Green roofs: the unexpected benefits

June 12, 2006
Green roofs are quietly cropping up across the nation. Find out the expected and unexpected benefits of putting a lawn on your roof through the eyes of successful implementers like the Ford Dearborn Assembly Plant.

If your plant has stormwater issues, is on a site with a lot of contamination, or if you’re looking for a way to reduce energy costs, a green roof might be the way to go. Green roofs have been quietly cropping up everywhere and they bring with them plenty of expected and unexpected benefits.

The Ford Dearborn Assembly Plant, Dearborn, Mich. has the world’s largest living roof, according to the Guinness Book of World Records. The roof is 454,000 sq. ft. (10.4 acres) says Donald Russell, with Ford’s Environmental Quality Office.

“You can’t even drive a golf ball from end of the roof to the other,” says Steven Peck, founder and president of Green Roofs for Healthy Cities (GRHC).

Ford employs 1,000 people and manufactures F-150 trucks at the site, says Russell. The roof was originally built in 2003 to coincide with the company’s 100th anniversary and hasn’t stopped reaping benefits since.

“As part of an overall site rehab, our main concern was to minimize stormwater discharge from the site,” Russell says. “If we had to build a treatment plant, it would be a lot of money and take land away.”

A green roof is an extension of the existing roof and involves a waterproofing and drainage system, filter cloth, a lightweight engineered soil called growing medium, and plants, explains Peck. At a minimum, structural support to handle nine pounds per square foot of additional roof load must be provided. That’s for the lightest-weight roof saturated with water, he says.

“It costs twice as much as traditional roofing, but there are paybacks,” says Peck. For example, by having the roof absorb rainwater, the size of retention ponds required on a site can be cut in half and stormwater fees can be reduced as well. Additionally, the roof insulates the building in winter and keeps it cooler in summer.

To build the Ford roof, a 15-acre industrial landfill was contoured to fit 1-meter squares of 13 varieties of sedum planted on 3-in.-thick mats of aggregate soil, explains Russell. The roof was initially designed to hold 25 pounds per square foot, but it actually weighs 11 pounds per square foot when the soil is saturated.

A traditional roofing company installed the mats on top of the building just as they would a regular roof, with overlapping flaps. At various times during the year, the different sedum plants bloom in a variety of colors. There were initial concerns about weeds blowing onto the roof, but the aggregate isn’t thick enough to hold them, says Russell.

There are positive psychological effects on the employees in the building too, says Russell. “The workers are kind of proud to work in a place like that.”

Low maintenance was one of the main reasons Ford decided to go with a green roof, explains Russell. The grounds crew, which takes care of the rest of the property, applies a slow-release fertilizer to the roof in spring, waters it on a couple of dry days in the summer, and does a visual inspection of the roof every two weeks or so.

“We do know the building is cooler [about 10°F cooler on hot summer days] than with a regular roof,” says Russell. “All indications are it will last longer because there is no UV degradation from heat.”

While initial costs to build a green roof are high, there are monetary paybacks. “If you have to replace a roof every 15 to 20 years, the material has to go to a landfill,” says Russell. “If you have to do that less, it’s another benefit.”

Thicker green roofs can be designed to provide for human access, says Peck.  In inner city areas, green roofs can relieve urban heat islands by reducing temperatures by as much as 15°F on a hot sunny day. The roofs can be designed to suit just about any part of the country.

In 2004, Ford received a “Green Roof Award of Excellence” in the Extensive Industrial Commercial category from GHRC. Pictures of the roof and a wealth of information about green roofs are available at

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