By James R. Kirby, AIA
Published in the August 2009 issue of Facility Executive
By definition, a vegetative roof system (often referred to as a green roof) is a roof area of plantings/landscaping installed above a waterproofed substrate at any building level located over habitable space. And yes, vegetation can safely be placed on top of roofs, transforming these structures into gardens of plants or a landscaped area to visit.
The structure itself consists of a waterproofing system and its associated components—such as protection course, root barrier, drainage layer, thermal insulation, and aeration layer—and an overburden of growth medium and plantings. System types are defined by the landscaping portion: extensive (shallow), semi-intensive (moderate depth), and intensive (deep).
So why would anyone want to have a vegetative roof system? There are a number of possible benefits best pondered if facility managers (fms) look at the positive aspects in two distinct categories: the possible benefits that are good for the building; and the possible benefits that are good for the environment.
Vegetative roof systems also can be a marketing and business tool. In other words, they clearly make the statement: “I own this building, and I care about the environment!”
Depending on location and sight lines, a vegetative roof system can not only benefit building occupants, it may also provide visual benefits on a larger scale. There is a general consensus that humans appreciate viewing nature and natural settings. And in some ways, a vegetative roof system is simply the relocation of the landscape that was displaced for the building and then moved from ground level to rooftop!
A vegetative roof system can be good for the building by increasing the thermal value of the roof assembly, reducing sound transmission, reducing rooftop temperatures, and creating additional usable outdoor space. Reduced rooftop temperatures help decrease the stresses placed on roof and waterproofing systems; this is a likely contributor to extending service life.
As a greening measure, a vegetative roof system could mean a more saleable or leasable building. If a building includes other green improvements like a photovoltaic (PV) system, integrating a vegetative roof system with a PV system can help keep the PV efficiency at its highest by bringing rooftop temperatures nearer to a PV system’s most efficient operating temperature (about 75?F to 80?F).
A vegetative roof system may also be used as a community green space. There are other environmental benefits of these roofing systems, including: the mitigation of heat island effects; improvement of air quality; creation of wildlife habitats and biodiversity; and management of storm water.
Storm water retention helps reduce and slow the amount of water that reaches the public storm water system. Reducing the flow of storm water into these systems can help reduce the need to upsize existing urban storm water management approaches. Most important, a vegetative roof system can be used to help keep water out of a building.
It’s All In The Membrane
The National Roofing Contractors Association (NRCA) recommends the following waterproofing membranes for vegetative systems:
- Hot fluid applied polymer modified asphalt membranes, minimum 215 mils thick, reinforced;
- APP and SBS polymer modified bitumen membranes, minimum two plies, reinforced;
- PVC membranes, minimum 60 mils thick, reinforced;
- EPDM (ethylene propylene diene M-class rubber) membranes, minimum 60 mils thick, reinforced; and
- One and two component elastomeric membranes, minimum two coats, reinforced.
NRCA recommends a waterproofing membrane be adhered to a solid substrate. Mechanically attached systems and loose laid and ballasted systems are not recommended for vegetative roof systems. NRCA also recommends the thermal insulation be installed above the waterproofing membrane.
The use of an adhered membrane with insulation above provides a waterproofing system that minimizes the potential for water to migrate laterally beneath the membrane. If a breach in the membrane occurs, horizontal water movement is not expected, therefore making repair of problems simpler and less labor intensive.
Quality materials, good design, and a proper installation and maintenance plan provide the basis for a successful vegetative roof system. Getting the job done properly will help fms ensure the possibility of a long service life of the roof.
Decisions made by the fm during the design and construction stages can affect the performance of the roof for years to come. Value engineering is not necessarily beneficial to the expectation for the long-term service life of a vegetative roof system.
In terms of planning, construction details should be conservative. This means fms should do whatever they can to protect the vertical membrane (including extending the flashings above the growth medium). Under any circumstances, in-house maintenance crews should be given access to flashings, and fms should feel free to dictate the installation of vegetation-free zones. It may also be wise for fms to ask contractors to install counterflashing.
Codes And Ratings
The International Building Code (IBC) 2009 Edition now includes specific language that “roof gardens” and “landscaped roofs” need to meet the requirements of the roofing chapter (Chapter 15, Roof Assemblies and Rooftop Structures). Fms should screen the team of designers, contractors, and manufacturers to make certain everyone is achieving appropriate building code compliance, especially for fire and wind resistance.
A green building—one perhaps with a vegetative roof system—may even speed up the building code permitting process. This of course depends on the local jurisdiction’s priorities.
Another point to consider is how much of the roof should be covered with vegetation to achieve the desired results? Municipalities and current building rating systems give credit (whether for monetary benefits or point accumulation) for covering 50% of the roof area. But will as many benefits be realized by only covering half the rooftop?
The bonuses that go along with reducing rooftop temperatures, protecting the roof or waterproofing membrane, managing storm water, improving thermal value, and cutting sound transmission may only be partially realized—unless they are eliminated entirely. Reducing rooftop temperatures for only half the roof area and protecting only half of the roofing or waterproofing membrane realistically means that “increasing membrane service life” should not be a factor when deciding to install a vegetative roof system.
Fms hoping to assume extended service as part of a life cycle cost analysis of a vegetative roof system will need to cover the entire roof area with the system. Twice as much storm water will be managed, and thermal and sound transmission values will be consistent across the rooftop.
The Role Of The Fm
Fms should be active participants in any vegetative roofing system project. They should make sure to participate in the following decisions:
Integrity testing of the waterproofing membrane. After installation of the membrane and before additional components and trades are allowed to proceed with work, a membrane integrity test should be performed. A membrane integrity test can be performed in a number of ways: a standing water flood test; a flowing water test; and electronic field vector mapping. A membrane should be determined to be waterproof before proceeding with any additional construction activities.
Post-installation maintenance. Although waterproofing the building and keeping water out is the overriding point of installing any roof system, a vegetative roof system also includes the key issue of plant success. Fms should look to the installing contractor(s) to establish specific waterproofing system and plant maintenance plans. If the plants do not survive, or if leaks occur in the future, the expectations of the building cannot be met. Working with the landscaping contractor (who may be a part of—or a subcontractor to—the vegetative roofing contractor) to establish a maintenance plan for at least the first year greatly increases the likelihood of plant success.
Plant maintenance in the early stages primarily means establishing a regular watering schedule to make sure the plants get the required amount of water. Waterproofing system maintenance is recommended at least twice a year—in the spring and in the fall—and after any significant weather events, such as high winds or hailstorms.
For any vegetative roof system project, fms should strive for an integrated design process, engage all participants, and be active participants in the process. The decisions made during the process affect the outcome and performance for years to come.
Kirby (firstname.lastname@example.org), AIA, associate executive director of the National Roofing Contractors Association (www.nrca.net), joined the staff of the NRCA in 1996. He is responsible for ensuring continuity of NRCA’s technical policy and assists with communicating it throughout the roofing industry.