Is that a Garden on Your Roof?

Sept. 14 - Got a green thumb? Gardens are for amateurs; consider applying your talents with plants to your roof. A movement with, uh, roots in Germany is picking up steam in the United States that aims to ameliorate ecological problems from storm water runoff to urban greenhouse warming. A "green roof" or "ecoroof" replaces traditional roofing with a lightweight, living system of soil, compost and vegetation. It's not about looking pretty (although it does) but rather creating a thin, green skin atop your building that gives a little something back to the world—and your pocket book. Apart from local environmental benefits, preliminary evidence suggests green roofs reduce roof maintenance costs and energy use by insulating buildings from extreme temperatures.

Most of the empirical data on the benefits of green roofs has been collected in Europe. David Beattie is hoping to change that. As director of Penn State University's Center for Green Roof Research, Beattie is looking into whether, and how, green roof technology can be applied in dense urban settings like New York and Atlanta. He recently spoke with NEWSWEEK's Brian Braiker about the costs and benefits of ecoroofs and how, exactly, one might mow them. Excerpts:

NEWSWEEK: In a green roof, the greenery actually becomes a living part of the roof. Can you explain how that happens?
David Beattie: Well, the two don’t actually become one, but it’s a thin layer of vegetation on the roof. That is the modern concept of what we would call an extensive green roof, to distinguish it from an intensive green roof, which is a deep one—maybe several feet [deep] such as you’d find on the top of Rockefeller Center. This is a technology the Germans developed about 30 years ago. It’s relatively thin, relatively lightweight and usually consists of fairly drought tolerant plants in the cactus family, called sedums.

Why put a garden on a roof?

Pat Little / AP 'Rottweiler tough:' Beattie conducts research at Penn State

Its principle use is for storm water amelioration. And in a climate such as we would have in the middle Atlantic states, one would expect it to retain 50 percent of the rainfall that falls on it. The other 50 percent is actually slowed down so when it hits the storm water system of pipes, it doesn’t overwhelm the pipes. This is the problem in all of the major cities in the East Coast now: You get a lot of rain and because of all the impermeable surfaces—parking lots, buildings, roads. All this rush of storm water hits the storm water system of pipes all at once and, bang, they can’t handle it. So you see a lot of these cities—just to name a few, New York, Washington, Pittsburgh, Cincinnati—all facing billion-dollar upgrades in infrastructure, because the way that this problem has been solved before has been to put a bigger pipe in. That’s the engineering solution. This is a soft approach, an environmentally green approach, to the storm water.

Would it work on a grand scale like in New York?
Sure. It's especially designed for built up areas and it’s about the only thing that you could put on Manhattan. When you’re in more suburban areas, if you put up a big box store you can put in a big storm water retention basin around the building. But in Manhattan you don’t have that option any longer. The only option left is to put the green roof on top of the building, especially  for Manhattan because the combined sewage system is at about 85 percent of  capacity. Every time the city gets a good, hard thunderstorm, it has to open up the sewage treatment plants and guess what floats into the Hudson.

Ew. Does having all that greenery also reduce the localized greenhouse effect in major cities?
Yes, it does have an effect on the heat island, to reduce that temperature envelope. There have been some estimates on cities like Atlanta and Toronto if you were to green a substantial part of the roofs in the city, the temperature could be reduced by 12 degrees.

What about the insulation benefits?
That’s a tricky one. We have shown that during the summer we can save about 10 percent in air conditioning costs. I think we’d have to look at it over a lot of more areas of the country. When you talk about energy, most people focus immediately on air conditioning. But when you’re talking about a building, there are a lot more energy costs than just the electricity costs. For instance, you have all the energy costs associated with tearing the roof off and putting a new roof back on. The best estimates that we have now is that a green roof will lengthen the life of the roof membrane two to three times. First of all, you protect it from temperature fluxuations that you get on top of a black roof. Secondly you protect it from the UV light which degrades the roof membrane.

Can you actually walk on a green roof or is it hands off?
If you have to walk across it to service something, that’s OK. It’s not the kind of a roof though that you can have a party on. Quite frequently you would have pavers up there for people to walk on, but these plants are Rottweiler tough and you can certainly walk on them.

How much does it cost?
If you were to put it on a two-story building, I’ve known some that have been installed for $6 or $7 a square foot. You design more or less for the same roof whether its for a one-story building or a 50-story building. The cost is going to be more related to how difficult it is to get it up there. [But] assuming a regular black roof would last 10 years, you could get 30 years out of the green roof. There are roofs in Germany that have been on for 50 years.

Wow, and this could go on any home?
Yes, if you were starting out and building a home. Most of these go on flat roofs, but you could certainly put them on a sloped roof that you would find on domestic housing, but you would have to increase the load-bearing capacity. Architects tell me that doesn’t really add that much to the cost of the roof. These are relatively thin—I am talking about a three-and-half to four-inch deep overburden for everything. So I’ve seen domestic housing roofs go in where we had an unusually deep, wet snow in the spring around here. So if you retrofit an roof you really do need to work with an engineer or an architect.

Do you trim it? How do you mow it?
[Laughs.] Good question. It is not something you put up there and absolutely walk away from. There is no such thing as a completely maintenance-free garden, roof or whatever you have any time you’re dealing with plants. You have to go up and inspect your drains to make sure nothing is in them. You probably need an inspection in the spring and an inspection in the fall. Most of the maintenance will occur in getting the roof established—for the first year you may have to have access to that roof if you put it on during the middle of the summer you’re going to have to provide some kind of irrigation to get it going. Even sedums, being Rottweiler tough, they would live but they wouldn’t thrive if you put them up there and didn’t water them for six weeks. Does it mean that you can’t put other plants on there? No. But we don’t know a whole lot about them and we don’t want a lot of things like grasses because they create a lot of biomass. 

What do you do at the Center for Green Roof Research? Are you looking at how they work? How to get more cities to adopt them?
The technology comes from Germany, so we feel we have to demonstrate that first of all it will, and secondly how  it will, work in our climate. Our mission is to provide numbers for engineers and architects that they could estimate how much water would be retained on the rooftop. We’re trying to interest the federal government in it, particularly the [Environmental Protection Agency].

Is it interested?
Oh yeah. But the gristmill in Washington moves very slowly. We’ve made some reasonably good progress to put green roofs as a technology on their radar screen so when it comes for funding time they’ll acknowledge this technology has some validity.

Well, there are converts out there. Chicago’s city hall has a green roof so does the Ford Motor Company. What other prominent cities are adopting them?
They’re just about ready to let the bids out on one at the Interior Department in DC. The Harford Community College [in Maryland] just put one in-one of my students was the contractor. Anne Arundel County [also in Maryland] has been very much interested and have adopted green roofs as part of its storm water treatment methods. The best estimate—and there are no hard figures—is that there is somewhere between 800 million and one billion  square feet in Germany. There are some cities like Stuttgart where you can hardly put up a building these days or go back and reroof a building unless you consider a green roof.

Well, it is beautiful, too.
Sure, there are lots of aesthetic advantages; [advantages for] wildlife and migrating songbirds. Think of if that kind of scene were to confront you outside your office window.

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