When School District of Lancaster pupils return to classes at three elementary schools next week, they may be wondering: Who's gonna water the roof?

That's because three of the schools — Lafayette, Wharton and Ross — installed "green" roofs this summer on new additions.

The vegetated roofs, which are designed to reduce rainwater runoff and conserve energy, are the first ever installed at public schools in Lancaster County, said Mary Gattis-Schell of the county planning commission.

Franklin & Marshall College, Lancaster Mennonite School and a few local businesses have installed the roofs in recent years, but SDL is the first public school system to try out the green technology on a large scale, she said.

The roofs were funded with a portion of a $479,000 "energy harvesting" grant the planning commission received from the state Department of Environmental Protection.

SDL received $118,710 for its two vegetated roofs at Lafayette and Wharton, each of which total 10,000 square feet. The district also received a $30,000 grant from the Lancaster Foundation for Educational Excellence for the 2,500-square-foot vegetated roof at Ross.

The grants offset the higher cost — about $7 per square foot, or a total of $157,500 at the three schools — of the roofs, said Greg Collins, SDL's coordinator of capital projects.

VIDEO: Greg Collins explains benefits of green roof

A vegetated roof also weighs more than a conventional roof and requires beefed-up structural supports to handle up to 50 pounds per square foot of extra weight when wet.

But green roofs can last up to twice as long as a conventional design, Gattis-Schell said, and they have multiple environmental benefits.

Their extra insulating properties cut heating and cooling costs and reduce the "heat island" effect, the buildup of heat from idling vehicles, pavement and the flat black roofs common to urban areas.

The vegetation also greatly reduces stormwater runoff and filters pollutants from the water that does end up in the sewage system.

A 2009 study by Millersville University found that a vegetative roof recently installed at Novelty Brush Co. in Lancaster reduced nitrates in rainwater runoff by 97 percent.

The green roof also reduced heat transfer by about 35 percent and is expected to cut energy costs by 25 percent.

SDL's green roofs will serve an educational purpose as well, Collins said. Science teachers plan to integrate them into the curriculum as environmental science learning laboratories.

What are the downsides?

The roofs require some initial weeding and will need to be watered during droughts lasting more than four weeks.

But that's about it for maintenance.

The roofs are planted with sedum, a hardy plant that, like a residential lawn, goes dormant in the winter and rebounds in the spring.

It also doesn't grow higher than a couple of inches, so there's no need for high-rise lawn mowing.

The roofs at Wharton and Ross include a layer of soil and crushed stone planted with sedum "plugs" grown by students at Lancaster County Career & Technology Center.

At Lafayette, workers installed the sedum in rows of 4-inch-deep plastic trays measuring 1 by 2 feet.

The trays allow the roots to poke out the sides but not the bottom, so they don't damage the roof membrane.

All three roofs are warranted against leaks for 20 years and equipped with electronic "vector mapping" leak-detection systems that pinpoint problems.

That's important, because before repairs could be made, the layer of sedum would need to be dug up or lifted out.

Collins hopes SDL's roofs inspire others in the city to use Mother Nature to reduce stormwater runoff, which overtaxes the city's sewage treatment capabilities by more than 1 billion gallons a year.

Much of that untreated runoff ends up in the Conestoga River.

SDL's decision to install the three roofs is "really remarkable," said Gattis-Schell, a senior environmental planner with the county agency.

With those roofs added to the mix, she said, Lancaster city now has almost one square foot of vegetated roof per resident — one of the highest per-capita rates in the country.

bwallace@lnpnews.com