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Strategies for Specifying Vegetative Roofs with Tim Pennigar


Tim Pennigar
Project Manager, Engineering & Operations
Duke University Health System
Durham, N.C.

A portion of the Duke University Medical Center roof features a vegetative system. How has the performance of that system impacted your decision to specify a vegetative roof for two new construction projects - one addition to Duke University Hospital and a new cancer center - that comprise an $800 million expansion?
The vegetative roof is growing over the main entry of Duke Hospital, which is about 7,000 square feet of roof area. We installed it in May 2008 to guide our roofing decisions on the new hospital expansion. Following some preliminary research and vegetation trials, we selected a pre-cultivated mat system for this demonstration project.

The mats arrived on site much like rolls of sod, but instead of grass, they were pre-planted with several species of sedums - the succulent plants most common in green roofing. Like some of the higher-quality modular tray systems, the plants already were mature with at least 90 percent plant coverage. That feature proved to be valuable on the highly visible roof area over the main entry to the hospital.

The green transformation of an otherwise unlovely roof area almost overnight produced a very vocal and positive response from our patients and staff. The beneficial role of nature and natural views in the health care environment has long been understood, but now we are hearing of this benefit from the commercial-properties sector, as well.
Some property managers now see a vegetative roof as an important key to attracting and retaining quality tenants. Seeing the psychological benefits of a vegetative green roof has been the biggest surprise of our experience, and this aspect likely will remain an important driver in our future planning.
We've experienced nothing so far that would discourage us from proceeding with future vegetative roofs. Having buy-in and ownership at the facility-operations level has been vital to our success.

Facility managers often are involved too late in the planning and design of new construction projects. They inherit a new green roof they did not ask for. Many times, they were not trained on maintaining the roof. When this happens, the outcome is predictable.
Obviously, if the new vegetative roof does not perform as a reliable roof and watertight barrier, no one is going to be happy. Duke's long-term experience with protected membrane roofing has been a real advantage. We have several sod-covered building sections that have performed flawlessly for more than 40 years.

What are the biggest challenges related to retrofitting a more conventional, existing roof with a vegetative system?
Assuming you have successfully navigated all the marketing madness and have settled on a competent design, product, and installation team, one of the biggest challenges on a retrofit is doing a thorough assessment of leak history and existing conditions in your building.
You do not want to be digging up your new green roof searching for an illusionary roof leak that just happens to be caused by an old exterior wall defect. You need to know how your existing building has leaked and how it might tend to leak in the future. You can do this by having a competent engineering assessment of your as-built drawings and actual site conditions.  This preliminary study is a good time to confirm your building structure can actually support the weight of a new vegetative roof because the saturated weight on some of these systems can be a bit scary.

How can managers learn to maintain vegetative systems to ensure the roof performs as its designers intended?
First, I would make sure your designer actually had intent, other than simply grabbing LEED points or award-winning views. It is critical for the facility manager to be actively involved in conception through design. Managers definitely should lean on the vegetative system provider and warranty holder for instruction and training on proper care and maintenance.

Small pilot or demonstration projects are an excellent way to learn. We conducted several small test plots many months before we actually installed our hospital entry roof. Sometimes, we tried to kill the vegetation just so we would know how.

What are three key maintenance requirements associated with a vegetative roof that managers do not have to consider with conventional systems?
Often, a conventional roof is out of sight, out of mind. You really do not want to employ that mentality with a vegetative roof.
There will be some limited irrigation required during the first year or so of plant growth. You need to think about irrigation needs early in design, not as an afterthought. Supplemental irrigation might an on-going requirement, depending on climate.

You also will need to control weeds occasionally. We have not found this to be a very demanding task. The vegetative roof will begin to choke out weed grow as the sedums mature and completely cover the roof. You also will want to monitor the health of your vegetation over time. Poor aeration, over-watering, and excessive foot traffic can threaten the long-term success of your project.

Duke is heavily involved with vegetative roofs. What is the next green roofing technology on the horizon for your organization?
We are planning some photovoltaic (PV) demonstration sites on the medical campus. We understand some truly amazing developments are on the horizon in the PV industry, but we hope the payback periods continue to compress.

We are working on new schemes for storm-water reclamation from roofing on the campus. The severe drought in the Southeast last year made us look more closely at the way we manage storm water.

We also have been involved in research and test plots of ornamental moss for green roof applications. As more managers begin to look at retrofitting existing buildings with vegetative roofing, they likely will deal with roof areas that are not ideal environments for traditional vegetation. They might be too shady, for example. We have several moss roofs on campus that are putting on a good show right now in shady courtyard roofs.

posted:  3/5/2009