Leaf Litter

Leaf Litter Talks with Robert Berghage

We chat with Dr. Robert Berghage, associate professor of horticulture and director of the Center for Green Roof Research.

By Amy Nelson

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Robert Berghage is an Associate Professor in the Department of Horticulture at Penn State. Dr. Berghage is involved in teaching, research and extension and is the director of The Penn State Center For Green Roof Research. Dr. Berghage’s research focus is on phytoremediation systems including green roofs, rain gardens, bioswales, constructed wetlands, and living walls. His extension focus includes these areas and greenhouse crop production and landscape use of herbaceous plants.

Dr. Berghage’s primary research focus is on the function and benefits of green roofs, particularly remediation of stormwater runoff, energy conservation, biodiversity benefits, and materials testing and evaluation for green roof systems. The Center for Green Roof Research has documented 50-60% annual reduction in stormwater runoff with a 4″ thick roof system, delay in peak runoff flows, reductions in acidity and some nutrient pollutants in runoff. Their research also shows reductions in air conditioning demand in buildings with green roofs. The Center is also evaluating green roof plant materials for pollinator food and habitat.

This research has resulted in an ongoing partnership with Penn State Physical Plant and the use of green roofs on many of the new campus building projects including the Forest Resource Building, The Cellar Market, The Student Health Services Building, The Dickinson School of Law Buildings, The Hershey Medical Center in Hershey, and the planned Life Sciences III Building.

How did you become interested in green roofs and living walls?

Within the university, you’re always hunting for money. The whole reason we got the green roof center started was because, about ten years ago, an industrial client came to us with a product they wanted evaluated for potential agricultural uses. It’s the spongy material that’s inside your car bumper. It was a sole use material; it was not used for anything other than car bumpers. Around 2000, GM had a strike, and this company had to shut down its factories because they didn’t have anything else to use this material for. They came to Penn State looking for alternative uses.

We tried a bunch of things with flow culture, hydroponics, underlayment for playground equipment, etc. – because the material has a spongy resilience. We also put it on green roofs. It worked for a lot of these applications, but it was a little too expensive relative to other, comparable systems. The only place where it is now used extensively is in acoustics. Because it has very good acoustic properties and is a plastic material that can withstand weather, it’s really good in marine engine rooms. So it’s used in the engine rooms of boats now.

That’s how our green roof research started. From there, we graduated into doing other work in green roof research. We got some funding from the Pennsylvania Department of Environmental Protection to set up the research center. In terms of green roof research in North America, we were probably the first group to get started. In Europe, of course, they’ve been doing this for years.

What were you doing/teaching before the Center for Green Roof Research was established?

My background is in phytoremediation. I started out with greenhouse and nursery industries, using constructed wetlands to treat wastewater. It’s an interesting wastewater stream because it’s high in nutrients and very low in carbon. Treating it is very challenging because you don’t have a carbon source for all of the microorganisms. It’s not like a traditional treatment wetland where you’re feeding sewage or some other good carbon source for the microbes to feed on.

If you think about what you’re doing when you construct wetlands for wastewater treatment, it’s not a big leap to do the same thing on a rooftop or wall. It’s all phytoremediation.

One of our water resources engineers wanted to know about soil specs for green roofs. I’d like to take it a step further and ask you to describe the anatomy of a green roof.

There are intensive and extensive roofs. An intensive roof is one that’s thick. An extensive roof is a thin soil layer of up to 6 inches. Some people even consider anything up to 12 inches extensive. They are very different animals.

The intensive group is basically landscape over structure. Anything I’d do with my landscape outside, I can do over a structure if I build a solid enough structure and put enough soil on top. The structured soils that go into an intensive roof are a bit different from the soils that go on an extensive roof.

Extensive roofs, which have a very thin soil layer, are put on top of buildings primarily for stormwater management purposes. Although there are energy benefits and other benefits, stormwater management is the real impetus for extensive green roofs.

In Germany, whenever it would rain – very much like it does here – all of this impervious surface in cities like Stuttgart, would cause a lot of runoff. This runoff would go through the sewer system because they’ve got one set of pipes under the city. Then it would dump raw sewage into the Rhine. So the Rhine was an open sewer every time it rained.

Alternatives for fixing this were very expensive. The idea was to distribute that stormwater management as much as possible -on as many impervious surfaces as possible. A rooftop is a great impervious surface to think about. It’s wasted space for the most part. The extensive green roof came about as a result of that.

The structured soils – or media – that we use for extensive roofs are designed to hold moisture. In terms of specifications, we usually want to hold something between 40-50% by volume moisture when it’s at field capacity. We also want about 10-20% air space at field capacity. We generally want a lightweight, aggregate structure. In North America, it’s usually something like a clay, slate or shale that has been run through a rotary kiln and popped like popcorn. It becomes a lightweight rock.

In Europe, they use much heavier materials, such as ground up bricks, roof tiles and slates.

They just build their buildings a little bit stronger than we do. In North America, we’re stuck in this lightweight aggregate mode. A lot of our buildings aren’t built well enough to have much weight on top of them.

As we progress with this industry and do more and more green roofs, I think you’ll see more compromises between the weight of the structural soil and the structure of the building — particularly from an energy standpoint. When you think about the carbon footprint of the green roof, a lot of the carbon costs of that roof is the embodied energy that goes into manufacturing the lightweight aggregate and then trucking it from, say, North Carolina, to the site where you’re going to use it. If we used heavier, locally sourced materials, we could get away from a lot of that embodied energy use. The whole LEED and green building idea makes sense: perhaps go with a little heavier grade structural steel and use a locally sourced material.

Out in the West Coast, they’ve got lava rock – pumice – everywhere. You just mine that stuff and use it directly. You don’t have to run it through a rotary kiln. It’s there, it’s ready, and it has all of the physical properties we’re looking for in terms of being lightweight and having large surface areas for microbial attachment and to hold water.

The aggregate material we use is usually very porous. We stay away from sand and silt particles for the most part. We want very low silt and clay percentages. We want it to drain very fast, because when it does rain, we want the media to work as a sponge, but we want the excess water to move off the roof as quickly as possible. We don’t want excess weight on the roof, we don’t want extra water sitting around, and we certainly don’t want surface flows of water because that would cause erosion. You want excess water to go through the roof, hit some kind of drainage layer, and then flow off the roof.

Have you ever seen coarse grade diatomaceous earth used?

No, but it would be a reasonable choice of material. There are a lot of materials you can think about using. Anything that looks like or acts like a gravel or golf course sand material, or some mix of those, is potentially suitable. The lighter it is, the better it is because of the structural constraints.

What do you think about the use of Styrofoam and other reuse materials as media on green roofs?

Styrofoam is very lightweight, which is good. But it is sterile and inert, which is stupid. It’s a non-natural material, which is somewhat problematic in my mind. I have the same problem with using tire crumb. I guess it’s a good idea to use waste products in some way, and I guess if you put a waste product upon a roof, it gets rid of it…sort of. But it really just pushes off the need to deal with it.

Have you done any research on any of these materials?

We’ve done some work with tire crumb. I really don’t like styrofoam. I don’t use it in my greenhouse nursery soils either. It’s just too lightweight. It floats away. I think it’s just an environmental nightmare.

To me, a more ideal solution for an old tire would be to grind it up and make a new tire.

So back to the anatomy…I assume the first layer of a green roof is the roof deck, right?

The first thing you have is a roof deck. It might be wood, metal or concrete. Any roof deck made out of concrete is usually capable of supporting a green roof. Most wooden roof decks – like the one on my house, which was built in the 1960s – are not capable of supporting much more than the shingles. In those cases, you’d have to have a structural engineer come in and do some kind of retrofit. Some metal roof decks are capable of supporting a green roof and some are not. Most fiberglass roofs are not capable of supporting a green roof. If it’s a new structure you can design the structure with a wooden deck to support the weight.

You have some kind of waterproofing layer on top of the roof deck. The waterproofing material is the same kind of material that we use on any other rooftop – the kind of rubberized, rolled sheet material. Most of the time, on a green roof, we like the waterproofing to be adhered to the roof deck. The advantage to an adhered layer is that if you get a leak, it’s easier to identify the source of the leak.

On top of the waterproofing, you need a root barrier. Some waterproofings are root resistant. Many are not. Any amount of asphalt will be attacked by roots, for example. What we do is lay a layer of plastic – basically construction grade polyethylene – overlap it so the roots cannot get through and tape the seams so that the whole system is integral and the roots cannot get through.

On top of that, things start to vary depending on whose roofing system you’re using, what can be gotten cheap, what they can charge you extra for, and all that good stuff. There will always be some kind of drainage layer. This will either be a coarse gravel type material with some kind of geotextile on top (to keep the media out so fine particles don’t fill the pore spaces and extra water can wick off) or it will be a geotextile sheet type drain. This sheet drain would be the same stuff we use at grade in landscapes for the same purposes. The drain layer is contiguous with whatever roof drains you have. If you have a flat roof with stand-up drains, the layer will be contiguous with that. If you’re sloping down to a gutter, your drain layer will be contiguous with the gutter connection.

On top of the drainage layer, you’ll have a layer of some kind of aggregate media. That aggregate media may be anywhere from two to four to eight inches thick, depending on the project. Four inches seems to be a reasonable compromise between weight and function, from a stormwater standpoint.

Have you learned that from your research at the Center?

We’ve learned it through research we’ve done, as well as research done in other places. You do get additional benefit from going beyond four inches. With a four inch layer, I can store 50-60% of the stormwater. If I go to a 12 inch layer, I’ll store 75% of the annual water. So it’s more, but I’ve added an awful lot of weight, a lot of extra structural materials, and a lot of cost associated with getting that stuff up there to get incrementally much smaller gains in my stormwater management.

Then you have plant material up on top. The material we select is usually what the people would consider rock garden plants. If you think about the environment on the roof and the kind of media we’re using, it is kind of like a rock garden. It is well drained, it’s usually open to a full sun environment very frequently, it’s often very windy or droughty, and so it’s either very dry or very wet.

Things like Sedum, stone crops like hens and chicks (Sempervivums), other species like chives, Dianthus and Campanula — anything you’d see typically growing in someone’s rock garden — is potentially a plant you could put on your green roof.

Depending on the climate, you might use different plants. Here in central Pennsylvania, I can’t use the ice plant type species Delosperma because they freeze out for me in the winter. Further south, in Washington, DC, there are a whole bunch of ice plant species you can use on a roof. They are very successful and they provide a lot of interest and color.

Typical of most roof systems is that plant diversity really follows soil depth. In places where the soil is not very deep you have very restricted diversity in the plant species. It’s mostly the Sedums, some of the Talinums, and some of the Sempervivums that have survived. When you go from four inches of soil to six inches, you suddenly get a lot more species that you can grow. Certainly, when you get beyond six inches, you have a lot of choices of plant species, including a lot of natives. There are some native species of Sedum, but most of them are not native.

If you’re designing a roof and you want to have a lot of plant species up there, you should probably use more than one kind of soil. If we go outside or in a wild area and look at where things are thriving and where they’re not, we find that where species are thriving, there is usually a slight difference in the soil type or soil structure or depth. If you think about putting something up on a green roof and you want a variety of plant material and an interesting plant palette, the way to make that a sustainable landscape is to vary the soils. The plants will naturally follow the varied soils. You can go up there and plant lots of different things, but if you don’t vary the soil, it’s kind of naïve to think that the plant design you put there will stay the way you put it without a lot of maintenance and intervention.

Does climate also affect the choice of media?

Absolutely. The media you’d use in Florida is going to be very different from the media we use here. Around here, we want a relatively low organic matter by weight. Most of it is an inorganic aggregate. If we used a potting soil, which is 100% organic, it’d disappear in a couple of years. This is even more the case as you go further south. If you go north to a much colder climate, you can probably get away with using a higher percentage organic matter soil because it’s not going to decompose as fast.

What types of research is going on at the Center right now?

Our emphasis is on trying to quantify the benefits of the green roof in terms of ecological services and other benefits. A lot of the work we’ve done in the past and continue to do centers on the stormwater management function of the roof. This is where the incentives are coming from.

If you think about putting a green roof on, it’s an expensive proposition. Why are you going to do it? What’s the return on investment? You’re either doing it because someone is giving you a grant and you can write it off, or there’s some other financial incentive. That incentive is probably going to be based on stormwater management. If you think about Chicago, New York, and any other city built before 1960, there is probably only one set of pipes underground to carry away all of the wastewater – storm and sewage. Every time it rains, you get too much water for the sewage treatment plant, and you dump raw sewage into the nearest open water supply. The whole idea of a green roof is to eliminate or reduce that. The municipality that has to deal with it has a choice. They can either store the water someplace, build a bigger treatment plant, or they can make people do something to manage their stormwater. There are a lot of different ways to manage stormwater: build cisterns, put aside ponds, set up mud puddles, etc. Depending on the cost of the land and the amount of stormwater management you need to do for a given site, a green roof may or may not be cost competitive. That’s why we’re focusing on stormwater. From a return on investment standpoint, it looks to me as though most of the incentives are going to come from regulations and legislation around stormwater.

I see. No one is going to give you a break because you’re providing bird habitat.

Not yet. It doesn’t make sense from a carbon trading standpoint either. Even if we get into carbon trading on a large-scale basis, I can plant two trees and store as much carbon as I can on an acre of green roof.

From a stormwater management standpoint, if I’m on a 10-acre site in an urban area and I want to build a new commercial center. It’s probably going to be 20-30% building space and about 70% parking lot with a few little islands in between. It’s mostly impervious surface. If I have stormwater regulations associated with that, then I’m going to have to take all of the water coming off that impervious surface and do something with it. If that is underground storage, detention basins, whatever, there’s a cost associated with that. I can offset some of that cost with a green roof.

We may also see some offset of costs coming as municipalities try to figure out how to pay for stormwater improvements. You’re going to see more and more stormwater utilities formed. As with sewer and water utilities, you’ll be charged for your stormwater. The charges are probably going to be based on impervious surface on your property. You will have a stormwater fee associated with the amount of your property that’s impervious, related to the amount of water that’s going to run off. If you’re paying for it, and you get a discount because your roof is no longer impervious, then you’re not paying as much to that utility.

This is all very quantity based. What about water quality?

At this point, from an incentive standpoint, there are areas where you get a stormwater quality credit for putting a green roof on. There is no requirement by any of those regulations to actually prove that it does anything.

The problem with stormwater quality is that it is very hard to come up with good data to prove that something is a stormwater quality feature. If you really want to prove it, you have to have data from multiple years, multiple storm events, and different kinds of roofing systems over the long term of the roof, relative to all of the maintenance that goes on the roof. Sampling is expensive, so the amount of samples we have is limited. The data sets are fairly limited.

Quantity, on the other hand, is easily proven. And you better be able to prove that it not only works, but continues to work. If it is part of your stormwater management plan and someone has signed off on that plan, you are legally required to maintain that structure and it must continue to function the way it is supposed to function.

What do we know about nutrient loading coming off of green roofs?

It depends on which nutrient you’re talking about. It also depends on what you compare it with. Let’s talk about acid rain. Acid rain is an issue in much of North America and in Europe. Because you filter that acid precipitation through a media that has buffering capacity, you neutralize the acidity. So the runoff from a green roof is basically neutral regardless of how acidic the rainfall is when it hits the roof. Looking at measurements we have taken on the roofs at the [Center for Green Roof Research] farm, runoff from the green roof will be 6 or 7 pH from a 4.5 pH input. So, from that standpoint, a green roof is a water quality feature.

Now let’s look at nitrate. Whether or not it is a water quality feature from a standpoint of nitrate depends on a number of things. It depends on how much you fertilize, and that depends on what plant species you put up there. If I put up a very diverse, landscape-like palette of plant material and it’s an aesthetic type garden, most of those species require a fair amount of nitrogen, so I’m going to have to fertilize them. This means that some of the fertilizer is going to run off. That runoff may be higher than my rainfall. If I put just a sedum meadow on a roof, and I keep them hungry and starve them, I might be able to get enough nitrogen out of my rain to keep the plants alive without additional fertilizer. In that case, I’m going to tie up nitrogen that’s coming from rainfall and my runoff is going to be no higher in concentration than the rainfall that’s hitting it. The mass balance is going to be in favor of the green roof. My green roof is, in that case, a water quality feature.

If I’m in an area of the country that has very clean rainfall, then I’m not going to get enough nitrogen from the rainfall to keep a sedum meadow alive. As a result, I’m going to have to fertilize some, and I’m going to end up with more runoff nitrogen than what’s coming in.

Let’s take phosphorous as another example. I’m always going to have more runoff from the green roof, in terms of phosphorous, than I’d get in my rainfall. In my rainfall, the runoff from the green roof will be something like 10 or maybe 100 times more concentrated. But when you’re talking about .02 parts per million of phosphorous in the rainfall and maybe .2 parts per million in the runoff from my green roof, that kind of keeps things relative, but there’s more in my green roof runoff. If I’m going to keep any healthy plant community healthy on that roof, there has to be phosphorous in the media and there is going to be more phosphorous in the soil solution than there is coming in the rainfall. Long term, I’d have to fertilize that roof to keep it functioning as my buffer of phosphorous is leeched out. In that regard, the green roof probably isn’t a water quality feature.

Now let’s look at zinc. There is relatively low zinc in rainfall. If you’re in or near an industrial site, you can get dry deposition of particulate material, so you can get quite a bit of zinc. Zinc is also in the soils. If I look at runoff from a green roof and I compare it to a plastic roof where there is no additional input except maybe the particulate disposition. I’m always going to measure the zinc in the runoff from a green roof, whereas often, I won’t even be able to detect it in the runoff from a flat roof because once I wash the particulate off, there’s no source of zinc. When I compare a green roof to a plastic or bare rubber roof and look at the bulk solution, the green roof is going to lose. But if I compare the green roof to a metal roof, the green roof is going to win hands down because the metal roof leeches far more zinc than the green roof does.

This is why water quality is hard to talk about. It really does depend on the individual nutrients, what you’re comparing it to, and where you are relative to sources of atmospheric deposition.

Aside from stormwater quantity and quality, what other types of research are underway at the Center for Green Roof Research?

Other things we’re working on now are energy. Again, we’re focused on return on investment. If I’m going to look at return on investment for a green roof, one of the places I can get that is in energy savings. A green roof acts as an evaporative cooler, it reduces air conditioning demand on a building. Our research suggests that on small buildings (like the 6′ x 8′ sheds at the Center), we reduce air conditioning demand by about 10%. If I were to scale that out to a larger building, I might save as much as 30%…or even more.

Is this true in newer construction, where the building is going to be more efficient anyway?

It depends. It depends on how much insulation you stick in. Our garden sheds at the Center have four inches of fiberglass insulation in the roof. That’s way less than what you’d normally call for in a normal building in this climate. The more insulation I put in, the lower the benefit of the green roof is going to be, depending on what else I have.

If I have a commercial building with a flat roof and I put in four feet of insulation in the rooftop, the green roof doesn’t do anything for the building itself. It has a huge effect on heat island, though, which is another story. For return on investment, I’m interested in my own building. Where that equation changes dramatically is when you perforate the roof and put in skylights. With “green” building, one of the things you try to do is get as much natural light in as possible. As soon as you start allowing natural light to get through that insulated layer, then you have thermal load coming into the building and that thermal loading changes. Then, my green roof starts to have a benefit from a total energy standpoint.

Can your research that is done on small sheds really be applied to larger buildings?

Obviously, there are differences. Not just with the thermal stuff, but with stormwater. Any time you’re doing research on a small scale and you say, “My 144,000 square foot roof on top of a strip mall is going to behave the same way as my 6′ x 8′ roof,” in terms of overall function, that’s a pretty risky leap. The data that we have suggests that the larger scale roofs are actually more effective than our small-scale roofs.

We are monitoring a 75,000 square foot roof right now in Chicago and it appears to be far more functional than my little roofs. That makes sense, because you don’t have the edge effects.

Do you foresee corporations becoming partners in green roof research?

For corporate users to put a green roof on building and do it routinely there has to be a reasonable return on investment. Any company that is interested in doing large scale green roofs on lots of buildings needs to be able to prove that there will be a return on investment. The things they’ll look for are stormwater quantity and peak flows. Both of those are regulated and easy to determine. Both can give a return on investment. Building energy consumption can also be a return on investment for the individual building owner. For commercial buildings, those are probably the best places to look for return on investment.

For an institution like Penn State, we can also look at roof longevity. When we build a building here, we expect to be in it for hundreds of years. A lot of commercial buildings are built with the knowledge that the initial owner may or may not be in the building ten years down the road, so the return on investment has to occur before that. For us, that’s not true. With governments, schools, and really any kind of institution where you’re building for the long term, you also get the benefit of having two to three times (or more) roof life as return on investment.

A lot of our research focuses on, “Where can I get return on investment and let’s prove it.”

Have you done any work on assigning dollar values to any of the green roof benefits?

We try all the time. It’s a very difficult problem. It is very site specific and very project specific. For any given project, if you give me data on energy consumption of the building, stormwater alternatives, etc. you can work out some kind of reasonable return on investment numbers. A lot of it is back in the envelope at this point, where you’re making some pretty gross assumptions.

Is the public relations/goodwill aspect of “being green” perceived as a return on investment?

That’s a really hard one to quantify. Can I say that? Absolutely. It’s probably a lot easier to quantify for the mixed commercial buildings where you can track occupancies. There’s at least some decent anecdotal evidence that occupancy in green buildings is better and you get better rents for your office space.

There’s a lot of good data – much of which is from Europe – that hospital stays and outcomes are improved if you have a green view from your hospital room. That green view could be a landscape or a garden, but it could also be a green roof. And that is something you can consider a direct return on investment.

Do you think LEED is furthering green roofs?

Definitely. LEED is driving a lot of these roofs. If you want to be LEED Platinum or Gold, you get a lot of points for putting a green roof on. If you ask me if we “chase LEED points” on green roofs vs. other things, the official response is “no,” but we are putting green roofs on all of our new buildings.

Is putting on a green roof a relatively economical way to get LEED points?

I don’t know if it’s as economical, but it’s a way to get points. Once you get into that green building thing and start talking about trying to be LEED certified at the highest level and becoming a leader in terms of green campuses or whatever, the economic return on investment becomes far less important than the good will factor that comes along with being “green.”

Ed Snodgrass of Emory Knoll Farms, a supplier of green roof plant material, mentioned that the pursuit of LEED points can sometimes lead to failed green roofs.

There are roofs that have failed because people didn’t think enough about what they’re trying to do with them. If the roof is supposed to be an aesthetically pleasing showcase and it doesn’t look like a showcase, then I’d consider it a failure. If the roof is a stormwater management roof and you have slapped it up on top of a strip mall and no one sees it and some stuff grows and some doesn’t but you have reasonable plant coverage. Even if it doesn’t look very good, is that a failure? No, I’d say it’s actually a success because it’s doing what it was designed to do.

Has there been any great failure from which you learned a great lesson?

The easiest way to kill a green roof – and most of the failures I have seen – is poor water management in the soils. The plant species that we use on the roof, the rock garden species, do not like to have their feet really wet. If you have a heavy media that retains a lot of water, what you will do is induce root rot and these plants will die. You’ll end up with a brown roof.

The second area in which I have seen a fair number of failures is in weeds and weed pressures upon the roof. It’s usually coming up in the media – the media is not clean to begin with. Maintaining it is usually not a high priority for the building owner so you end up with a roof that’s nothing but weeds.

In the case where it’s not a showcase garden that people will see, is that really a failure? As long as you have vegetation growing, isn’t that roof still providing ecological services?

That approach is being used in Europe, and it’s not a bad approach. In some cases, what they’ll do in Europe is push aside several inches of topsoil when they’re putting up a building, and then they’ll put it up on the roof and let whatever can germinate in that seed bank germinate and grow on top of the roof?

Our president, Keith Bowers, recently wrote something on our blog expressing his frustration about, “…green roofs consisting of plastic trays filled with three inches of soil planted with three varieties of non-native sedum.” I realize this comment raises two issues for debate: the use of non-native sedum and plastic trays?

Let’s start out with the native vs. non-native question. Just as with my landscapes at grade, I don’t usually make the distinction between native and non-native. I’m more interested in function in the environment for what I’m trying to do. That being said, the biggest argument for native plant material is to increase or maintain the biodiversity in an area. You can do a lot more toward maintaining biodiversity by planting a lot of different species.

Regarding trays with three species, that’s a stupid idea. Yeah, okay, I have a little diversity, but it’s not much. If I really want biodiversity, I want some short stuff, medium stuff and relatively tall stuff. I want a few simple grasses like some fescues. I want flowers that bloom from spring to fall, so I have a food source for my pollinators. Providing habitat and food for pollinators is a good thing. Our urban areas are basically deserts for most pollinators. A lot of our native pollinators are also in decline – not just honeybees suffering from colony collapse disorder. One of the reasons is habitat destruction.

You can take a green roof that is a sedum meadow, like our Forestry Building roof. It has five different species of Sedum, which bloom at different times. It has campanula, dianthus, lavender, a few grasses. By having that diversity of plant material up on the roof, you provide food, shelter and habitat for organisms. In my mind, what you want to do is select species that are going to be successful on this rooftop environment, and you want to select a variety of species to provide the ecological function that you’re looking for.

There’s nothing really wrong with sedums. If you look at almost anybody’s list of pollinator-friendly plants, you’ll find sedums on that list. They produce nectar and pollen. They end up being a food source, so it is a pollinator-friendly plant. Mix it with a few other things, and the more diverse the palette, the more biodiversity you’re going to encourage.

Unless you leave it alone and let it go to a native weed patch, a sedum meadow like the one on the Forestry Building roof, is not a stable climax community. A stable climax community in Pennsylvania is a forest. So some degree of maintenance has to go into that to maintain the kind of community you want to keep up there.

Now let’s talk about plastic trays. Plastic trays have a place. They provide you with the ability to provide an instant green roof. Here in North America, a lot of us are into instant gratification. I want to be able to have my green roof, and the day that the contractor is finished with it, I want it to look like it has been there for 10 years. In order to achieve that, my vegetation has to be fully grown before I move it up to the roof, and it has to be transportable to the roof. How do we do that? There are two choices. It’s either going to be a vegetated mat or a plastic tray. Both allow you the option of: Boom. I’m done. My roof is green. There are places where that is what’s going to sell.

From a cost advantage standpoint, the plastic trays may make sense on some small jobs where you’re not going to achieve any kind of economies of scale by doing something different. The plastic trays can make sense in some areas where you’re dealing with logistics issues. If it’s really hard to get to the area where you’re going to put this roof, and everything has to go through a freight elevator and then you have to crawl through a window to get it on the roof and spread it around, the plastic tray might make sense. You grow it somewhere else, you haul it up the elevator, you pass it through the window from one person to the other, you lay it down, and you’re done. So that makes some sense.

Do the plastic trays make sense on a 150,000 square foot commercial building? To me, no. When you start talking about a stormwater roof on a 150,000 or 200,000 square foot building, you want to fluidize the installation process as much as possible. You want to blow the media up. You want to blow cuttings over it. You want to spread hydromulch. The fewer things I have to touch, carry, handle and move on a roof like that, the better, if I’m to get the cost down to the point where it becomes a reasonable stormwater management feature. For a building like that, I don’t think you could ever get the cost low enough to make it a competitive stormwater feature using trays.

Would trays make more sense in a situation where you had a highly sloped roof and needed structure?

You need something. The tray can be a structure, but you can do something else for structure. I don’t think the tray is the only solution to that, but you do have to have some kind of structure to hold the soil in place. There are sloped applications where that makes sense.

I read that a major part of the Center’s mission is to “promote the use of green roofs through education and outreach.” What are some of the key points you are trying to communicate? Who are the audiences you are trying to reach? What have you found to be the most effective method of communication?

We do everything we possibly can! At this point, it’s still a lot of speaking engagements at ASLA, Greenbuild, and other large conferences. There is a real thirst for information and knowledge about green roofs. There are a lot of people doing green roofs who don’t have a lot of experience and don’t really understand the whole system.

Is there one common misperception among those audiences about green roofs?

The biggest misconception I come across – and the one that bugs me the most – is that extensive green roofs are being promoted as no maintenance roofs. There is no such thing as a no maintenance landscape, and a green roof is a landscape – unless we’re talking about a weed patch.

When I looked at the syllabus for the EcoRoof Technology course taught by you and Bob Cameron, I was surprised to read, “There is no appropriate text for this course unless you read German.” Is this really such a new academic area in the U.S. that there isn’t even an appropriate textbook in English?

Ed Snodgrass’ book Green Roof Plants is good, but there is no good practitioner type book yet. There are a number of books, and there are more coming out all the time. But in my mind, they are all pretty picture books. They are full of case studies of pretty buildings. You can learn a lot from that, but are they appropriate textbooks for anything other than a landscape architecture class? Probably not.

What is one thing you hope your students come away from the EcoRoof Technology course with?

An understanding of the maintenance issue, of course, so they are not promoting green roofs blindly. The other thing I hope people come away from my course – or any seminars or anything else where they are learning about green roofs – is that the one-size-fits-all roof or one-size-fits-all media mentality is inappropriate. You need to really think about where the system is being installed, what its purpose is.

Are there any other universities, in the U.S. and abroad, who have something similar to the Center for Green Roof Research.

A fair amount of work that has been done over the years has been done in Germany. The University of Von Steffen in Germany has done a lot of stuff. Technical University of Munich, Weinstephan campus.

Here in North America, there are a number of places that have done work. Michigan State University did some work at one point surrounding the Ford Motor Company’s River Rouge Plant Their program is less active than it once was, though. We have worked closely with Columbia University on the energy management stuff. We have worked with North Carolina State University on all kinds of stormwater management projects. There is a green roof research center at the British Columbia Institute of Technology. We are working with the University of Maryland. There is a research group at the University of Central Florida that is doing some very interesting things, which makes a lot of sense to me. Florida is such a different environment. Nothing I do here translates there. Everything from media to plant species selection.

What is the best clearinghouse for information about green roof research?

There are two I recommend. The first is the Green Roofs For Healthy Cities Research Links. The other is Greenroofs.com, a commercial site compiled by Linda Velazquez. Both of those sites provide good collections of what information is out there.

The green roofs must bring an educational benefit to Penn State. What are some of the lessons your students have learned on top of the green roofs that they couldn’t learn in the classroom?

Just getting up on a roof…seeing it, touching it, putting plants in it and working on it at any level is a real benefit, as opposed to just showing pictures. When we first started doing this ten years ago, I couldn’t show students anything other than my little research roofs out at the [Center for Green Roof Research farm. The only other roof within a reasonable driving distance was done at a senior community in Tyrone, PA called Epworth Manor. The green roof was put on around 2000. That was the only other green roof around for a long time.

In your travels, is there one standout example of a green roof that was done right?

I have seen a lot of impressive green roofs. I tend to like the meadow type of roof. It’s managed, but lightly managed, and there’s fair amount of biodiversity. I’m not really into the monoculture carpet roof. I think those are boring and ugly.

While I like the intensive rooftop gardens, those are too maintenance prone for me. The Church of Jesus Christ of Latter-Day Saints Conference Center has a beautiful green roof. If you ever get up on top of it, it’s incredible. But it takes a lot to maintain.

Speaking of intensive green roofs, what do you think about growing food on rooftops?

You can do it if you want to. Certainly people in downtown Manhattan have had pots of tomatoes on their balconies and rooftops as long as there have been buildings there. Does it make sense for large scale, commercial food production? I don’t think so. You have a bunch of little postage stamps, when you think about it. None of them are big enough to gain in terms of economies of scale. Access is problematic. Getting materials up and off of the roof is problematic.

If you are trying to use your roof as a water quality feature and you are growing food on it, those are diametrically opposed purposes.

A good exception to having food produced up on the roof is apiculture. If you have a convention center or shopping mall where you have four acres of green roof and you plant it with species that are attractive to pollinators, why not put some pollinators up there and harvest the honey? That makes sense to me as a cottage industry and a potential additional economic benefit to putting the roof up there. If you think about keeping bees in an urban or suburban environment, they’re kind of protected.

Our folks up in our Great Lakes Bioregional office want to know what you think about some of the roofs that are popping up in Chicago.

I like what Chicago is doing to promote green roofs. They have lots of issues with stormwater and it’s a hot place. Reducing urban heat island and reducing stormwater by using green roofs is a good thing. It’s also great for the city in terms of PR. I grew up in Grand Rapids, and I remember going to Chicago to visit museums and it is so much prettier and more livable now than it was 40 years ago. They have done great things with trying to green up that urban space.

Many of our readers are practitioners and project managers involved in ecological restoration, conservation planning and regenerative design. What is one thing you’d like to say directly to them?

Design to purpose.

For example, if I’m in Atlanta, Georgia, and I want a roof that will reduce air conditioning demand on the building, I am going to install an irrigation system and I’m going to put some cisterns into the system so I can capture the excess water and pump it back onto the roof. It just makes sense. It’s going to be so much more functional and effective when the roof is wet and you’ve got evapotranspiration occurring. If my primary purpose is stormwater management, then design to stormwater management.

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