Vegetated Roof Systems

Vegetated or green roofs are not a new concept in the roofing industry, but it is only recently that they have started to gain popularity. For the purposes of this unit, we will refer to vegetated systems and green roof systems interchangeably. What is a vegetated or green roof? A green roof is a rooftop designed specifically to foster growth of plant life. Because they consist of water retention and drainage systems, waterproofing membranes, soil and plants, they can support everything from vegetable gardens to shrub-lined walkways. These systems can be installed either as fully built-up systems intended for extensive or intensive green roof designs, or as lightweight, interlocking modular trays that reduce the installation time and offer pregrown vegetated options

From industrial facilities to private residences, vegetated roofs can be installed on a wide range of buildings. When installed, green roofs provide a number of tangible and intangible benefits. They can provide space for commercial gardens, recreational space and more, but beyond these aesthetic and social benefits, vegetated roof systems provide numerous benefits for building owners and inhabitants.

In this unit, you will learn about the history of vegetated roofing, advancements in legislation, the numerous benefits of   this type of roof system, the types of vegetated roofs, components of vegetated roof systems, and things to consider when thinking about installing a green roof.

Vegetated Roof History

From the famous hanging roof gardens of Babylon to the modern-day roof-top garden of Music Center in Nashville, Tenn., the roof garden has been a part of civilized society for a surprisingly long time. Through the years, roof  gardens have been used as extensions of living rooms, displays of wealth and social status, gathering places for the elite and places to escape the summer heat. They’ve also been used as insulation, a means of flood control, a place to grow food and as camouflage.

Ancient roof gardens were installed as far back as 2020 BCE in Mesopotamia, an example of which is the Hanging Gardens of Babylon—one of the seven wonders of the world. During this time, green roofs consisted of cave-like structures or sod roofs covered with soil and plants commonly used for agriculture, dwelling and ceremonial purposes. These structures provided protection from the elements, with good insulation during the colder months and a cool place to stay during the summer.

The modern trend of green roofs began in Stuttgart-Weilimdorf, Germany. These green roof systems were initially developed to try to reduce the problems associated with runoff from impermeable roofs. German green roofs were the first to include technology for sophisticated irrigation and protection against root ingress for rooftop gardens. Growth of the use of green roofs throughout Germany was encouraged by state legislation and municipal government grants. Over time, Germans became experts in green roofs and used their broad experience to develop guidelines for the construction of this type of roofing. These guidelines, called Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau (FLL) or The Landscaping and Landscape Development Research Society, are not only used in Germany, but throughout Europe.

Popularity continued to grow decades ago overseas, an example of this is the vegetated roof at Hotel New Otani in Tokyo, Japan. Built in 1965, on the site of a villa and garden, much of the 10-acre garden was preserved and adapted to the needs of the hotel. The garden contains a large waterfall fed by a large pond. Much like traditional Japanese gardens, the Hotel New Otani roof garden contains raked gravel, rocks, a selection of plants and water, creating a calming atmosphere for hotel guests to enjoy.

In the 1980s, lighter and less expensive green roof systems that could be applied to large flat roofs were developed. Today, vegetated roofs are becoming increasingly popular in the United States with examples found at the Chicago City Hall, Target Center in Minneapolis, Minn., and Music City Center in Nashville, Tenn. These cities lead the country in green roof installations as a response to rainwater management initiatives and related incentives. Many green roofs in North America today are based on the German FLL standards.  

Benefits of Vegetated Roofs

There are many social, environmental and economical benefits to installing vegetated roofs on your building. While all vegetated roofs have similar functions, each installation is unique and technical performance can vary by region, climate, building and green roof type and design. The benefits described below can be achieved by virtually all green roof systems  and designs.

Rainwater management—a green roof acts like a sponge, taking in water as it rains and releasing it to the plants in the system, which prevents spiking in water levels in cities’ rainwater runoff systems. For example, Portland, Ore., utilizes numerous rainwater management solutions, including vegetated roofs. Washington, D.C., New York City and Philadelphia are also leading the way using vegetated roofs as a rainwater management tool.

Rainwater management is actually the most important benefit of modern-day green roofs. Urban environments contain large amounts of impervious surfaces. Most large cities have combined sewers, so your rainwater and sewage go into one pipe. With heavy rain events the single pipe becomes overburdened. This overburden causes Combined Sewer Overflow events (CSO’s). However, green roofs can assist with rainwater management and mitigate any possible issues.

Runoff comparison—gallons of water are collected on a roof during a six-hour storm event. Not only do vegetated roofs retain water, but they also detain. This is just as important, as it lessens the burden on the rainwater infrastructure. Vegetated roofs are designed to decrease runoff-related pollution of  natural waterways.

Convert CO₂ to O₂—vegetated roofs also convert carbon dioxide (CO2) to oxygen (O2). Plants help remove particulate matter from the air while adding oxygen to the environment, improving our air quality and combatting greenhouse effects.

Natural water filtration—natural water filtration can be affected by the use of a vegetated roof. Heavy metals carried by the rain are bound in the roof instead of being discharged. With green systems, 95 percent of cadmium, copper and lead are filtered out of the runoff.

Protect your investment—vegetated roofing also protects your investment. This type of system guards roof membranes against harmful ultraviolet radiation, extreme temperature fluctuations, heat aging and physical damage. As a result, this type of roof structure can require less maintenance, saving the owner money in replacement costs over the long-term life of the roofing system. If the roof is well maintained, it will last longer than a standard roof, making up some of the upfront costs of installation.

Noise reduction—noise bounces off of flat, hard surfaces, but covering these surfaces with plant life can dampen that sound. Green roofs have excellent noise attenuation, especially for low frequency sounds. The substrate blocks lower frequencies, while plants can block higher frequencies. A 4-inch layer of growing medium on this type of roof system can reduce sound up to 20 dB.

Reduce heat island effect—many cities deal with urban heat island effects when sunlight and attendant heat energy are absorbed into materials with dark surfaces. Green roofs help mitigate this by cooling the air through a process called evapotranspiration. On a 90-degree day, a traditional roof can exceed 160 degrees Fahrenheit. However, under the same conditions, a green roof will typically emit a temperature of only 95F. Lower rooftop temperatures mean less smog and improved air quality.

Energy efficiency—the greater insulation and surface performance offered by green roofs can reduce the amount of energy needed to moderate the temperature of a building, as roofs are the sight of the greatest heat loss in the winter and the hottest temperatures in the summer. For example, research published by the National Research Council of Canada found that an extensive green roof reduced the daily energy demand for air conditioning in the summer by over 75 percent (Liu 2003). This energy efficiency was achieved by increasing the R-value of the roofing system through the components of the green roofing system and the effects of evapotranspiration to lower surface temperatures. Evapotranspiration is the process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants. This evaporation process can create a cooling effect on surface temperatures and can offer performance results similar to a highly reflective roof surfacing.

There are resources available to assist in the estimated energy savings of a green roof—The Green Roof Energy Calculator co-developed by Green Roofs for Healthy Cites (GRHC) with the University of Toronto and Portland State University allows you to compare the annual energy performance of a building with a vegetative green roof to the same building with either a conventional roof or a highly reflective roof.

Temperature fluctuation differences—green roofs modify temperature fluctuations of the membranes, thereby reducing thermal stress and possibly extending membrane life. On one hand, a dark, heat-absorbing roof surface increases demands on mechanical systems, making it more difficult to adequately cool a building. On the other hand, a green roof reduces the temperature of the roof’s surface, and therefore, the building itself. The layers of a green roof decrease the amount of heat being absorbed by the building.

Civic benefits—besides water management, there are additional civic benefits of installing green roofs. Secondary benefits of green roofs include more opportunities for urban farming, creation of urban wildlife habitats, and formation of environments that are simply more pleasant to interact with than traditional commercial rooftops.

Green roof requirement—some cities, especially in Europe, require newly constructed commercial buildings to have green roofs to help address urban environmental problems and foster sustainable development. As previously mentioned, Germany is considered a pioneer in green roofing. At least 150 cities in Germany already require vegetated roofs with new construction projects and requirements exist throughout many other countries as well. If the pending bill in San Francisco is passed, it may not be long before we see more U.S. cities joining  this list.

Incentives—while green roofs can be required by some municipalities, they are more commonly encouraged simply through financial incentives. When incentives are combined with the inherent benefits of vegetated roofs, these sustainable choices become even more appealing. For example, Portland, Ore., incentivizes the development of green roofs by offering grants and highly visible campaigns featuring rooftop events and installation of green roofs on public buildings. Cities like Washington, D.C., New York, Chicago and Philadelphia lead the country in green roof installations as a response to rainwater management initiatives and other related incentives.

Environmental stewardship—companies like Ford, Walmart, FedEx and others, install green roofs as part of their corporate sustainability initiatives or to create goodwill with stakeholders. In south Chicago, a plant that produces Method cleaning products features a large produce growing operation on the rooftop. At Columbia University, where research pertaining to the benefits of green roofs takes place, tens of thousands of square-feet of on-campus buildings boast green rooftops.

Types of Vegetated Roofs

Green roofs are often chosen based on their soil depth and structural requirements, intended purpose and maintenance needs. There are three main types of green roofs that differ by cost, depth of growing medium and plant types. 

Extensive Systems

Extensive systems, also known as compliance roofs, are considered the “work horses” of vegetated roofs. They provide a lightweight, low-maintenance green roof solution and as such, are the most specified and least expensive of all green roof options. Typically, this type of vegetated roof is selected for large, flat-roofed buildings and is a good option for low-sloped roofs and retrofit styles. Installation methods can vary, but extensive roofs can often be installed in a very short period of time to provide an instant green effect.

Extensive green roof systems are the shallowest type of system at 3 to 6 inches in depth and can weigh up to 42 pounds-per-square-foot when saturated. This type of vegetated roof is not usually intended for general public access, rather, they are chosen mostly for their ecological benefits. Types of vegetation used on these roofs include sedums and certain drought tolerant perennials and grasses. Typically, they do not require irrigation in most climates, except in the south and southwest, and in high desert areas. 

Semi-Intensive Systems

Semi-intensive systems are a combination of both extensive and intensive types, providing a “best of both worlds” solution. This type of system opens up the plant palette tremendously. They include a richer, deeper substrate and drainage solution compared to extensive roofs, which enable the use of a wider range of plant mixtures. Perennials, taller grasses and even some small shrubs, can all be specified. This type of roof can also store and/or detain a lot of rainwater. Unlike an extensive system, it is quite common to see an irrigation system with a semi-intensive green roof.

These systems are 6 to 10 inches in depth and many times defined up to 12 inches in depth. When they are saturated, semi-intensive systems can weigh up to 84 pounds-per-square-foot. With deeper growing depths and the expanded plant palette, these systems offer much more design flexibility than extensive vegetated roofing.

Intensive Systems

Intensive systems are designed to replicate what is typically found at ground level in the natural landscape, as well as in green spaces like parks or cultivated gardens. You are likely to see intensive systems used on commercial buildings where owners want to have large green areas that incorporate all sizes and types of plants. They are made for those who have the time and the mindset to put in fully landscaped rooftop gardens, as well as those who do not mind the regular maintenance that  it requires.

Intensive systems can include perennials, groundcovers, shrubs, trees and even turf grass, along with the proper irrigation system. Tables, benches, planter boxes, ponds, fountains and greenhouses can be installed on an intensive green roof to offer places for people to relax, dine or work in a picturesque park-like setting. As far as the irrigation and drainage systems are concerned, they must operate at peak efficiency to reduce the chance of overloading the roof’s structure.

Vegetated Roof Components

Just as the strength of any building is largely dependent on the building’s foundation, the success of a green roof depends greatly on its compatibility with the roof structure it is built on. Green roofs consist of many components that promote vegetation growth. When constructing a green roof, special attention must be paid to the materials and design in order to ensure that the integrity of the roof’s structure is not compromised. The construction of this type of roof typically includes layers of drainage material and planting media on a high-quality waterproof membrane, along with other components.

Waterproofing Membranes—waterproofing membranes are the most important component of the vegetated roof. Not only must waterproofing membranes prevent water from entering the building from the outside, but they must also be capable of resisting mechanical damage from tools and the penetration of plant roots. In addition, these materials should be capable of lasting many years without repair or replacement, as the roof garden will need to be deconstructed to perform any repairs.

Waterproofing membranes are available in options including SBS-modified bitumen, hot rubberized asphalt, polymethyl methacrylate (PMMA) liquid membrane and polyvinylchloride (PVC). Waterproofing membrane systems can be installed in a standard configuration over insulation and recovery board, or in an inverted roof membrane assembly (IRMA), known as protected membrane roofs (PMR).

Both types of assemblies have pros and cons. PMRs are protected, but are harder to repair or troubleshoot when there is a leak because the membrane is buried. IRMA green roofs contain many of the components explored in this unit, including root barriers, XPS insulation, a drainage layer, growing medium and plants. Conventional assemblies provide easier access to the membrane, but are potentially more susceptible to  physical damage.

Root barrier—simply put, root barriers protect the waterproofing membrane from root penetration. The FLL has established a standardized method for investigating and determining the root penetration resistance of waterproofing products used throughout Europe. Root barriers are used to create a physical boundary between plant roots and the waterproofing membrane.

SBS-modified bitumen, hot rubber and PMMA require root barriers. Typical root barriers are made from LLDPE (linear low density polyethylene). Seams are either heat welded or taped with polyethylene or butyl seam tape. Thicknesses range from 20 mils to 40 mils depending on the vegetated roof plant types. Root barriers can also be made from PVC or ethylene propylene diene terpolymer (EPDM).

Insulation—insulation for PMR assemblies is made from extruded polystyrene (XPS). Generally, insulation layers are 1 to 4 inches in thickness and are installed directly over the root barrier. Insulation typically includes drainage grooves and channels or installers can insert a synthetic drainage in lieu of drainage channels.

Drainage layers—drainage mats combine drainage and protection functions for the green roof and are available in four types—dimpled drain, entangled net, drainage plate and lightweight aggregate. The first three are lighter in weight compared to the lightweight aggregate and are synthetic (typically polyethylene or polypropylene).

Capillary mat or moisture retention mat—sometimes extra water-hold capacity is needed and many drainage layers will have an optional integrated capillary or moisture retention mat for this reason. Capillary mats are used in green roof construction to help distribute water evenly throughout the roof system. The moisture retention layer can also be a separate component.

Edge restraints and drain access boxes—edge restraints and drainage access boxes provide a physical separation between the vegetated roof and the vegetation free zones or drains. Most are made from aluminum, stainless steel or high impact  resistant plastic.

Growing medium—the soil media provides a structure to which roots can attach, retains moisture after rainfall, provides minerals and nutrients, and allows for gas exchange. When it comes to soil used on a vegetated roof, there are many variables to consider. The right depth for the growing medium will be determined by site conditions, such as local precipitation patterns, and sun and wind exposure. The type of vegetated roof chosen can also impact the kind of growing medium needed. For example, the extensive vegetated roof generally has the largest aggregate sizes and uses the least amount  of organics.

Soil considerations—in addition to soil varying by the type of vegetated roof chosen, it can also vary based on the plant selection. When selecting soil, special consideration should also be given to the soil’s ability to retain volume, drain adequately and supply nutrients. Also consider that the soil must use a filter fabric and not contain any silt that could potentially clog the fabric.

Soil content—engineered soil ratios and mix designs utilize both mineral and organic soils and vary by region and supplier. A typical mix of soil contains lightweight aggregate, organics and graded sand.

Minimum soil depths—depending on the type of vegetation chosen, soil depths will vary. Generally speaking, sedums will require 3 to 4 inches of soil, grasses need 6 to 10 inches, and shrubs and trees generally require 12 inches or more of soil.

Growing medium delivery methods—soil can be delivered to the vegetated roof in a number of ways, including blown in place, delivery by totes or delivery by cubic yard bags. Blown-in-place growing medium delivery eliminates the need for equipment such as cranes, man lifts and wheelbarrows, and allow for quick installation. Bags or totes can be palletized and wrapped to be brought to the location.

Vegetation—given adequate growing medium depth and irrigation, just about any plant can be grown on a green roof. It is really up to the building owner to determine the aesthetic they are looking for when choosing vegetation. Whether choosing sedums, grasses, annuals, perennials, shrubs or trees, or any combination of those, the possibilities are endless.

Vegetated mats and tiles—vegetated mats and tiles can provide an instant vegetated roof. The mats and tiles come in either coconut coir or nylon entanglements, and sedums are typically used as the plantings. Mats and tiles are rolled out or placed onto the growing media, creating an instant green roof.

Modular systems—modular systems are good for extensive systems, especially on logistically challenging projects. However, they can be expensive. These systems can be made of recycled or virgin high-density polyethylene (HDPE) or  polypropylene (PP). Tray systems can be pre-planted with typical sedum plantings, and all trays feature a drainage component, growing medium and plant components. Since the plants are already established, there can be fewer issues with replacement plants that do not take root or thrive. However, in most modular systems moisture cannot flow between modules, increasing the impacts of wet and dry spells on the plants.

Vegetated Roof Considerations

Beyond having a strong understanding of the benefits of a vegetated roof and the components that comprise one, there are some additional considerations to keep in mind when thinking about designing a building to have a green roof. In order for a green roof to flourish, it must be properly designed and  constructed.

Condition of existing roof—the additional weight loading from a green roof should not be overlooked. The existing  structure ought to be able to bear the extra weight. In  addition, it needs to have sufficient drainage which will help reduce loads and prevent excessive saturation of the growing substrate.

Structural capacity—over most concrete roof decks, structural modifications are not necessary. However, for metal and wood roof decks, there may be some structural modifications involved. This is one factor that will guide designers, architects and other building and design professionals, regarding the scope of a vegetated roof project. This is the number one item that needs to be first addressed when designing a new building or retrofit.

Access to the roof—a green roof will require regular safe access for maintenance. It is also important that this access follows all safety procedures and the proper fall protection measures are put in place to allow for safe inspection and maintenance of the roof.

Project location and hardiness zones—when choosing the appropriate plants for the region, it is important to review the U.S. Department of Agriculture’s Hardiness Zone Map, which will indicate a plant’s tolerance for cold. Planting plants, shrubs, trees and more, that are appropriate for the growing zone in which your project is located will ensure that vegetation stays healthy and thrives in the environment.

Purpose of the roof—the purpose the green roof serves is a consideration that takes place during the design phase of the project. Is it for environmental purposes or is the green roof a space that people will go to enjoy? This will inform decisions on the type of green roof installed, as well as the  vegetation planted.

Material availability—the variety of materials available to complete the project is numerous and there are new materials entering the market every day. Understand which materials are best suited for the type of vegetated roof you are working on and select those that accomplish the structural and aesthetic needs of the project.

Budget—the creation of a green roof can initially be more expensive than traditional roofing materials; however, there are many benefits that make the investment worthwhile. The price paid for a green roof largely depends upon the style of the roof system chosen.

Maintenance—although these types of roofs can be low-maintenance that does not mean that there is no maintenance involved. Just like a residential garden, vegetated roofs will need to be weeded and fertilized on a regular basis. Building owners should inspect their roofs at least once a month for weeds and seeds, and remove the unwanted material. Pesticides and herbicides are not recommended. Building owners will also need to trim plants and shrubs and keep in mind that the grass may require cutting. During inspection, owners will also want to check whether the drainage system is still functioning properly and remove any leaves blown into the gutters and drains.

They will also want to consider installing automatic or supplemental irrigation for watering of vegetation and check the drainage regularly in order to avoid any water pile-up or water damage to the building’s structure.

LEED certification—valuable green building certifications, such as LEED, may award points for green roofs. The wide variety of benefits associated with green roofs is captured to varying degrees in the U.S. Green Building Council’s LEED rating system. Alone, a green roof will not qualify a building for LEED certification, but it will contribute to the LEED score, along with a set of other measures. Even without legislation in place, green roof installations are picking up steam due to this.

The widespread adoption of green roofs is on the horizon. Green roofs hold keys to addressing several pressing challenges facing modern cities, and they are both accessible and easy to install. A few scattered green rooftops will not be enough to create truly sustainable cities; however, a more holistic embrace of these structural elements is necessary.  Whether through government requirement, financial incentive or otherwise, the more vegetated roof adoption can be encouraged, the more our urban environments and the people who inhabit them will ultimately benefit.

As climate change makes regions hotter and urban areas grow, cities with green roofs will experience an array of benefits that will improve quality of life for their citizens, and help propel cities into the next century and allow them to thrive. The history of green roofs shows us that they were originally conceived to regulate the temperature of their environments and assist with rainwater runoff control. Today, they still serve these purposes and can provide even more benefits on top of these. Vegetated roofs show us that we can take wasted grey space, and create a useful green space that generates economic, social and environmental benefits.