Extensive Green Roofs

by Charlie Miller, P.E.
Roofscapes, Inc.

Last updated: 05-17-2009

Introduction

Green roofs, also known as vegetated roof covers or eco-roofs, are 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 the loss of open space.

Green roofs are divided into two categories: 1) extensive green roofs, which are 6 inches or shallower and are frequently designed to satisfy specific engineering and performance goals, and 2) intensive green roofs, which may become quite deep and merge into more familiar on-structure plaza landscapes with promenades, lawn, large perennial plants, and trees. This guide addresses only the more shallow extensive green roofs.

The challenge in designing extensive green roofs is to replicate many of the benefits of green open space, while keeping them light and affordable. Thus, the new generation of green roofs relies on a marriage of the sciences of horticulture, waterproofing, and engineering.

The most common vegetated roof cover in temperate climates is a single un-irrigated 3- to 4-inch layer of lightweight growth media vegetated with succulent plants and herbs. In Germany, this simple design has demonstrated the highest benefit-to-cost ratio. In most climates, a properly designed 3-inch deep vegetated roof cover will provide a durable, low maintenance system that can realize the many benefits that green roofs have to offer.

Description

A. Features

All well-designed green roofs include subsystems responsible for:

• Drainage: Green roof drainage design must both maintain optimum growing conditions in the growth medium and manage heavy rainfall without sustaining damage due to erosion or ponding of water
• Plant nourishment and support: The engineered medium must meet exacting requirements for grain-size distribution, void ratio, moisture retention, etc. and
• 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.

A wide range of methods can achieve these functions. For instance, drainage layers may consist of plastic sheets, fabric or synthetic mats, or granular mineral layers. Similarly, the physical properties and performance characteristics of growing media (engineered soils) and plant materials may vary with the climate, plant community, or engineering requirements. Figure 2 shows a generic cut-away of a common type of green roof assembly that utilizes a lower granular drainage layer in combination with an upper growth medium or substrate.

The selection of a particular approach may depend on performance-related considerations, such as runoff control, drought-tolerance, biodiversity, appearance or accessibility to the public. While many pre-engineered systems are currently available, it is frequently necessary to customize these systems to satisfy specific performance objectives.

B. Ancillary Features

1. Waterproofing

Many premium waterproofing materials have a proven track record when used in combination with green roof installations. These include, but are not limited to polyvinyl chloride (PVC), thermal polyolefin, EPDM rubber, polymer modified bituminous sheet membranes (e.g., SBS membrane), liquid-applied rubberized-asphalt, and coal tar pitch. Other materials are likely to enter the industry as their suitability is proven in certification testing and prototype installations.

Worldwide, polymer modified bituminous membranes and PVCs are the most common. Many of these installations have now been in place for over 30 years and continue to perform as designed.

In all instances, materials, methods of installation, and quality assurance procedures must be more stringent when green roof installation is involved. Waterproofing material that cannot withstand decades of root and biological attack unaided must be protected with a supplemental root-barrier layer. For information and standards pertaining to waterproofing materials, consult the National Roofing Contractors Association (NRCA) or American Standard Testing Methods (ASTM).

2. Pitched Roof Installations

To install extensive vegetated roof covers on pitches steeper than 2.5:12 (12 degrees) supplemental measures will be required to prevent sliding instability. Varied building systems have been developed to support vegetated covers on steeply pitched roofs. Pitched roof systems merge into vertical fa¸ade greening techniques.

3. Wind Resistance Systems

Due to the unique physics of the green roof profile, attaching the elements of the green roof to the underlying structure is not usually important. The biomass bonds with the fabrics to create a unified cover, and the plants themselves create enough surface wind turbulence to foil potential uplift—the converse of an airfoil. If the green roof will be located in an unusually-high wind area, such as a high-rise building or a coastal region, appropriate ballasts should be specified. Guidelines for ballast requirements are available from European green roof providers.

4. Modular Systems

Modular systems involve installing the green roof system inside plastic trays. Use of these systems does not relieve the designer from responsibility for considering the integrity of the underlying waterproofing system, nor does it make location of damaged waterproofing easier. However, these systems can be useful when designing small gardens on residential property or terraced commercial roofs. They also preserve flexibility to re-arrange landscape designs in the future. Owners who wish to engage in active gardening will find modules a convenient way to do this without damage to their homes' waterproofing. Two companies in North America currently offer modular, or tray, components, GreenTech and Weston/ABC Supply.

5. Electric Leak Detection

Inexpensive methods for locating damaged waterproofing underneath vegetated covers are available. These include the electric field vector mapping (EFVM) procedure. This method works by charging the moist media layer of the green roof with electricity and then looking for electrical grounds caused by moisture in contact with an underlying steel or concrete deck structure.

C. Benefits

There are many potential benefits associated with green roofs. These include:

• Controlling storm water runoff
• Improving water quality
• Mitigating urban heat-island effects
• Prolonging the service life of roofing materials
• Conserving energy
• Reducing sound reflection and transmission
• Creating wildlife habitat, and
• Improving the aesthetic environment in both work and home settings.

As a result green roofs may be appropriate as an addition to many types of buildings, including commercial, industrial, institutional, and residential settings.

1. Controlling Storm Water Runoff

The rapid runoff of storm water from paved areas and roofs contributes to destructive flooding, erosion, pollution, and habitat destruction. The capacity of green roofs to moderate this runoff through both retention (water holding) and detention (flow-slowing) properties has been well-documented in Europe and increasingly in the United States. Green roofs share many engineering features with conventional storm water management basins, and compared to many at-grade storm water management practices, vegetated roof covers are unobtrusive, low maintenance, and reliable. Green roofs may offer the only practical "at-source" technique for controlling runoff in areas that already are highly urbanized.

Vegetated roof covers are particularly effective at controlling runoff on the large roofs typical of commercial and institutional buildings. They can be designed to achieve specified levels of storm water runoff control, including reductions in both total annual runoff volume (reductions of 50 to 60 percent are common) and peak runoff rates for storms.

Reliable techniques for predicting the rate and quantity of runoff from vegetated roof covers have been used successfully to design integrated storm water management measures in Germany, where large zero-discharge developments that rely heavily on green roofs are already operating. For example, the Bondorf transportation center (PDF 473 KB) in Sindelfingen achieves net zero storm water runoff discharge, largely through the use of 516,000 square feet (11.8 acres) of green roofs.

2. Improving Water Quality

By reducing both the volume and the rate of storm water runoff, green roofs benefit cities with combined sewer overflow (CSO) impacts. In cities with combined storm and waste water sewer systems, storm water dilutes the sanitary waste water, rendering treatment less efficient. During heavy rainfalls these systems also overflow, discharging raw sewage mixed with runoff into the receiving streams-resulting in ecological damage and human health hazards. Therefore, important water quality benefits are achieved by controlling runoff.

In addition, in urban areas, up to 30% of total nitrogen and total phosphorus released into receiving streams is derived from dust that accumulates on rooftops. Acting as natural bio-filtration devices, green roofs reduce this water contamination. In the Potsdamer Platz (PDF 205 KB) district of Berlin, extensive green roofs have been employed on a large scale in an effort to reduce pollution of the River Spree. This program has demonstrated that extensive green roofs can achieve large reductions in nutrient releases from roofs; however, the research also shows that the correct choices of growing medium and plant types are essential for success.

3. Mitigating Urban Heat-Island Effects

Covering dark conventional roofs with green roofs can significantly reduce the temperature above the roof. Green roofs have been shown to out-perform white or reflective roof surfaces in reducing the ambient air temperature. If sufficient urban surfaces are covered, this cooling (and attendant improvement of air quality) can have significant positive effects on human health, especially for the young and elderly in congested urban areas.

4. Prolonging the Service Life of Roofing Materials

Thirty-five years of experience with green roofs in Germany have demonstrated their value in protecting waterproofing materials. The multiple layers of the green roof protect the underlying roof materials from the elements in three ways: by protecting from mechanical damage (mostly from humans, but also from wind-blown dust and debris, and animals); by shielding from ultraviolet radiation; and by buffering temperature extremes, minimizing damage from the daily expansion and contraction of the roof materials.

A roof assembly that is covered with a green roof can be expected to outlast a comparable roof without a green roof by a factor of at least two, and often three. Although modern green roof systems have not yet been in place longer than 35 years, many researchers expect that these installations will last 50 years and longer before they require significant repair or replacement. For a building owner with a long-term investment in the roofing system, this benefit factor goes a long way toward paying back the initial investment in a green roof.

5. Conserving Energy

Not all benefits will be equally important in every project or climate. For instance, the capacity of green roofs to reduce heat flow, and therefore energy demand in buildings, is mostly a warm season phenomenon. As a result, this benefit will be realized most fully in warm climates, where energy expenditures on air conditioning are an important concern. Energy-related benefits will also be less important in multi-story buildings, due to the low ratio of roof area to the total of exposed building skin. Because green roofs are more complex than simple insulators, project-specific building envelope analysis is required to predict energy conservation under specific project conditions.

6. Reducing Sound Reflection and Transmission

Green roofs can absorb a portion of the sound that otherwise bounces off hard roofing surfaces At the Frankfurt International Airport, green roofs were employed successfully as a means of sound abatement along new runway approaches. A simple 3-inch deep vegetative cover can be expected to reduce sound transmission by a minimum of 5 decibels. Sound abatement of up to 46 decibels has been measured on thicker roofs.

7. Creating Wildlife Habitat

Green roofs can be used to create wildlife habitats to supplement or replace diminishing open space in developing areas. With thoughtful planting and avoiding pesticides, a mature, self-sustaining ecosystem will teem with insects, spiders, snails, and songbirds. Using native species can recreate lost prairies, as at the Oaklyn Branch Library in Indiana.

8. Improving the Aesthetic Environment

Green roofs offer interesting new opportunities for architectural design. A green roof can allow a structure to merge with the surrounding landscape, provide a dramatic accent, or reinforce the defining aspects of the structure's geometry. In Germany—and increasingly in the United States—green roofs are frequently integrated into the design of hospitals and care facilities in order to provide a more restful and restorative environment for patients. Similarly, multi-unit residences and hotels will find that green roof-tops views substantially enhance property values. In commercial settings, job satisfaction and effectiveness can be enhanced by providing window views of meadows or flower beds or relaxing garden areas for breaks or meetings.

Applications

A. Design Factors

There are many interactive factors that green roof designer must take into account, balancing many considerations for optimal performance in each setting, including:

• Climate, especially temperature and rainfall patterns
• Strength of the supporting structure
• Size, slope, height, and directional orientation of the roof
• Type of underlying waterproofing
• Drainage elements, such as drains, scuppers, buried conduits, and drain sheets
• Accessibility and intended use
• Visibility, compatibility with architecture, and owner's aesthetic preferences
• Fit with other "green" systems, such as solar panels
• Cost of materials and labor

B. Integration With Other Green Design

Green roofs can be designed in conjunction with solar panels and also work very well in combination with other 'low-impact' development measures, such as infiltration beds, rain gardens, bioretention systems, cisterns and rain barrels. It is common place in Germany to find large developments that have zero runoff discharge. In these developments, rainfall is captured on the green roofs, returned to ground water through infiltration, and re-used for irrigation, toilet flushing, etc.

C. Examples of Extensive Green Roofs in North America

Thirty-five years of German experience and research indicates that extensive green roofs will succeed in most climates, if properly designed. With appropriate plant selection, sufficient drainage, and adequate structural support for the additional dead weight, green roofs will survive winter ice build-up. However, buildings in arid (desert) zones may not be good candidates for extensive green roofs, due to the difficulties and expense of water distribution and retention.

In North America, examples of extensive green roof projects are present in most climate zones, including New England, Mid-Atlantic, Gulf Coast, Midwest, Pacific Northwest, and Southern California regions. Ten or fifteen of these were built prior to 2002, and can be expected to have reached maturity by spring of 2003.

Because the few North American roofs that have been built to date demonstrate such a wide variety of settings and approaches, it is impossible to highlight "representative" case studies here. However, many updated case studies of green roof projects, including both extensive and intensive designs, are available at Greenroofs.com.

Relevant Codes and Standards

In the United States, green roof designs are generally regulated using existing standards for ballasted roofs. The International Code Council (ICC) code, formerly the BOCA code, used for guidance by many municipal authorities, recognizes roof gardens. It requires that the 'wet weight' of the green roof be treated as an additional dead load. It also supplies live load requirements for maintenance-related foot traffic and for regulated pedestrian access. One limitation of the ICC standards is that it does not specify the testing methods to be used in satisfying the code. ICC also provides standards for parapet heights and requirements for railings.

Trade organizations such as National Roofing Contractors Association (NRCA) are developing guidelines for waterproofing with green roof installations in mind. In addition, American Standard Testing Methods (ASTM), through the Green Roof Task Group E06.71, is in the process of developing guidelines and testing procedures specifically for green roof products. For more information, see the Roofscapes web site.

However, at present, the only accepted guidelines for green roof construction are those developed by Forschungsgesellschaft Landschaftentwicklung Landschaftsbau. e.V. (FLL), in Germany (Guidelines for Planning, Installation, and Maintenance of Green Roofs, Richtlienien für die Plannung, Ausführung und Pflege von Dachbegrünungen, Forschungsgesellschaft Landschaftentwicklung Landschaftsbau. e.V.). These standards and guidelines are comprehensive, and include industry standard tests for medium weight, moisture, nutrient content, grain-size distribution, etc. The 1995 edition of the guide is available in English. The English translation can be purchased directly from FLL or through Roofscapes, Inc.. FLL also certifies laboratories to conduct critical tests such as the root penetration resistance of waterproofing membranes. Many green roof products available in the United States have FLL certification.

Federal Mandate

• Energy Policy Act of 2005 (PDF 1.9 MB, 550 pgs)

Additional Resources

WBDG

Design Objectives

Sustainable—Optimize Site Potential, Sustainable—Optimize Energy Use, Sustainable—Protect and Conserve Water

Products and Systems

Roofing Systems
Federal Green Construction Guide for Specifiers:

• 07 10 00 (07100) Dampproofing & Waterproofing
• 07 33 63 (02930) Vegetated Roof Covering
• 07 50 00 (07500) Membrane Roofing
• 07 55 63 (07530) Vegetated Protected Membrane Roofing

General Information

Non-commercial organizations that can provide current lists of green roof service providers and are a useful source of up-to-date information, include:

In addition, some regional groups and agencies have distinguished themselves in the promotion of green roofs. These include the Earthpledge Foundation in New York City, Northwest Eco-Builders Guild, and Cleveland Green Building Coalition.

Design and Analysis Tools

The following table provides links to key analysis, simulation, and research evaluating and predicting the performance of green roofs.

German universities with significant on-going research in the science and engineering of green roofs include:

• Weihenstephan Fachhochschule
• Bayerische Landesanstalt für Weinbau und Gartenbau (LWG), Veithöchstheim
• Universität Hannover
• Techniche Universität Neubrandenburg
• Universität Essen

Publications

• Building Technologies Associated with Rooftop Greening for Better Environment—The Building Centre of Japan also has a comprehensive and explanatory publication. Portions of the site and document are only available in English, but a translator may be required for the other parts in Japanese.
• Dach + Grün—the most respected green roof publication worldwide, a German language quarterly published by FBB (Fachvereinigung Bauwerksbegrünung e.V.). To subscribe contact Verlag Dieter A. Kuberski GmbH, Postfach 102744, 70023 Stuttgart, Germany (Fax 011-711-2388619)
• Green Roof Infrastructure Monitor—the most comprehensive English language periodical dedicated to green roofs is published quarterly by Green Roofs for Healthy Cities (GRHC). A quarterly web-based publication, Green Roof Infrastructure Journal, is available to GRHC members.
• Introduction to the FLL-Guidelines (PDF)—a brief overview of the FFL guidelines written by Peter M. Philippi of Green Roof Service, LLC.

Commercial Resources

Two national companies in North America offer complete green roof building systems, independent of any particular waterproofing product:

• Roofscapes, Inc., and
• Tecta America Corp.

In addition, vegetated roof covers may now be purchased in conjunction with most conventional waterproofing systems, some of which have been tested by FLL for compatibility with green roofs. At least ten North American roofing companies offer green roof assemblies as standard auxiliary products, and more companies are entering the field all the time.

Software

• RoofNav—RoofNav is a free Web-based tool developed by FM Approvals™ that provides fast access to the most up-to-date FM Approved roofing products and assemblies.