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A design that has been popular in other countries such as Germany and Switzerland is taking root in the United States and its tasteful appeal is helping it grow rapidly.

Green roofs are not only beautiful to look at but are also environmentally friendly.

"There are two kinds of green roofs. One is called an intensive green roof and that is six inches of soil or more and it means that you can grow just about anything on the roof top. It probably means there's access so there are benches and water features -- it's basically a roof-top garden," says Jim Mumford, owner of Good Earth Plant Company.

The other type is called extensive. Mumford says these types of green roofs are developed more for the ecological benefits than for having access to a garden setting.

According to the Green Roof Association, there has been a more than 25 percent growth rate over 2005. The survey, which used statistics from its corporate members, showed that more than three million square feet of green roof were installed in 2006. Intensive green roofs grew by 110 percent in 2006. These types of green roofs typically include larger plants.

Some cities are contributing significantly to the green-roof movement. According to the association, Chicago implemented the most square feet of green roofs in 2006 of any city. The city also has policies that help support green roofs and urban greening. Washington, D.C. is second on the list followed by Wildwood Crest, NJ; Dulles, VA' Kansas City, MO; Phoenix, AZ; Milwaukee, WI; New York, NY; Portland, OR; and Columbus, OH.

Mumford says there are many reasons for the increased interest in green roofs for commercial and residential properties.

He says an eco roof or a green roof will extend the life of the roof by two to three times. "That means it costs the owner less to replace the roof and there is that much less material going into the landfill," says Mumford. He adds that, "A green roof will absorb 60 to 80 percent of a storm event."

What isn't absorbed is filtered. "So dust, pollen and particular matter are taken out of the water. What water does go through [the gutters] is cleaner and it also slows down," explains Mumford.

Yet another benefit says Mumford is that green roofs absorb heat and help to insulate a home. Earlier this year on his commercial building in San Diego California, Mumford began constructing his own green roof; he expects to see a reduction in his air-conditioning bill by about 20 percent.

For some, getting a garden started on the ground can be challenging enough, let alone trying to plant a garden sky-high and have it thrive. But Mumford says the process isn't that difficult especially if you bring in the experts to make sure you begin with a good solid roof.

"The first thing I do is send a roofer out to see if it's possibly a viable project. If the roofer says, 'Yeah, we can go forward with it,' then you have to hire a structural engineer; that's about $500," says Mumford.

The structural engineer will make sure that the roof can withstand the weight of the green roof.

"An extensive green roof [weighs] about 12 to 15 pounds per square foot which isn't too bad, it's pretty lightweight actually," says Mumford.

Then you have to decide how you'll contain your green roof. "What I did is what's called a built-up layering. Above the waterproof membrane there is a protective mat, there's a drainage layer to allow the water to get off easily, and then there's a roof barrier and a soil barrier [put in place] before you put the soil down," explains Mumford.

He says you can also do a modular-unit green roof which uses two-foot by four-foot shallow plastic trays. "You're basically filling up the entire roof-top with a grid of these trays," says Mumford.

Mumford says what you put in your green roof is very important. He says you should choose plants that are shallow-rooted and non-invasive root plants. "They need to be drought tolerant. They need to be bullet-proof tough. The rooftop is a unique environment; it's hot, it's windy -- it doesn't reflect the rain and instead it wants to burn things," cautions Mumford.

In many areas, Mumford says irrigation is not that vital because the amount of rain is sufficient, but in San Diego, it's a big concern for the survival of green roofs.

"In Southern California because it's so dry so much of the year, we're guessing that [green roofs] have to be irrigated somewhat year round which is why I choose to use native plants. I am hoping that once these native plants are established, I won't have to water as much or at all," says Mumford.

He chose to use pop-up sprinklers to irrigate, but he says, "You could use soakers, you could use micro-spray, and you could use drip. But because I am trying to get this to be as low-maintenance as I possibly can I use the pop-ups."

For Mumford, his own green roof is a learning process that enables him to teach and work with others to get their own started. You can see his progress at greenroofsandiego.com.

If you're looking for a return on investment by installing a green roof, Mumford says, "Right now we're looking at between five and seven years."

The concept is very much in its infancy in many areas such as San Diego, but Mumford is optimistic that green roofs will blossom into a thriving industry.

However, he says if you're planning on staying in your home, then, "You're going to see that investment be returned down the road." And in the meantime, you'll enjoy a garden that's above all the rest.

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