Green roofs are here to stay. But aesthetics and energy savings must be balanced with the practicalities of structure, water, and wear. PWL Partnership Landscape Architects Inc.
ARCHITECT Magazine
By Aaron Seward, Published November, 2011
*Excerpts from The Grass Ceiling. Download the PDF to read the full article.
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 York's 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 sizeable 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 roof's 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 historic approach, there is much to consider.
"You have to decide for your client whether you want a design solution or product solution," explains ED Snodgrass, a leader in horticulture 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 we've got you covered so you don't 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 client's objectives."
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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 project's 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. "It's 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. It's 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 (238 pounds) 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 Fraser River, organic matter, and lava rock. The project also employs a mat drainage system, the better to contend with Vancouver's wet winters, and a triple-ply liquidized-rubber waterproofing layer topped with a granulated cap sheet for protection during construction.