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Raise the green roof

February 2011 » Features » SPECIAL FEATURE
Experts weigh in on the benefits of vegetated roofing
Randy Hofbauer

Green roofing certainly isn’t a new concept. But as time passes, more and more building owners are jumping on to the sustainable-roof bandwagon. According to Thomas Cooper, principal and director of Glenview, Ill.-based Green Roof Solutions, Inc., roofing products are “notoriously toxic” and appear to most people as being unsustainable.

“However, the roof envelope offers … one of the best vantages of a low-impact development strategy on the surrounding landscape,” he adds. “The roof is the most exposed portion of the building — it receives the most UV energy and storm water of any feature, except perhaps large areas of paved infrastructure.”

The ultraviolet energy and storm water both especially help one particular kind of green roof that has been gaining popularity in the states: the vegetated roof.

For those less familiar with vegetated roofs, it’s important to note that several types exist. Most experts would agree on two: intensive and extensive. They also would agree that extensive green roofs have shallow soils (3 to 6 inches deep) and must be planted with vegetation containing shallow root systems, and intensive green roofs are similar to terrace gardens with shrubs, trees, and perennials, requiring deep soils (at least 18 inches) and strong structural support.

Jim Lindell, national marketing manager for GreenGrid Green Roof Systems, a division of Vernon Hills, Ill.-based Weston Solutions, Inc., mentions a third type of vegetated roof: the modular-style green roof.

“These green roofs are made up of a series of pre-planted modules or trays,” he points out.

Weston Solutions provides such a product in GreenGrid, which is pre-planted and can be grown out in a nursery prior to delivery so as to provide a mature green roof upon installation. Not only does GreenGrid allow for more efficient installation, Lindell says, but it also provides builders with the ability to easily access the underlying roof structure post-installation if repairs or maintenance are required.

What’s the advantage?
According to Lindell, strong benefits can be reaped from roofs predominantly covered in vegetation; such roofs reduce heat gain — typically more effectively than reflective roof coverings — and reduce storm water runoff.

“Green roofs have been utilized for decades in Europe,” Lindell adds,” but in the last 10 years, [they] have grown in popularity in the U.S. for their many benefits.”

A single ply roof with an intensive vegetated green roof can be considered sustainable, as the roof membrane is away from ultraviolet energy exposure, slowing the aging process, and the system flows water runoff from the roof. However, structural engineering is needed to verify that the roof assembly can take the added weight of soil media when saturated.
Chicagoland Roofing Council

Bill McHugh, executive director of the Chicagoland Roofing Council (CRC), Hillside, Ill., notes that vegetated roofs were introduced in Chicago a little more than 10 years ago, when Mayor Richard M. Daley had such a structure installed atop City Hall.

Introducing vegetated roofs seems to have been a natural move for Chicago. McHugh points out that the city, along with areas in Europe that have been using vegetated roofing for some time, share a similar problem: A combined water/sewer system meant municipalities were seeking methods to reduce water runoff into the sewer system during rainy spells.

“Plus, with a heavy clay under the city and not much slope on land or in rivers to carry water away quickly, we are susceptible to flooding,” McHugh explains. “This has intensified [during] the past 50 years as city and suburban sprawl has taken open space that used to hold water away. That’s why the city of Chicago invested billions in water management.”

McHugh adds that CRC members have installed more than 2,000,000 square feet of vegetated roofs from the deck to the soil media surfacing with intensive, semi-intensive, or extensive vegetated toppings.

Speaking of media, when it comes to growing and maintaining a vegetated roof, media type is critical to its effectiveness. According to the website of Skyland USA, the Avondale, Pa.-based manufacturer of rooflite certified green roof media, growing media can be defined as “an engineered soil that both mimics and improves on the properties of native soil.” Although native soil works fine in its natural environment, it cannot be used on green roofs because of its excessive weight, inability to retain enough moisture, and failure to drain excessive amounts of moisture appropriately.

“On the other hand, green roof-growing media faces several additional requirements, since it ultimately must promote plant life in an environment [that] is very different from the natural surroundings of these plants,” the company added.

Skyland’s rooflite brand offers several products — including intensive mc, semi-intensive mc, and extensive mc — that are certified to comply with the German FLL Green Roof Guidelines, which the company says are the “acknowledged state-of-the-art [rules] for successful green roof construction worldwide.”

Grab your partner
In the end, McHugh believes building owners and managers need to make sure their buildings can handle the load of all elements that make up a vegetated roof. He explains that structural engineers are needed more today than ever before — and that they should partner with the professional roofing contractor.

“Building owners and managers are … wondering, ‘Can this structure take the weight of the soil media, the water held in it, and the water retention boards — or not?’ Working with the professional roofing contractor adds new opportunity for structural engineers in a tough new construction market.”

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