Updated: Nov. 7, 2019

What is Green Roofing?

A green roof system is an addition made to the roof of an existing building for growing plants on the roof. Depending on the type of green roof you install, the plants may be modular or have drainage layers. However, all green roofs include a few important features, such as waterproofing and root repellent, to keep the underlying structure safe and undamaged.

Different Types of Green Roofing

There are two primary forms of green roofing: intensive and extensive. These are differentiated by the amount of vegetation utilized. While extensive green roofs can support up to 25 pounds of vegetation per square foot, intensive green roofs can hold 150 pounds per square foot.

Another factor that differentiates these styles is the amount of labor required to maintain them. Intensive green roofs, as the name suggests, take a lot more work to manage, as they involve keeping the various types of plants separate and maintaining an actual garden. Extensive green roofs, on the other hand, are left to grow naturally and without restriction, taking care of themselves as they would in the wild, and thus only requiring yearly weeding and fertilization.

For homeowners, the intensive format allows you to pick and choose which plants and flowers you would like to grow, enabling you to sculpt the aesthetic you desire. Extensive green roofs are designed only to be entered for their yearly maintenance, so they become more naturally overgrown than their intensive counterpart. This makes extensive green roofs harder to navigate, meaning individuals can't walk through the space to enjoy the flora.

Both green roofs use a layering system, which allows the use of various soils, mats, and other materials to retain the nutrients while forcing out any waste by-products. Through these layers, a natural soil profile is mimicked, thus creating a drainage process that allows liquid to be filtered through safely while also nourishing the plants.

There is a more recent hybrid form of green roofing, known as comprehensive roofing. This format is made to support a wide variety of plants, which is commonly found in intensive roofing, while allowing for a greater amount of vegetation to be supported, like extensive roofing.

Benefits of Green Roofing

There are a variety of reasons why an individual would want to pursue green roofing, both for the betterment of the community and the home.

Community benefits include:

  • The installation of a green roof reduces the need to manage any stormwater that accumulates, as well as the stress put on local sewer systems.
  • In urban settings, the inclusion of green roofs reduces the overall heat conducted by the buildings, covering many of the surfaces that would generate the most warmth.
  • The plants prevent the distribution of smog and dust while also catching many of the pollutants found in the air.
  • Green roofs can be public spaces, such as gardens or recreational areas, to be enjoyed by the community.
  • Due to the amount of work and money that goes into the creation and installation of green roofs, the use of them in a neighborhood can result in an increase of jobs and revenue for local businesses.

There are also personal benefits to a homeowner, whether you live in an urban or rural setting, for installing a green roof, such as:

  • By creating a natural barrier, green roofs can reduce the amount of noise that comes into your home.
  • Since your roof is covered by plants, the amount of it exposed to the elements is limited, enabling the roof membrane to last much longer than if unprotected.
  • Green roofs reduce the amount of energy your house needs for heating, saving you money.
  • The fire retardation provided by green roofs helps protect your home.
  • Since green roofs utilize waste products that would otherwise end up in landfills, as well as extending the lifespan of your other household products like an HVAC system, the use of one reduces the amount of waste generated by your home.
  • By adding a green roof to your house, you'll increase your property value due to the enhanced efficiency of your appliances and your home becoming more aesthetically pleasing.

Negatives of Green Roofing

While green roofing can benefit you and the community, it is a costly process that can lead to some unforeseen expenses:

  • Insurance for green roofs is important, due to the potential of droughts or unforeseen storms, but also is expensive.
  • The weight of the green roof may result in liability issues if the pressure of it causes sagging.
  • Any damage the plants might do to the building, such as the roots growing into the shingles, will result in costly repairs.

Have questions about how much it costs to repair or replace your roof? You may be interested in our roofing cost guide.

The Development of Green Roofs

The first green roof was developed long ago, with the earliest recorded instance being the Hanging Gardens of Babylon, created in 500 B.C. At the time, the structure was built on stonework, using tar and reeds as the lower protective layers.

The modern method of building green roofs, however, was formed in Germany during the 1960s, creating the layering system that is implemented today. While fairly common in Europe, this practice is only now starting to gain popularity in the United States.

Modern green roofs also differ from those made in ancient times because the technology has greatly advanced. The layers utilized to create today's iterations include soils, three different fabrics, and drainage plates and mats. The drainage systems also make these stand out from previous versions, utilizing the soils' natural tendencies to guide the water in ways that help better maintain the system.

Scientists and engineers are still researching the best methods for green roofing. This is primarily focusing on how green roofs might be built in different climates and environments, as well as the scale in which these can be made.

The Cost of Green Roofs

The cost of a green roof varies between $15 and $25 per square foot for intensive roofs, and between $10 and $20 for extensive roofs. Much of this cost comes from the soil and growth concentrates, as well as the plants. This cost increases depending on whether you'd like your plants to be individually planted as opposed to pre-grown via a vegetation mat, and may also affect the ability and convenience of getting the materials needed. While none of these components are particularly hard to find, transporting them to your location may be expensive.

This is starkly juxtaposed to the cost of a green roof in Germany currently, which sits at between $8 and $15 per square foot. This is due to the newness of the industry and technology in the U.S. and the increase in custom projects associated with the market here.

When establishing a budget for a project like this, one should consider the long-term maintenance involved as well. While this may be sporadic if one is getting an extensive green roof, it is still an expensive process depending on the size. For some companies, this is a free inclusion that comes with their services, but generally, it's between $0.75 and $1.50 per square foot.

Design Standards

When your green roof is being installed, three key points should be closely considered to ensure your product lasts many years without issue or extensive maintenance costs.

The first of these is wind uplift, which can cause severe damage to both the green roof and the structure of your house. If there is too much air pressure caught underneath the green roof when the wind blows overhead, it may cause a pressure shift, pulling the greenery off the roof. To avoid this happening RP14 Wind Design Standard for Vegetable Roofing Systems was created to guide and protect buyers from this issue.

Secondly, while green roofs provide fire retardation for your building, there is still a risk of the greenery catching fire due to natural events in your area. This potential disaster can be minimized through the implementation of VF-1 Fire Design Standard for Vegetative Roofs, a document made to guide the "prudent design and mandatory maintenance regiment" of each unit.

Lastly, one of the biggest issues green roof owners face is the harm caused by the roots of the plants burrowing into the roof of the house, tearing up the material and compromising the overall structure. The VR-1 Procedure for Investigating Resistance to Root Prevention presents guidelines on how to safely check the efficacy of barriers protecting your home and how they can be repaired or improved. This is prominently for shorter-term effects and should not be relied on when studying the long-term effects of chemicals or barriers.

Installation Process

The installation process for a green roof is deeply involved, even with the help of skilled contractors. While much of this work, i.e., the creation of the various modular containers, will be handled by your contractor, there are some tasks that, if you are comfortable and physically capable, can be done yourself.

Please note that, if you choose to do any of the work yourself, you do so at your own risk, potentially harming your roof or yourself in the process. Be especially sure you can safely and thoroughly complete any of these steps before attempting them yourself.

An important consideration to make when deciding if green roofing is right for you is whether or not your roof will be flat or slanted. When slanted (commonly referred to as "pitched"), the design reduces the risk of eventual water deterioration, thus requiring fewer protective layers.

The first of these steps is hiring a structural engineer to assess if your home or building is capable of supporting the weight of the green roof you intend to install. For instance, a single modular container can weigh up to 40 pounds, which might lead to irreversible damage if your roof isn't structurally sound enough to support all of that weight. It's also worth asking whoever you hire what additions you can make to allow your roof to more easily and safely support that weight.

Once you are given the necessary data and approval to continue with the project, a layer of pitch (a resin commonly used for planting) needs to be laid down. This will divert any excess rain to the drainage systems, preventing negative moisture build-up that would otherwise damage your products. It is recommended that each of these layers of pitch be "at least ΒΌ inch per foot of run."

Next is the inclusion of the plant trays. Each of these need to have been grown for at least four months before the installation, allowing them to be appropriately cultured for the green roof. Assuming you purchase these through a contractor (which is recommended), they'll be shipped to you, covered in plastic soil elevators to protect them from the heat as they are transported.

After placing protective edgings along the roof in preparation, the trays can be installed. Trays are laid out in rows starting at the lowest section of the roof. If working on a slanted roof, contractors will work their way up the incline. Otherwise, they will select an edge based on how you would like the flora to grow aesthetically, building in the same methodical fashion. After this, the trays can be partially covered by said edging.

Once you've begun to place the planters, you can attach them to one another through the use of the plastic lips on the edges of the trays. This will ensure that your green roof remains secured and in one piece, but allows you to move each of the planters as you see fit. These trays are also equipped with feet, which keep the plants elevated off the roof so as not to sit directly on top of your house.

Depending on how well the planters fit, you'll likely have to cut off parts of the final tray in any row to allow the flora to sit snuggly and tightly across your roof. How this is traditionally done is, after measuring out the space, a wax pencil is utilized to mark the areas that need to be cut, then trimmed through with a concrete saw. The saw you choose is crucial, as the planters are filled with dense aggregate, meaning that a lesser tool will get ruined if utilized for the same task.

At this point, the soil elevators should be removed. These are used to moderate the height to which the plants grow before installation, and allows for water, nutrients, and other healthy organisms to pass from planter to planter. Note that, if doing this step yourself, you will need to pull the soil elevator out at an angle to avoid damaging the plants.

Once all the trays have been set, the edging needs to be secured to the roof, and the plants should be watered. While you won't normally need to provide these plants with water, it helps to settle them into their new home.

LEED Credits

For those unfamiliar with Leadership in Energy and Environmental Design (LEED), it is a program dedicated to rewarding those who build and/or maintain highly green structures. Receiving certificates from LEED will financially assist with projects that fall under their definitions and regulations of what is environmentally positive.

By installing a green roof onto your building, you may qualify to receive credits from LEED that can lead to financial assistance. There are a variety of aspects of your green roof that can help you qualify, including:

  • Employing a variety of plants that promote biodiversity while requiring little or no upkeep.
  • Decreasing the amount of storm runoff from your building by 60 percent or more, or by removing large amounts of pollutants.
  • Extensive reduction of the heat signature of your building, thus reducing your contribution to the Heat Island Effect.
  • The landscaping commonly required to keep the plants watered and irrigated is reduced by at least half.
  • Reducing the amount of waste your building creates by using it to fuel your green roof.
  • Utilizing high amounts of recycled material in the creation of your green roof.
  • Lowering the amount of energy generated by your building.

Who to Talk To About Green Roofs

If you're interested in building a green roof, there are a few individuals that you should contact to ensure the safest, most cost-effective installation.

Firstly, bring in a structural engineer to not only ensure that your roof can handle the weight of such a job but to better understand what you can do to further protect your home in the process. If the weight of your green roof shifts or changes, you may need to take the time to restabilize portions of it, moving planters around for better support and safety. Restabilizing portions of your roof may make an important difference in protecting yourself and your investments.

Secondly, reach out to a trusted contractor to see if they complete green roof jobs, as well as to get a quote. Even if you're considering doing the work yourself, it's wise to get the information for all your possible options before selecting one.

Third, you'll want to speak with an organization that grows plants specifically for green roofs. The cost of these materials will be important to consider, whether or not you're using a contractor. Considering this will also give you an idea of what the transportation charges average, depending on how far away the group lives from where you are located. You can also take this time to ask questions about the intensive or extensive formats, helping you to make a decision on which type you will be working with moving forward.

Lastly, once you have all the information you need and have a good idea on the direction you want to take the project in, you should talk to those who live in your building or family and friends if you intend to do this yourself. If so, you will likely be able to use the assistance of those around you to make the process easier, whether it be physical labor the day of the installation, any tools you may need to collect for the process, or for future maintenance of the green roof.

RoofingCompare is provided "as is" and without warranty of any kind, either express or implied, including, but not limited to, the implied warranties of merchantability, fitness for a particular purpose, or non-infringement.

What Happened to GreenRoofs.net?

Screenshot from GreenRoofs.net on June 1, 2011

GreenRoofs.net was a website dedicated to increasing the popularity of green roofing. (In industry terms, "green roofing" is a roof that is designed to allow plants to grow on top of it.)

The site was run by a 501(c)(6) not-for-profit industry association called Green Roofs for Healthy Cities (GRHC). GRHC was established by a group of private companies in March of 1999 to "foster the development of a market for green roof products and services".

The organization published the following mission statement:

To develop and protect the [green roofing] market by increasing the awareness of the economic, social, and environmental benefits of green roofs, green walls, and other forms of living architecture through education, advocacy, professional development, and celebrations of excellence.

The GRHC's first website was GreenRoofs.ca, which it launched in the beginning of 2002. The initial group of companies that established the GRHC were located in and around Toronto, which probably explains the Canada-specific .ca top level domain name. To make the domain soup more complex, the organization launched both GreenRoofs.org and GreenRoofs.net in 2003.

GreenRoofs.net was initially an empty page with a simple link to GreenRoofs.org. However, by 2004 the GRHC was maintaining an actual website on GreenRoofs.net in parallel with the GreenRoofs.org site. The two sites appear to have been mirrors of each other, containing content about upcoming GRHC conferences and events, research and information for the news media. This duplication of content continued until June 2011, when the site displayed a simple "This site is temporarily unavailable" notice. By July the site was showing a 404, "Page Not Found" error.