neighborhood," he said. "A lot of educational institutions are doing this."

Phillips is using Centerbrook Architects and Planners from Connecticut to design their roof.

This might be their first green roof, but Phillips Academy has been working at making their campus green since the 1990s. They even installed an environmentally friendly dehumidification system in the Harrison ice rink last summer.

"We've been trying to make our buildings much more sustainable," Williams said. "We want our buildings as energy efficient and environmentally sensitive as possible."

Materazzo said once the roof goes in, he will bring town officials over for a little presentation, saying he envisions town buildings using these someday.

What is a green roof?

A green roof is made up of many layers, including plants, soil or growing material, a layer to catch roots, crushed stones for drainage, and a layer impermeable to roots all on top of a traditional rubber roof. Other layers control vapor and provide insulation.

Source: GreenRoofs.org