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Vegetative Roofing Systems to Green Your Building

Dec. 2, 2010

A vegetative roofing system can help green your building – in more ways than one

In your quest to green your facility, you’ve upgraded your HVAC and lighting systems to be more energy efficient, switched to green cleaning products, and installed occupancy sensors. But as you’re greening your facility from top to bottom, don’t forget the green potential of the very top – the roof.

"A green roof can serve several purposes, so it is a good idea to know which of those purposes is driving the notion that you need a green roof," says Kelly Duke, vice president of pre-construction services with ValleyCrest Landscape Development, Inc. "For example, in some cities it may be a permit or code requirement to hold or treat storm water onsite, and a green roof may be the most viable or attractive way to do so. For some owners, the achievement of LEED certification may be important and the implementation of a green roof may score credits in multiple categories. For some, the aesthetics of a green roof may make the property more attractive to customers or tenants. For others, it may be simply a case of environmental one-upmanship that gives them something to talk about."

The benefits of vegetative roofs are numerous, including obtaining sustainability program credits, reducing the urban heat island effect, adding green space to your property, and even lowering your building’s heating/cooling costs due to the roof’s insulating qualities. But before you start landscaping your roof, there are several important factors to consider.

Look at the Lifecycle
One of the first things to consider when contemplating a green roof is the lifecycle of your current roof. Adding a green roof to your building shortly after you replace the membrane will ensure you get the greatest longevity of the membrane – and the green roof, too.

"Generally, we recommend that when someone puts a green roof on a building, they do so when they’re also replacing their waterproofing," explains Steven Peck, founder and president of Green Roofs for Healthy Cities. "That’s because one of the principal benefits of a green roof is that it can extend the life expectancy of the waterproofing system underneath it by anywhere from doubling the life expectancy to as high as 40, 50, or perhaps 60 years."

Structural Strength
Another consideration is the structural loading capacity of your roof. "Because green roofs involve additional layers – the plants, the growing media, the drainage layer, and other aspects that can be incorporated – they require that you have excess loading capacity above and beyond what the minimum core requirements for your area are," Peck says.

To determine what, if any, additional loading capacity your roof has, an architect or structural engineer can look at architectural or engineering drawings, if they are available. If it’s an older building and this documentation isn’t available, the roof’s capacity may need an investigation by a structural engineer, roofing consultant, or architect.

Going for Green
After you evaluate the lifecycle of your roof and its additional loading capacity and decide that a green roof is in your near future, you should determine which type of green roof to implement. If your roof doesn’t have a lot of additional loading capacity, you may be limited to an extensive green roof, which is defined as having 6 inches or less of growing media. "Extensive green roofs are characterized by the use of relatively low maintenance, drought-tolerant plant material, such as sedums, sedges, or some ornamental grasses or low-growing groundcovers," Duke says.

If you have ample additional loading capacity, you have the choice of an extensive system or an intensive system. Intensive systems generally have 6 inches or more of growing media. This deeper soil profile will support a wider range of plants – including woody shrubs and even small trees. Intensive vegetated roofs often incorporate human use and are what many people would consider a true roof garden.

No matter your reason for implementing a green roofing system, the benefits are numerous – for both the environment and your bottom line. More green in your community can equal more green in your pocket.

Kylie Wroblaski was an Associate Editor of BUILDINGS.

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