GREEN ROOFS – A Primer By Heather Ellis, FLC Environmental Center ‐Fall 2007‐ OVERVIEW Green Roofs are essentially thin layers of living vegetation installed on top of conventional flat or sloping roofs. Green roofs protect conventional roof waterproofing systems while adding a wide range of ecological and aesthetic benefits. They are a powerful tool in combating the adverse impacts of land development and loss of open space. A well‐planted roof absorbs solar radiation and CO2, decreases storm‐water runoff, provides valuable insulation, and numerous other benefits (economic, environmental, and aesthetic). Green roofs can take a thousand different forms depending on location, but their structure is pretty basic: a waterproof layer over a standard roof, topped with drainage materials, soil, and finally, the plantings, which vary from region to region. (New, modular systems allow one to create a green roof from premade blocks, greatly simplifying construction). Your green roof may be a thick layer of grass, or a garden with food, to a forest‐like rooftop. BASIC TYPES OF GREEN ROOFS Types of green roofs fit into two basic categories: (1) intensive or (2) extensive. This categorization is based on soil depth and also determines a variety of factors including weight, cost, maintenance, plant selection and function. Intensive green roofs require a minimum of one foot of soil depth and generally require more effort for the maintenance of plants and proper watering schedules. Intensive green roofs involve a greater diversity of plants, including trees and shrubs. These roofs emphasize the use of space and therefore raise higher aesthetic expectation than more functional extensive green roofs. There are certain degrees of intensiveness in the approach to rooftop greening. Intensive green roofs are generally more expensive than extensive green roofs. Extensive green roofs are roofs that call for a more passive approach than intensive. The vegetation used within an extensive roof is extremely resilient, providing a lush carpet‐like foliage. Plants for an extensive green roof are those that require only a few inches of growing medium, from 2.5 to 6 inches. Extensive green roofs can be established on a very thin layer of “soil” (most use specially formulated composts): even a thin layer of Rockwool laid directly onto a watertight roof can support a planting of Sedum species and mosses. Sedum species are plants with a waxy leaf that retains moisture. An extensive green roof will only require minimal maintenance and irrigation will only need to be addressed in drought conditions. Extensive green roofs are designed to be virtually self‐sustaining.
Table 1. Characteristics of Intensive and Extensive Green Roofs. Characteristics of Intensive and Extensive Green Roofs Intensive Extensive Characteristic Green Roof Green Roof Requires minimum of Requires only 1 to 5 inches of SOIL one foot of soil depth soil depth Accommodates large Capable of including many VEGETATION trees, shrubs, and well‐ kinds of vegetative ground maintained gardens cover and grasses Adds only 12‐50 pounds per Adds 80‐150 pounds per square foot depending on LOAD square foot of building soil characteristics and the load to structure type of substrate Regular access Usually not designed for ACCESS accommodated and public accessibility encouraged Annual maintenance walks Significant maintenance MAINTENANCE should be performed until required plants fill in Includes complex Irrigation and drainage DRAINAGE irrigation and drainage systems are simple systems Questions: What is the soil depth required of different native species? What is cost savings in terms of energy from the solar absorption and insulation effects of a green roof? Is there enough CO2 absorption to qualify as a carbon offset? By how much can they extend the life of a roof? BENEFITS of GREEN ROOFS Green roofs have numerous benefits, from ecological to aesthetic to environmental benefits. Green roofs are an attractive roofing option that can reduce urban heat islands by providing shade and through evapotranspiration, the release of water from plants to the surrounding air. Green roofs can also absorb air pollution, collect airborne particulates, and store carbon. They protect the underlying roof material by eliminating exposure to the sun’s ultraviolet (UV) radiation and extreme daily temperature fluctuations and can thus extend the roof’s life. Green roofs also serve as living environments that provide habitats for birds and other small animals. They can also be an area for food production: for
example, the Fairmount Waterfront Hotel in Vancouver uses their green roof to grow herbs, flowers, and vegetables, which saves its kitchen an estimated $30,000 a year in food costs. They offer an attractive alternative to traditional roofs, addressing growing concerns about urban quality of life. Green roofs reduce noise transfer from the outdoors. They insulate a building from extreme temperatures, mainly by keeping the building interior cool in the summer and warm in the winter (and can save on heating and cooling costs). Green roofs play a large role in mitigating storm water runoff as well. As impermeable surfaces like buildings and pavement replace open space and vegetation, green roofs can play an increasingly important role in storm water management. During rainstorms, green roofs act as a sponge, absorbing much of the water that would otherwise run off, carrying numerous pollutants with it. Researchers estimate that just three to five inches of soil or growing medium absorbs 75% of rain events that are one‐half inch or less. Green roofs can also filter pollution from rainwater. This is achieved by the root systems’ bacteria and fungi, which utilize the natural filtering processes of bioremediation and phytoremediation. As a result, the non‐point source pollutants, nitrogen and phosphorous, are broken down and detoxified. This beneficial process increases over time as rooftop plants and root systems mature. The economic benefits of green roofs are numerous as well. Green roofs have the potential to reduce the size of HVAC equipment on new or retrofitted buildings (capital and operation savings); the potential to reduce the amount of standard insulation used; the potential to incorporate cooling and/or water treatment functions; the potential for local, regional, and national market exposure depending on the uniqueness of the project; the potential to reduce or eliminate roof drains; the potential to meet regulatory requirements for stormwater management; as well as the potential to reduce community resistance to new development. GREEN ROOFS and LEED Credits Green roofs can facilitate a significant improvement in the LEED rating of a building, contributing as many as 15 credits under the system, depending on design and level of integration with other building systems. In some instances, while green roofs may not contribute directly to achieving points under the system, they contribute to earning LEED credits when used with other sustainable building elements. For example, green roofs can earn direct credits under the following: (1) reduced site disturbance, protect or restore open space, (2) landscape design that reduces urban heat islands, Roof, (3) storm water management, (4) water efficient landscaping, (5) innovative wastewater technologies, and (6) innovation in design. FEATURES of GREEN ROOFS A green roof that is designed well must include subsystems responsible for the following:
(1) Drainage: Green roof drainage design must both maintain optimum growing conditions in the growth medium and manage heavy rainfall without sustaining damage due to the erosion or ponding of water. (2) Plant nourishment and support: The engineered medium must meet exacting requirements for grain‐size distribution, void ratio, moisture retention, etc. (3) Protection of underlying waterproofing systems: Green roof assemblies must protect the underlying waterproofing system from human activities (including the impact of maintenance) and biological attack. Principal Green Roof Technology Components Source: National Research Council, Institute for Research in Construction Basic Diagram of Green Roof System: source http://www.Greenroofs.org IMPORTANT INFORMATION/DATA to HAVE BEFORE CONSTRUCTION One will need to know the slope, the structural loading capacity, and existing materials of the roof, as well as the nature of any drainage systems, waterproofing, and electrical and water supply in place. You should also consider who would have access to the roof, who will do maintenance, and what kind of sun and wind exposure the roof gets. One should also research the types of plants that do well in your specific environment. Although information about green roofs is available, installation specifics are mostly proprietary and performance data are largely anecdotal. In either case, there is little actual research information available for the United States. Much of the popular semi‐technical literature is only written in German, which effectively slows its adoption by Americans. GREEN ROOF COSTS: General/Approximate Costs The cost of a green roof varies considerably depending on the type and factors such as the depth of growing medium, selected plants, size of installation, use of irrigation, and whether they are to be
accessible on inaccessible‐ intensive or extensive. Intensive green roofs typically require greater investment but confer the benefits of accessibility. An installed extensive green roof with root repellant/waterproof membranes may be installed for $10‐$24 US per square foot. While green roofs typically require a greater initial investment, it is important to keep in mind that they can extend the life of the roof membrane and reduce the heating and cooling costs of the building. It is useful to speak to a qualified green roof professional about the range of costs and benefits for different green roof systems and designs. I contacted Mr. Tom Mitchell from Weston Solutions (their Colorado Springs office) whom installs extensive green roof modular systems. He gave me an estimated cost for their 4” modular systems to be approximately $14 per square foot for our area with delivery from a nursery in Fort Collins. This company installed the modular green roof system on the EPA headquarters building in Denver, CO last fall. According to the United States Environmental Protection Agency, currently, the up‐front cost of an extensive green roof in the United States starts at about $8 per square foot, which includes materials, preparation work, and installation. In comparison, the cost of a traditional built‐up roof starts at about $1.25 while cool roof membranes start at approximately $1.50 per square foot. Extensive green roofs cost more than traditional roofs because they require more material and labor for installation. Another factor affecting price is that green roof contractors are limited in number. As the demand for rooftops gardens increases in the US, and as additional contractors come into business, up‐front costs will likely decrease. However, it is widely known that up‐front costs do not tell the whole story. Taking into account future summertime energy savings at the time of purchase brings the price of a green roof closer to that of a tradition roof. Depending on local construction codes, it also may be possible to do without storm water infrastructure investment. As one example, the city of Chicago states that their 10,000 square foot green rooftop saves them $10,000 per year in energy costs! Another factor reducing the cost of a green roof is that vegetation can extend the life of a roof. This is because less solar energy reaches the roof substrate, limiting damage from Ultraviolet radiation as well as daily temperature fluctuations, which cause repeated contraction and expansion. One should keep in mind that the cost of a green roof will depend on the type of roof, the structure of the building, and what plants can grow on the material that is on top of the roof. One should also look in to local, regional, state or nationwide incentives for constructing a green roof and potential donations from nurseries that could help with the initial cost of installation. GREEN ROOF CASE STUDIES
(1) The Colorado Architecture Partnership In Colorado Springs, the Colorado Architecture Partnership had an extensive green roof system installed on its 3,000 square foot roof with plants supplied by Green Roof Plants and Emory Knoll Farms. (2) The United States Environmental Protection Agency Denver, CO Office In 2006, the US EPA installed an extensive green roof system on its 19,396 square foot roof office building in Denver Colorado. The upper levels of the building feature a three‐level green roof. The extensive (3‐4 inches) planting scheme consists of grasses, perennials, and groundcovers selected in accordance with the United States Department of Agriculture hardiness zone classification in 2’x2’ or 2’x4’ modules with 4 inch soil depth. This green roof is expected to minimize heat island effects, absorb carbon dioxide, and contribute to an estimated 26.7 percent reduction in stormwater runoff. Below is a picture I received from Tom Mitchell of Weston Solutions who was responsible for the green roof modular system used on the EPA headquarters building in Denver (installed last fall). (3) Manchester, New Hampshire’s first Green Roof: The University of New Hampshire’s Cooperative Extensive Program teamed up with the city of Manchester to plant a green roof on Manchester City Hall’s Connector building as a demonstration project. The green roof is GreenGrid System which began in 2002. The GreenGrid (http://www.greengridroofs.com) system is geared toward the needs and requirements of industrial, commercial, and government facilities, offering a sleek, flexible, and cleverly simply designs. GreenGrid modules arrive at the site planted and ready for installation. These modules are composed of recycled plastics and can be placed directly on the roofing membrane or on any other surface. The system is lightweight and flexible (2‐inch module‐saturated weight= approximately 10lbs per ft2; 4‐inch module – saturated weight= approximately 15 lbs per ft2; 8‐inch module – saturated weight = approximately 28 lbs per ft2). With the GreenGrid system, the modules are pre‐planted with plants of the color and type selected by the client. The 4‐inch depth modules (Extensive systems) will
support grasses, sedums and wildflowers, while the 8‐inch modules (Intensive) will support a variety of large ornamental plants.