Do I really have to bring the lawn mower up to the roof?" asked the facility manager after his company had just installed a 4000 sq ft green roof.
The direct answer is "No." But, regular maintenance is just as important to green roofs as to any other roofing system.
With vegetated roofs having gained strong public interest in the United States over the last decade, there is widespread appreciation for the intricacies involved in building a green roof. Now, though, facility managers, engineers and others interested in and/ or involved with this emerging technology have new questions, including: "How do I maintain my green roof once it's installed? "
Green roofs are living systems. Thus, regular and proper maintenance, on an ongoing basis, is vitally important in order for them to survive and succeed. This is especially crucial in the first two years after a green roof installation. In fact, green roofs must be attended to much more frequently in the first two years. Furthermore, because such a system is a roof environment, all safety precautions and OSHA regulations still need to be implemented.
The initial cost for installation of a green roof can be one-and-a-half to two times the cost of a traditional roof. With proper maintenance, however, a green roof can double the life expectancy of a roof. Add to that the cost savings for heating and cooling a green building and, amortized over the life of the roof, green ones—if properly maintained—come out on top economically.
Once installed, property management staff can be left a bit perplexed as to what to do, or who to hire to carry out the job of green roof maintenance safely and correctly. Green roof installers are often a good place to start. Some of them offer green roof maintenance as part of the recommended ongoing preventive maintenance that is imperative to the care of any roof system.
If your organization is considering a green roof for your facility—or if you've already installed one—here are some things you'll want to keep in mind going forward.
Green roof maintenance 101
There are two basic types of green roofs—extensive and intensive.
Extensive green roofs are lightweight veneer systems with thin layers of drought-tolerant, self-seeding vegetated roof covers requiring little or no irrigation or fertilization after establishment. They are built when the primary desire is for an ecological roof cover with limited human access.
On extensive green roofs, vegetation should grow to cover the soil surface, usually within two years after it is installed. Extensive vegetated roofs generally have three to six inches of engineered growing media and are designed to be self-sustaining over time. Drought tolerant plants, usually succulents, are planted and grow quickly over the soil surface. Most of the succulents—Sedum—have adventitious roots, meaning they can form new roots at the stems and leaves. Cutting back healthy plant material, distributing across the bare areas of the roof and irrigating for a few weeks is an economical method of re-establishment.
Engineered growing media is comprised of lightweight aggregates and minimal amounts of organic matter. The growing media is designed to be lightweight, not decay over time, and needs little amending to provide adequate nutrients to plant material.
The vegetated system can be walked on from time to time, but should not be used in a highly recreational setting. Walkways made of pavers or gravel ballast may be installed to guide maintenance workers to mechanical equipment.
Many times, the primary reason extensive green roofs are integrated into the building is to capture storm water. Calculations are done on a project basis to satisfy local ordinances, or to apply for green building, such as LEED, incentives. In this instance, it is important for as much of the roof to be covered with vegetation as possible. Overall, green roofs can retain and detain 60-100% of rainfall.
Intensive green roofs are more elaborately designed roof landscapes, such as roof gardens and underground parking garage roofs that are intended for human interaction. The growing media starts from about 8-12" and can range to 15' or more, depending on the loading capacity of the roof and the architectural and plant features that the building owner desires. Maintenance will need to be more frequent, resembling the needs of a typical ground landscape.
Aesthetics and usage…
Visually, one should expect the green roof to behave similarly to the landscape of the surrounding area. For example, the plants will go dormant in the winter around the same time the tree canopy loses its leaves. Some plants will die back and others are evergreen, but colors change to dark reds and browns. In the spring, growth will resume with warm days and rain showers, and plants will bloom throughout the growing season.
One matter that should be resolved between the owner and the facility staff ahead of the planting of the green roof is the expectations of the vegetated roof, including usage (as in, who will be visiting the roof).
A roof system that is only visited by roofers and mechanical crews providing periodic maintenance will not need to be maintained as frequently for aesthetics as one that is viewed daily through office windows or entertains frequent visitors.
Extensive systems may be designed with a specific pattern, often achieved from a bird's eye view. For example, representation of a theme for the building or client may be incorporated in the design. Many succulent plants are aggressive growers in this setting, and more frequent maintenance is imperative to achieve the desired aesthetic goals.
A newly installed green roof should be maintained monthly, as necessary. Temporary irrigation should be available for the first few months, and should saturate the system at least two or three times a week. Thereafter, irrigation should be weaned, with the intent that the vegetation will remain self-sustaining within the first year.
During hot and dry spells, the system should receive water. While irrigation seems counter-intuitive in a roof designed to capture and detain stormwater, irrigation is mandatory in order to have a healthy and functioning green roof system long-term.
Plant and media concerns…
Initially planted and allowed to fill in over time, there is an opportunity for unwanted plants to germinate, grow and seed themselves on the roof. For projects in temperate climates, weed pressure begins in early spring and continues throughout the year, including winter. In small green roofs, hand weeding may be the fastest and most effective method of removal. However, for larger projects, protocols should be agreed upon for use of alternative weed management techniques or approved chemicals.
Approved growing media is comprised of approximately 20% organic matter. Over time, German green roofs have shown the organic content is reduced to 7%. Within the first several years, additional fertilizer should be applied. The FLL German standards, as well as ASTM, recommend a very low rate of application, using slow release fertilizers. Commercially available organic fertilizers are an option.
The growing media should be evaluated to ensure proper drainage throughout the green roof system, and off the roof. Yearly pH testing will tell when the growing media should be amended with lime. One sign that the green roof is too acidic is the presence of moss. It is up to the owner whether to keep the moss. It is probably harmless, and can create a lush green color in the cool season, but it may not be desired by the roof owner.
Spring responsibilities include broadcasting with a slow-release fertilizer. Removing leaves and branches is recommended, but not necessary. Periodic weeding in the summer season will keep weed pressure low. In preparation for winter, irrigation water lines on green roofs need to be drained and cleaned before a freeze. During a mild winter, weeds should be pulled before they are allowed to flower and set seed.
Green roof maintenance is as critical to the success of a green roof as plant selection, climate and other installation criteria. Without regard for the care or maintenance of a green roof once it has been installed, building and facility managers may not be adequately prepared to protect their building's asset long term. Moreover, they may not be able to reap the inherent benefits associated with green roofs and the role they play in sustainability. UM
Angie Durham is a green roof specialist with Magco, Inc. For more information on green roofs, contact her directly through Tecta America Corp. Telephone: (866) 832-8259; or visit www.tectaamerica.com or www.greenroof.com