Asumag 597 Thebigtop

The Big Top

July 1, 2008
Tips for installing and maintaining school green roofs.

Schools and universities throughout the United States now appear to be embracing green roofs as one of many environmentally friendly initiatives. This living rooftop, with all its associated benefits, has been something many have long wished for and has finally become a reality.

Once a green roof is in place, school managers soon realize they are not the same as typical landscaping. Although they tend to be relatively low-maintenance, the location and highly specialized plant types of a green roof mean that facilities personnel must learn how to care for and maintain these new systems — an aspect that sometimes can be overlooked during planning and installation.

Because a green roof is a living part of the building envelope, occasional to regular maintenance (depending on the scope of each project) is as vital to the success of the green roof as the objective, design, plant selection, climate and other project specifics. The question becomes: How do schools most effectively maintain a green roof?

This question has financial implications — the initial investment to install a green roof can be as much as two times that of a conventional roof. Proper maintenance increases the likelihood of a successful green roof project, making the return on investment well worth the upfront cost of the roofing system. Further, when the cost savings for heating and cooling a facility crowned with a green roof enter into the equation, a green roof becomes an even better value — but again, only when maintained properly.

Preliminary considerations

Care and maintenance concerns begin before a green roof is installed. The contractor, engineer or green roof manufacturer will have to conduct a preliminary inspection to determine whether the existing roof can support a green roof. The wet weight of a built-in-place green roof can range from 15 to as much as 50 pounds per square foot; the wet weight of a modular system, a newer technology, often is lighter, ranging from roughly 15 to 18 pounds per square foot for a 4-inch-deep module, to 30 to 35 pounds per square foot for an 8-inch-deep module. Whichever system is used, an expert should be brought in to ensure the green roof's weight will not jeopardize the structural integrity of the building.

Furthermore, especially when installing a built-in-place green roof, some type of irrigation system (typically temporary, but occasionally permanent) may be required to ensure proper plant rooting and survival. Depending on the location and project, some green roofs such as modular systems allow for pre-establishment, potentially minimizing or eliminating the need for regular irrigation during the first four to six weeks after installation.

To maximize plant survival and maintain reasonable levels of post-installation maintenance, including watering, the best time to install a green roof in most temperate parts of the country is between April 1 and October 15. Southern climates have a longer growing season, thus a wider window for green roof installation; however, areas that routinely experience midsummer heat waves or droughts may want to avoid green roof installation at this time. Pre-established modular systems can be more flexible with installation dates if the plants are well-rooted. To avoid costly rework or repairs, especially with a layered, built-in-place system, a green roof should be installed only over a new roof or one that is determined to be in good condition.

Initial maintenance

Immediately after a green roof is installed, it's important to address plant irrigation. Temporary irrigation either in the form of an installed system or hand watering should be available for the first growing season and should thoroughly moisten the growing medium at least twice a week in the absence of natural precipitation. The vegetation then should be allowed to adapt to its environment by decreasing the watering frequencies to a monthly occurrence or only during drought conditions, depending on plant selection. The objective is for the vegetation to become self-sustaining in six months to a year. Availability of water when necessary is essential for the long-term health of a green roof system.

One common concern with having a living roof is the presence of undesirable vegetation (weeds). In time, it will be natural for some weeds to germinate and grow in a green roof system. Given the nature of most green roof media, hand weeding often is the most efficient and environmentally benign way to remove weeds and allow the intended plantings to flourish. Some school officials may think that any plant, weed or otherwise, on their green roof is acceptable, but it is important to realize that if weeds begin to take over the designed plantings, the likelihood of failure increases.

This happens for a couple of reasons. First, weeds generally are annual, and second, they typically are not as drought-tolerant as green roof plants. As a result, when the weeds die at various times during the year, significant portions of the green roof could be lost and require replanting. The root aggressiveness of some weeds, such as fast-growing trees (cottonwood, maple, etc.), also is a legitimate concern. Owners should be sure to obtain plant guides with pictures from their green roof provider so they can be sure which plants belong and which ones don't.

In the spring, some green roof systems may benefit from an application of slow-release fertilizer, preferably organic, to prevent excess nutrient runoff or the buildup of soluble salts. Cutting back ornamental grasses and removing leaves, branches and other unwanted debris is recommended so the green roof system can perform at the highest possible level. In anticipation of winter, school workers may need to drain and clean irrigation water lines prior to a freeze. Workers also should pull weeds before they flower and seed during the course of a mild winter.

It will be necessary to inspect the roof for debris and oversee its general well-being, including all standard roof maintenance and inspections (roof drains, flashing, etc). Some vegetation may need to be replaced or transplanted during the course of a year; climatic cycles will influence which of the green roof plants survive best. Generally, green roofs can thrive for years with minimal attention given the proper soil media and a mix of complementary vegetation.

Plants and growing media

Maintenance needs also will be determined by the selected growing media. Engineered growing media for green roofs has been refined in the United States in the past 10 years. It is made up of lightweight aggregates, micro- and macronutrients, and small amounts of organic matter. The growing media is designed to be lightweight; well-drained yet with adequate water-holding capacity for irrigation; physically stable over time; and able to provide sufficient nutrients to plant material. By contrast, traditional garden soil may not contain the proper amounts of nutrients, soil moisture properties, organic matter and other characteristics to promote plant vigor and ensure plant survival in the extreme environment of a rooftop in such shallow depths. Garden soil also will have a much wider weight range between dry and wet states, with wet weights dramatically higher than green roof media.

In Germany, where more than one of every 10 buildings has a green roof, studies show a slow reduction in the soil's organic matter each year. As a result, additional fertilizer may be needed on some green roof systems. A low rate of application using slow-release fertilizers is recommended by both the FLL (the Research Society for Landscape Development and Landscape Design: Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V.) German standards, as well as the American Society for Testing and Materials.

Yearly pH testing will indicate when the growing media should be amended. One indication that a green roof is too acidic is the establishment and growth of mosses. School facility managers will need to determine whether the moss is desirable. It can create lush green color during the cold season, but facility managers may not want to allow the moss to continue growing if it appears to be stunting the growth of desirable green roof plants or creating areas of exposed soil media if weather conditions result in moss death.

Most growth on a green roof will occur during the spring and fall months. Plants will bloom in the growing season, and plants can be selected to create a beautiful, multi-colored landscape that changes throughout the warm months. As fall approaches, some plants will change to common autumn colors; others — except for semi-evergreen varieties — will go dormant during the winter. All primary plants in a green roof system will be perennial and return when the weather breaks in spring. If winter aesthetics are a concern, a number of proven green roof plants that maintain winter interest can be included in the green roof design.

Sidebar: Types of green roofs

Maintenance decisions for green roofs may be influenced by the type of system installed: extensive (or low-profile) and intensive (or high-profile). Extensive green roofs, the most common type installed today, are lightweight systems with thin layers of drought-resistant, self-propagating vegetation, usually requiring little or no irrigation or fertilization after establishment.

Vegetation for extensive green roofs should grow as a groundcover to cover the soil surface, usually within two years after being planted. While 4 inches of soil media is the industry standard for extensive green roofs, they may vary in depth from 2 to as much as 6 inches of engineered growth media. Aesthetic decisions can influence the overall design, but the primary goals of an extensive green roof are for it to be self-sustaining over time, with limited routine maintenance and relatively low capital costs. Drought-tolerant vegetation, usually composed of various succulent types, is planted and grows quickly over the soil surface. Because of the self-propagating nature of many varieties, one economical method of re-establishment or addressing bare spots is to distribute cuttings from established plants across the bare areas of the roof and keep them moist until roots have developed.

Intensive green roofs most often are intricately designed roof landscapes and may require all the expected attention and maintenance concerns of ground-level landscaping. The growing media for intensive roofs ranges from about 8 to 12 inches, but can measure beyond 10 feet, depending on the loading capacity of the roof, and the architectural and plant features desired by the building owner. Regular maintenance will be required for these applications.

There are built-in-place systems where the green roof essentially is built on top of an existing roof, as well as modular systems available for both intensive and extensive green roofs. With a modular system, the growing and plant media are preplanted into 100 percent recycled plastic modules that are then lifted and installed directly onto a roof for an instant green roof. The process tends to be more streamlined and, as a result, less labor-intensive, thus potentially lowering installation costs. Built-in-place systems often are selected where very deep soils are required, on steeply sloped roofs, or in instances where extremely curvilinear designs are specified.

For most green roof applications, however, a modular system should be evaluated given the potential cost savings and ease of future waterproofing maintenance. A building actually can be reroofed and the green roof easily staged for reuse when a modular system is installed.

Sidebar: It pays to be green

Three of the key benefits of a green roofing system:

  • Reduced energy costs

    When the outside air temperature reaches 80°F, surface temperatures of traditional black rooftops can be as high as 140°F. Rooftop heat loads affect the amount of energy necessary to cool buildings. Because of their shading properties and active cooling through evapotranspiration, along with the additional thermal mass of the soil media, green roofs have consistently and significantly reduced rooftop heat loads in warm seasons, even more than reflective white roof membranes.

  • Stormwater management

    Especially in urban areas, more than 75 percent of rain can become surface runoff, depositing pollutants into waterways. Many older cities have combined stormwater and sanitary sewers where, in the event of even moderate (half-inch or less) rainfall, the municipal treatment facility can become overwhelmed, resulting in discharge of raw, untreated sewage into local waterways. By both reducing the quantity of runoff from roof surfaces and delaying runoff that does occur, wide adoption of green roof technology in a municipality can limit these overflow situations and protect local rivers and streams, as well as public health.

  • Reduced urban heat-island effect

    The urban heat-island effect occurs in most large cities. Roads and buildings absorb a significant amount of heat during daylight hours. This heat, in turn, is radiated into the atmosphere, causing further warming. To compound the problem, additional heat is emitted by vehicles and HVAC systems that are working hard to cool these heated buildings. The urban heat-island phenomenon actually has been shown to change weather patterns in some of these cities and also can contribute to dangerous conditions for the elderly or children living without air conditioning. By cooling rooftops and the surrounding air, green roofs can help counteract urban heat islands.

Markham is a LEED-Accredited Professional at Weston Solutions, West Chester, Pa., manufacturer of GreenGrid green roofing systems. 

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