By Matt Barmore
Vegetative roofs create a rich variety of benefits and paybacks for facility owners and their occupants. These benefits include readily accessible green environments; habitats for birds, butterflies and insects; and even new spaces entirely, like playgrounds at city schools or childcare facilities. They help mitigate the urban heat island effect by providing shade and removing heat from the air through evapotranspiration; this ultimately leads to a cooler air temperature above a roof’s surface. Green roofs can also serve as outstanding tools for stormwater management.
With a rapidly growing number of municipalities now offering financial incentives for installing vegetative roofs, it comes as no surprise that their prevalence is rising right along with their associated monetary and environmental savings. What is astonishing to some building owners and facility managers is the increased considerations necessary to ensure their investment is properly installed and maintained. Questions that facility professionals should keep in mind include:
- What modifications may be necessary to the current building to ensure a leak-free vegetative roof installation?
- What materials and procedures will be needed to maintain the installed vegetative roof?
- What contingencies should be created to protect the roof from future leak and damage expenses?
- Where can I turn to for help maintaining the vegetative roof and roof system?
These issues are addressed below, as general considerations where green roofs are either planned, or already installed.
Vegetative roof weights can range from 16 to 60 pounds per square foot, depending on the depth of the growth medium, types of plantings, and accumulated weight of other additions, such as pavers or water features. Always ask a manufacturer or installer to provide you with the maximum saturated weight of the total green roof system. While the water-holding capacity of green roof growing media greatly benefits stormwater management, it does add considerable weight.
Can vegetative roofs be “too light?” Yes. Materials used to create extremely lightweight systems—16 pounds per square foot or lighter—can actually be detrimental to long-term plant health and viability. For example, while volcanic rock is light and fluffy, not all plants will thrive in it. Why spend the time and money to install and maintain a vegetative roof only to have the plants die after just two years?
2. Structural Considerations
Structural enhancements are not necessary for vegetative roof installations over most concrete decks; very seldom needed for metal roof decks; and sometimes required for wood decks. These factors will guide the choices made by designers—architects, landscape architects, or other building and design professionals—regarding the scope of a vegetative roof project including depth of the media and types of plantings. For instance, a planned “intensive” vegetative roof system with a depth of 8 or more inches may need to be modified to a semi-intensive or extensive system to meet weight and load parameters.
Always consult a structural engineer, preferably early in the pre-design process of a vegetative roof project. Structural engineers can best consider the factors that will affect building performance and integrity (including structural load) before providing building-specific requirements for a vegetative roof system. Make sure your building architect includes the structural engineer’s requirements in the project specification.
3. The Underlying Roofing System
Design decisions such as the placement of drains or the size of non-vegetated areas can affect an underlying roof system. The system’s care should begin with a thorough inspection by a manufacturer’s technical representative prior to installing a green roof. The inspector would determine any issues that could lead to roof leaks, and in turn, provide their feedback to the roofing contractor before assembly. The actual installation of a vegetative roof system should also be overseen by a roofing professional to ensure the underlying roof remains in good condition and is not damaged during the process.
4. Installation: Landscaper or Roofer?
Both offer benefits. Landscapers know plants and installation techniques, but aren’t typically familiar with the precautions vital to keeping a leak-free roof. Roofers know how to protect roofs during installations, but generally are not familiar with installing vegetative systems.
Roofing manufacturer warranties may also require that their licensed contractor installs the green roof. Consult manufacturers for recommendations. There are also specialized green roof installers —sub-contractors who are experts in green roofs and understand both plants and how to protect roofing systems from damage.
5. Routine Maintenance
Caring for a vegetative roof takes proper planning and consistent attention, including the following:
- Drains should be checked monthly to ensure they remain clear of debris and moisture is flowing freely.
- Irrigation systems should be checked monthly for leaks in the system and to verify the amount of water dictated in the irrigation plan is being administered.
- Plants should be inspected frequently—weekly during the first quarter after installation, less often thereafter—to ensure they are rooted properly and receiving the right amount of moisture.
- Growing media should be tested every three years for the correct chemical balance.
6. Long-term Best Practices
Design plans should include considerations for the removal of a vegetative roof system to allow for the investigation of leaks. The outlined approach may include both a roofing and green roof professional to guarantee that each system is treated carefully during the removal of vegetative components. Eliminating modular vegetative roof systems is generally faster and less disruptive to plantings than removing built-in-place systems. Likewise, replacing the modules following any leak repairs or maintenance to vegetative roof components will also generally be quicker.
When planned thoroughly and managed effectively, a vegetative roof can be a tremendous economic, environmental, and social asset for a facility. Facility executives’ input during the processes of designing, installing, and maintaining the green roof can ensure that a building’s new vegetative roof system achieves all the intended benefits.
Barmore is product manager, system enhancements at Firestone Building Products. In this role, he directs the company’s green building envelope product lines. Prior to joining Firestone, he served as an officer in the U.S. Air Force and deployed to the Middle East. He holds a bachelor’s degree from Indiana State University, a master’s degree from Bethel Seminary, and is completing his MBA at the San Francisco Institute of Architecture.