In the global push to lower buildings’ carbon footprint and energy and water use, some look to blue and green roof construction. These roofs can help combat urban environmental challenges such as the heat island effect and rapid inundation of sewer systems from stormwater and are a small but growing market for geosynthetic materials.
While green roof policies are not widespread, some major cities have laws or incentives supporting their construction, including New York, N.Y., Chicago, Ill., Portland, Ore., and Toronto, Ont., Canada.
Giuliana Frizzi, technical consultant and project manager at ZinCo Canada, which specializes in green and blue roofing technology, moved to Toronto in 2022 because of 2009’s Green Roof Bylaw there. “This is where the green roof sector was growing the most, where the most research was happening,” she says.
“This has resulted in a lot of municipalities outside of Toronto asking how to do [this],” says Jeremy Wright, who oversees business development project management for commercial roofs throughout Canada for ZinCo. “It’s why you’re starting to see these types of systems employed from the East Coast to the West Coast, and Canada is very quickly becoming a staple green roof market.”
It’s not easy being green
However, Wright adds that these roofs are not as simple to construct and maintain as some may think.
“Green roofs are often thought of as the roofs on the top of the building,” Wright says, “whereas a lot of the time, because of the structural limitations of the top roofs—and where the opportunity for geotextiles is right now—is on focusing on the actual podium level and the lower, what we call, like, amenity spaces or intensive terraces.”
In creating usable green space, “you are always going to be building a very heavy roof, but ZinCo works to make them lighter, with less aggregate, using our geotextiles and green roof underlayments, while also maximizing the stormwater management potential that [Frizzi] specializes in, then turning them into stormwater tools,” he says.
Jody McGuire, principal architect at SALA Architects in Minneapolis, Minn., had to confront weight issues in a residential rooftop sauna project (see page 44). McGuire turned to Omni Ecosystems in Chicago, Ill., to help design the project’s biodiverse rooftop garden.
Working with Omni, McGuire says she found the options were “unlimited” but notes that to create these gardens requires “smart, specific planning, especially when you’re trying to place a green roof on an existing structure” because of the weight limitations, “particularly when wet.”
Her rooftop sauna project was a collaborative one from the outset. However, that’s not usually the case, Frizzi says. Because roofs are usually the last thing builders take into consideration on a project, “it’s why we’re a very big proponent of a built-in-place system,” she explains, where all layers are installed sequentially: the root barrier protection, mat drainage, soil and plants. This contrasts with less flexible modular systems grown years in advance then brought up to the roof whole. One geosynthetic alternative to modular systems is what Frizzi calls the mat system.
“It’s just a retention fabric made from recycled clothing parts or even car fabrics,” she says. Two layers are used for water retention, and then an almost sod-like structure of sedum is rolled out on top of it. Because these systems don’t contain soil, they are lightweight, but they “require a lot of irrigation and fertilizer,” she says.
Omni Infinity Media®’s living infrastructure system is also lightweight “relative to other engineered growth media,” says Michael Davenport, director of technical support at Omni Rewild. “Omni Infinity Media [OIM] introduces microbial communities that quickly colonize sterile mineral aggregates, which in turn rapidly form living soil. Plant growth is robust, and we have documented over 800 plant taxa that have been successfully grown in OIM.”
He cites OIM’s ability to grow switchgrass—a deep-rooted prairie species—to maturity in just 3 inches of media depth. Davenport shares that Omni favors marketplace materials that “can be reused or recycled, have a lower carbon footprint and have a good track record of long-term high performance. We want our green roofs to outlast the lifespan of the waterproofing membranes below and hopefully extend that typical lifespan.”
True blue roofs
Blue roofs store stormwater to prevent rapid runoff and flooding in urban areas. The water can be released slowly into storm drains or be kept for uses such as irrigation or gray water systems instead of drinking water. Blue roofs can be independent or work underneath a green roof, like in many ZinCo projects.
The company manufactures its blue roof assemblies and corresponding green roof geotextiles (including its proprietary Filter Sheet PV, Filter Sheet SF, Filter Sheet TG and TSM-32 protection mats). It has 20 systems with a “holistic approach to virtually anything being constructed on top of a roof,” Wright says, “that also allows us to ensure the roof can have a stormwater management influence on it and can be designed so that it can be tied into the overall site planning and architecture of the civil stormwater infrastructure.”
The general composition of ZinCo’s combined blue and green roof systems include a root barrier, a protection mat or slip sheet, and a retention/detention spacer followed by a final filter sheet.
“Depending on what landscaping is going to occur on top of the blue roof, we will specify the final filter sheet to either support concrete or substrates,” Wright says, although he notes that the specifications the company is regularly asked to accommodate “is wicking fabric that uptakes the water from the blue roof.” For this, ZinCo has the DV-40 wicking mat.
“You can’t necessarily factor in a return on investment for a higher price if you have a blue roof,” Wright says, “but by isolating and concentrating your stormwater management features on your roof, you’ll start shrinking the size of these cistern tanks below grade, which more often results in a higher return on the amount of parking spaces.”
Competitive market
New systems popping up are indicative of a competitive, growing market, Wright says, but he’s concerned about shortcuts being taken by some players “because they see dollar signs to make a quick buck to satisfy cheaper budgets.” The problem with this approach is that “they don’t last long term, and it puts the industry at risk.”
Frizzi says it’s essential to put science and nature back into these systems “because in the end, they’re designed to benefit nature and the environment. You don’t want to put too much ‘man’ in these man-made products. We need to be mindful of how we’re designing them and making sure that our goals are in the right place.”
Kelly Hartog is a freelance journalist in Los Angeles, Calif.
One area where green roofs are seeing a boon is urban farming.
“The research is very prevalent that vegetables from a carbon sequestration standpoint hold a lot more value to the environment than the typical perennials and grasses and sedums,” says Jeremy Wright, who oversees business development project management for commercial roofs throughout Canada for ZinCo. of Toronto, Ont.
That said, urban farming comes with ancillary costs “that a lot of people can’t quantify when they start budgeting jobs,” he says.
ZinCo has worked with rooftop gardens at Toronto Metropolitan University where “there’s a full team of people managing those roofs and harvesting the vegetables doing really calculated crop science and yields,” he says. “It’s an important item in the overall green roof toolbox, and we’d like to see more of it.”
Jody McGuire, principal architect at SALA Architects in Minneapolis, Minn., agrees. She says she’s excited about the future of green roofs being used to add a whole new dimension to rooftop designing.
She notes, “Access to fresh produce in an otherwise urban area can be a game changer, addressing food security, health, wellness, air quality and fostering community.”
CASE STUDY: Rooftop sauna and garden
Michael Davenport is the director of technical support for Omni Rewild. He says the SALA Architects rooftop sauna build used one of Omni’s Meadow™ products, which re-creates native meadows in urban landscapes.
“There were also many plugs of live native species planted along with the meadow seeds,” he shares. “Omni did adjust the original design by replacing the tray system/capillary fabric with a composite drainage layer, which allowed faster installation, better drainage performance and unimpeded root growth for plants.”
He adds that the company “believes strongly that if a project is designed with an aim to create a habitat that supports local wildlife, all other benefits of green roofs—stormwater capture, temperature moderation, pollution abatement, etc.—will follow.”
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