Home Landscape Design The Grass Ceiling


GREEN ROOFS ARE HERE TO STAY. BUT AESTHETICS AND ENERGY SAVINGS MUST BE BALANCED WITH THE PRACTICALITIES OF STRUCTURE, WATER, AND WEAR.
to some extent, green roofs have been around forever. The fabled hanging gardens of Babylon and the still - existent sod roofs of Scandinavia predate recorded history and show that, at least in some places on Earth, humans have always found reasons to grow plants over their heads. The advent of the high - rise took this idea of skygardens to new levels. In the 1930s, elaborate roof gardens were put in place on top of the plaza buildings in New Yorks Rockefeller Center, and other examples of tower - topping putting greens, herb patches, and sculpted hedges abound. Other than their altitude, these green roofs, known as "intensive" in the industry, differ little from gardens at grade. Almost always installed over waterproofed, reinforced - concrete slabs with drains, they feature soil depths of up to 10 feet, deep enough to grow sizable trees.
Intensive green roofs require a good deal of time and money, both in their design and construction as well as in their upkeep. Their motivating impulse tends to be aesthetic and recreational. However, a newer type of green roof has become more and more prevalent in this country over the past 10 years. Known as an "extensive" green roof, it is a system that has been engineered and developed in Germany since the 1970s, specifically for building performance and environmental sustainability. These roofs manage stormwater runoff, mitigate the heat - island effect, and create habitats for birds, insects, and other critters. They are also known to double the lifetime of a roof by acting as a barrier between the roofs waterproofing layer and the elements.
There are many types of extensive systems, each suitable for a different set of project parameters. What separates them from intensive green roofs is primarily that they tend to feature soil depths of 3 to 6 inches and are lightweight enough to be installed on top of existing structures, opening up the possibility of "greening" vast swaths of the built environment. There are now several companies in North America offering off - the - shelf extensive green - roof systems for building owners looking to meet increasingly stringent stormwater retention codes with a minimum of effort and maintenance. But for those seeking a more holistic approach, there is much to consider.

The Grass Ceiling


"You have to decide for your client whether you want a design solution or product solution," explains Ed Snodgrass, a leader in horticultural consulting for green - roof projects and co - owner of Street, Md.- based Emory Knoll Farms, a perennial nursery specializing in green - roof plants. "For a product solution, there are a number of companies that would say, if you want a green roof weve got you covered so you dont have to think about any design questions at all. They will come with a shiny brochure and you pick your option. If you want a design solution, you have to look at all of the layers that make up a green - roof system and decide what is best for your clients objectives."
Analyzing the Layers
The first consideration for any green - roof designer is the loading capacity of the roof itself. This is especially true when the project is to be applied to an existing building, in which case the designer must team with a structural engineer to determine how much weight can be added and where. At Chicago City Hall, which was completed a decade ago as a pilot project to determine the benefits and feasibility of green roofs on city municipal buildings, the Elmhurst, Ill., office of ecological - design firm Conservation Design Forum custom - tailored a system to meet the varying capacities of different parts of the historic structures roof. "Part of our approach on any green roof that we do is to maximize the habitat and therefore the depth of soil," explains David J. Yocca, principal landscape architect at CDF. "In most retrofit projects, such as city hall, we adapted the green - roof system to the characteristics of the structure, varying the thickness of the growing medium based on available loading."
Once a roofs loading capabilities are known, the specifics of the extensive system - which is composed of several layers of materials, each performing a different function - can be determined. The first of these is waterproofing, which is typically either an asphalt derivative, or made from nonasphalt material such as polyvinyl chloride or thermoplastic polyolefin. It is important to understand which material is used because while plants cant consume PVC and TPO, asphalt - derivative products can become food for plants and bacteria. If asphalt is used, then an engineered - fabric root barrier must be added to prevent the roofs living organisms from feeding off of the roofs waterproofing.
Leak - detection systems are often incorporated into green roofs so that if a leak does crop up, it can be quickly located and fixed. The most common form of leak detection is electric field vector mapping, which uses a low - voltage current to create an electrical potential difference between the nonconductive waterproofing membrane and a conductive substrate. Water atop the membrane serves as a conductive medium. If there is a leak, the water passes through the medium and onto the substrate, creating a ground - fault connection, or vector. Technicians can detect these breaches with pinpoint accuracy.
The next layer installed after waterproofing is drainage. While part of a green roofs primary function is the retention of stormwater, or at least the slowing of runoff, it must still fundamentally act like a roof, and that means shedding water. There are two types of systems used for this purpose: One, a drain mat, involves plastic or fiber channels that direct water filtering through the soil horizontally to drains in the roof. The other, granular drainage, is made from single - sieve aggregate, basically little stones that are all the same size. Each system has its benefits. Granular drainage has better horticultural performance because the plants roots can get down into it, however it does not move water off the roof as quickly as a drain mat because the grains form a kind of obstacle course; the mat provides direct paths for water to follow to the drains.
Above the drainage layer goes a separation fabric, a nonwoven geo - tech material that lets water pass through, but not soil or growing media. This layer does allow roots to pass through, but does not itself break up or degrade in the process. Its primary function is to keep the growing media from clogging the drainage.
Next comes the engineered growing medium, which is mostly stone and sand aggregate with very little organic matter. In fact, when installed, the surface looks more like a driveway than a garden. There are two main reasons for using this type of material. One, it does not compress like soil, and therefore allows water to drain through. Two, it doesnt dry up, blow away, or decompose as organic soil would. Since the return on investment for a green roof is 20 or 30 years, it pays to use a material that will last. Sometimes irrigation systems are also integrated into the growing medium, either to get a roof through periods of drought or to help it along in its first year of growth.
The final, and most visible, layer of a green roof is the plants themselves. Many factors must be considered when choosing what to grow, including the local climate and the roofs exposure to sun and wind. Those factors will determine what can actually live there. Then there are the clients goals to consider. If they want to establish an ecology, plants must be chosen that have pollen for insects and birds and good structure for spiders. If they care about stormwater retention, then plants must be chosen that behave well for that purpose. If they care about the way it looks, then there are ornamental considerations. And then, perhaps most importantly, theres the question of maintenance. A roof can be planted in a way that will require only a once - per - year checkup to make sure that the drains are clear and the flashing and penetrations are in good condition, but if clients care about maintaining a more complex ecology or a rigorous aesthetic, then they must be prepared to hire someone to weed, mow, and generally ensure the sculpted quality of their roof garden.
There are also different ways of delivering plants to the roof, each with its own level of installation cost. The cheapest is to start with seeds or sedum cuttings, with which you can cover a large area very quickly, though it is difficult to organize them in any precise pattern. The next option is to begin with small starter plants - i - inch - diameter - by - 3 - inch - deep plugs that include the plant with a bit of propagation medium. Those can be arranged in whatever pattern you wish, but must be planted one at a time, incurring a higher installation cost. The most expensive option is to start with mats and modules, which have been grown for a year in a nursery. The preinstallation growing time costs money, but the mats and modules are also more burdensome, and thus more costly, to transport and lift into place. On the other hand, putting in fully grown plants has the benefit of obviating the risk of wind scarification, which can plague freshly seeded soil. This method will also deliver an immediate vegetated roof, an important factor if the project is in the public eye, since in its first year a seeded roof may appear more like a dirt roof than a green roof.
Learning from Applications
The largest North American green roof of the last five years is that of the Vancouver Convention Centre in Vancouver, British Columbia, Canada. Aside from being very big - 261,360 square feet, or six acres, to be exact - the green roof is also integral to the ground - up projects overall ecological mission and LEED - Canada Platinum rating; in essence, it recreates a coastal grassland ecosystem, typical of the Pacific Northwest. "The design of the living roof is ambitious in integrating into the broader ecology and landscape of the Vancouver waterfront," says Mark Reddington, FAIA, of design firm LMN Architects. "Its big enough to house an ecosystem. There are a number of different local plant types, as well as birds, field mice, and other creatures who live there. Its also home to 250,000 bees in hives, whose honey is harvested and served in the building."
The roof, which undulates in elevation based upon the needs of the interior space it shelters, features a number of custom - designed aspects, many of which were locally sourced by PWL Partnership Landscape Architects. The plants, for example, were derived from seeds, sedum cuttings, and plugs that were collected from Pacific Northwest grasslands. It was the first time that many of these were ever grown commercially in a nursery, or used for a green - roofing application. In all, the project used 400,000 plugs, 108 kilograms of seeds, and 80,000 sedum plants. The growing medium - which was applied 8 inches deep across the roof and settled to 6 inches deep - was also locally sourced and engineered from sand dredged from the nearby Frazer River, organic matter, and lava rock. The project also employs a mat drainage system, the better to contend with Vancouvers wet winters, and a triple - ply liquidized - rubber waterproofing layer topped with a granulated cap sheet for protection during construction.
The Vancouver Convention Centre system was designed to impose no more than 39.6 pounds per square foot upon the roof. But at the Target Center in Minneapolis, Kestrel Design Group was asked to deliver a system on top of an already existing building that would impose no more than 17.4 pounds per square foot. While that is an unusually low number for a public building, the arenas original designers never expected the roof to have to bear anything other than snow, and so delivered the barest minimum of structure to serve that purpose. But when it came time to replace the roof, Minneapoliss sustainable - minded city council members wanted to go green. "Some city council members strongly encouraged putting on a green roof for stormwater management," says Kestrel designer Nathalie Shanstrom. "If we had any less loading capacity at all, though, they would not have considered it."
To minimize weight, Kestrel kept the growing medium as thin as possible - 2 3/4 inches deep in the middle of the roof and 3 1/2 inches deep at the perimeter. They also added a thin layer of a recycled geo - tech material designed to retain water and a drip irrigation system to give the plants the most favorable conditions possible in the meager soil. The plants were delivered with the medium in pregrown vegetation mats that included basic green - roof sedum augmented with plugs of 20 native prairie species adapted to shallow soil, drought, and windy conditions. The mats were pregrown in a nursery for two years, instead of the usual one - long before work began on replacing the roofs waterproofing membrane - in order to ensure a solid green cover the moment they were installed. Beneath the plants, medium, and water - retention layer, the project features a mat drainage layer, PVC waterproofing, and a leak - detection system.
While weight was of paramount concern at the Target Center, in calculations for the roof of the Laurie M. Tisch Illumination Lawn at Lincoln Center in New York, the weight of a different variable had to be accounted for: people. Diller Scofidio + Renfro and FXFowle wanted to encourage visitors to walk on top of the hyperbolic - paraboloid - shaped grass lawn set atop a restaurant. Making this green roof fit for occupancy began with the roof itself, a 6 - inch - thick concrete - on - metal deck slab that is rated for the dead load of the roof system and a live load of 100 pounds per square foot. Atop this substrate is a 14 - inch - thick roofing system that includes a green roof.
The system features a waterproofing membrane sheet, a 1 - inch - thick root barrier, 4 inches of insulation, and a 1/2 - inch - deep drainage mat with filter fabric. The growing medium itself is 81/2 - inches deep. Since the lawn is at a significant slope, between flat and a 1:8 pitch, the growing medium incorporates an integrated cellular confinement system, basically an expanded plastic mesh that keeps the soil from rolling down the hill. "We tested a lot of different grasses, looking at things like durability, appearance, water retention," says FXFowle partner Heidi Blau, AIA. "We wanted something that would be green as long as possible during the year and also comfortable under a bare foot." They ended up choosing a blend of tall fescue and Kentucky Blue grasses.
While its durable enough, the lawn has attracted more visitors than expected and within the first couple of weeks the grass got trampled. To prevent a dirt trail from forming at the favored entry spot, the designers worked with Lincoln Center to set up barriers to change this entry point from time to time, thus giving the grass time to recover. "Grass is a natural material," Blau says. "It can get destroyed. Lincoln Center is learning as each season goes by how to care for it."
Measuring the Benefits
As with any piece of building technology or infrastructure, the feasibility of installing a green roof - whether on a new building or as a retrofit - comes down to cost - benefit analysis. One problem with conducting this analysis,
however, is that there are no well - defined metrics for understanding the performance characteristics of these systems. For example, green roofs are often cited for improving an enclosures insulation values. In reality, you cannot apply an R - value to them because, while a green roof may deliver some insulation when dry, when wet, temperature will move right through. On the other hand, studies at Chicago City Hall have determined that the greening of the roof lowered the air temperature on the roof in summer to 85 F as compared to 110 F on a typical black - tar roof. That means that intake air has to be cooled that much less by the buildings air - conditioning system. "Green roofs will have an ecological benefit from an aggregation of services; there are very few discrete services that will justify the cost," says horticultural consultant Snodgrass. "Those are things that are hard to calculate. The formula is so long and difficult that engineers just dont want to think about it."
Perhaps the easiest way to measure the cost - benefit of green roofs is the extra life they lend to the integrity of the shelter. "We did a life - cycle cost analysis of a green roof versus a regular roof," says Shanstrom of the Target Center project. "It ended up being very close in cost when you considered the extra life span the green roof gave to the roofing membrane. Actually, the green roof was a bit more favorable.
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