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Home and Garden

Green roofs a growing trend

By Mary Collister
Published May 18, 2007


In researching a recent column about Earth Day, I came upon a good bit of discussion about green roofs.

Generally speaking, a green roof is partly or completely covered with vegetation and soil.

It might also include additional layers, such as a root barrier and drainage and irrigation systems. Container gardens on roofs generally are not considered to be true green roofs, although this is an area of debate.

You might also think of a "green roof" as one using some form of "green" technology, such as solar panels or a photovoltaic module.

What is photovoltaic?

Looking in Merriam-Webster online dictionary, I find photovoltaic means "of, relating to, or utilizing the generation of a voltage when radiant energy falls on the boundary between dissimilar substances (as two different semiconductors)." I might never use that word in everyday conversation, but at least I do have a basic understanding.

Green roofs are an intriguing concept. Wouldn't it be great to look around and see vegetation on everyone's roof instead of the hard surfaces we now see? Of course it is not practical for every roof to be green.

The advantages include reducing the city "heat island" effect that makes a metropolitan area significantly warmer than its surroundings.

You might also see reductions in carbon dioxide impact, summer air conditioning costs and winter heat demand.

Green roofs also have long life spans, can treat nitrogen pollution in rain and negate the acid rain effect.

Let's not forget about the aesthetic and psychological benefits of being surrounded by garden-like settings.

I don't have the scientific background to be able to completely concur with all these advantages, but it would appear that a green roof would be worth thinking about, especially on commercial buildings.

The Green Roofs for Healthy Cities organization lists Chicago as the city with the most green roofs for the second year in a row. It would be nice to see Tampa in the running in the not-too-distant future. If you want to see some pictures of the green roof on Chicago's City Hall, go to

And Dearborn, Mich., is not to be left out. The world's largest living roof is on Ford's Dearborn truck plant, according to Guinness World Records. The 10.4-acre living roof is part of Ford's redevelopment of the Ford Rouge Center, which includes a number of progressive environmental initiatives.

My wanderings on the Internet took me to, a Maryland nursery that appears to trade only in roof plants. It is interesting to see how they reuse, recycle and grow organically as much as they can. The Web site also clearly defined the difference between extensive and intensive green roofs. "An extensive green roof has low lying plants designed to provide maximum groundcover, water retention, erosion resistance, and respirative transpiration of moisture. Extensive green roofs usually use plants with foliage from 2 to 6 inches and from 2 to 4 inches of soil.

"An intensive green roof is intended to be more of a natural landscape, installed on a rooftop. Intensive green roofs may use plants with foliage from 1 to 15 feet and may require several feet of soil depth."

The closet project to Tampa that this nursery helped with is one in Fort Myers, and unfortunately there are no pictures on the Web site.

If anyone knows of a green roof project in our area, please let me know. I'd love to see how one would fair in our climate.

Although green roofs are probably not quite ready for suburbia - or more accurately, suburbia is probably not ready for green roofs - I wonder how much of an impact would be measured if a green roof was atop every shed in Hillsborough County! Or if the county stepped in and constructed a green roof on even half of the county buildings. It can be done.

What about the Hillsborough County School District?

Why not add a green roof to all those new schools that are being built?

Take a look at the Calhoun School in New York City, the first educational institution in New York City to design an eco-friendly green roof that offers multiple environmental benefits while providing programmable space for outdoor science and environmental studies. Go to for more information and pictures.

The list could go on, but I think you get the point.

Someday I hope have my own green roof to tend.

[Last modified May 17, 2007, 07:20:30]

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