Design and Construction, Green Building

Why Your Next Upgrade Should Be a Green Roof

Green roof, living roof, or vegetated roof—whatever you call it, this feature is quickly becoming recognized as a critical component of a green building. If your building doesn’t currently have a green roof, don’t worry—a green roof can be installed on many existing buildings.

A green 'living' roof on a public library

jgolby / Shutterstock.com

As a facilities manager, you know the importance of keeping heating and cooling costs low and managing stormwater, as well as the value that sustainability can add to the perception of your company. A green roof can help accomplish these goals, and some local governments—eager to be part of the green roof boom—will even provide financial assistance or tax credits to those installing one.

What Is a Green Roof?

A green roof supports the growth of vegetation and is generally made up of many layers, including the plants, growing medium, filter fabric, drainage layer, root barrier, waterproofing layer, and structural deck. Some green roofs also have an integrated irrigation system; a leak detection and monitoring system; or additional drainage systems for rainwater capture, storage, and reuse.

There are two types of green roofs, each determined by the thickness of the growing medium being used and the types of plants it is able to support. An intensive green roof has a deep layer of growing medium. Due to the weight of this system, it is most feasible for new construction that integrates the green roof as part of the initial design. Most existing buildings will install an extensive green roof that has a thinner, lighter layer of growing medium. One drawback is that this growing medium can’t support a wide variety of vegetation. However, sedum (leaf succulents) mats that roll out like sod are a popular, low-maintenance option suitable for most climates.

The Benefits of a Green Roof

Installing a green roof can have a positive impact on the environment and, possibly, your bottom line.

You may have heard of the urban heat island (UHI) effect in which urban areas are hotter than rural areas nearby. According to the EPA, cities with populations of 1 million or more can be 1.8°F–5.4°F warmer in the daytime and up to 22°F warmer in the evenings. The higher temperature in more densely populated areas is caused by several factors: (1) With less vegetation, the city has a reduced ability to evaporate water (so, less heat is moved away from the land surface to the lower atmosphere); (2) less shade allows more of the sun’s energy to reach the land surface; (3) paved roads, sidewalks, and building materials absorb heat (especially if they are dark-colored); and (4) the heat released as a direct result of  human activities (such as cars, air conditioners, and industrial facilities). There’s also a feedback loop: When it’s hot outside, we increase the air-conditioning inside, which increases the temperature outside.

Green roofs keep buildings cool in summer because less heat is absorbed by plants than by conventional roof surfaces (especially if the roofs are dark-colored). They are also insulating and keep a building warmer in winter. So, by installing a green roof, you can save on your electric bill and decrease your contribution to the UHI effect.

Effective stormwater management is an important component of green buildings. One way to accomplish this is by using  low impact development (LID), like a green roof, that mimics natural drainage processes. With conventional rooftops, stormwater travels to a storm drain (and, in many cities, these drains lead to a combined sewer system). According to a U.S. General Services Administration report, green roofs can catch and retain the first ½ an inch to ¾ of an inch of rainfall in a storm, reduce the rate of runoff from a roof by as much as 65%, and reduce the number of combined sewer overflows. In addition, the water quality of stormwater that has passed through a green roof system is better than that of runoff from conventional roofs because the water doesn’t reach a high temperature, and some pollutants may have been filtered out.

Where to Start?

If you’re interested in installing a green roof, here’s what you should consider before getting started.

  1. Applicability. Whether your building is a good candidate for a green roof depends on factors such as slope (Does it have a low slope?); structural loading capacity (Can it bear the additional weight of heavy layers?); the type of roofing material currently installed; and the presence of drainage systems or other roof-mounted equipment, such as heating, ventilation, and air conditioning (HVAC) units. You must also consider roof access—in general, green roofs should not support foot traffic. And, any components and equipment on the roof may need to be mounted on paving pads to separate them from vegetation, making them easier to service.
  2. Permitting and regulations. Some local and state building codes make it difficult to install a green roof. For instance, Florida does not recommend installing green roofs due to the potential for uplift during high winds from hurricanes. However, in jurisdictions with some permitting obstacles, with careful planning and patience, the right candidates may still be able to install a green roof. Check local regulations and building codes to anticipate any difficulties, before you start the process.

Conversely, in some places, green roofs are encouraged or even required by law on new construction (see, for instance, Toronto’s Green Roof Bylaw).

  1. Consider the cost vs. savings. When considering investing in a green roof, you should conduct a life-cycle analysis—weighing the costs and savings over the lifetime of the roof. Be aware that the costs of the construction and maintenance of a green roof are generally higher than for conventional roofs. Although costs can vary significantly depending on the system you install, Penn State Extension estimates that a green roof can cost $20–$30 per square foot.

Despite higher up-front and operational costs, buildings that have green roofs may see savings over the lifetime of the roof from the following:

  • Longer life expectancy of the green roof. The U.S. Green Building Council has stated that a green roof can outlast a conventional roof by up to twice as long. Investing in a long-lasting roof can help you avoid frequent replacement costs.
  • Reduced energy usage and a lower electric bill. In general, buildings with green roofs are cooler in the summer and warmer in the winter. Although the reduction in building energy demand will vary depending on climate and the characteristics of the specific building and roof, you can check out Arizona State University’s green roof energy calculator for an estimate of possible energy savings.
  • Government incentives or tax breaks. Many local governments have incentives to encourage businesses to install a green roof. For instance, Philadelphia has a Green Roof Tax Credit businesses can claim that is 50% of the cost to construct the green roof, with a limit of $100,000. To take advantage of any of these programs, do some research about the initiatives in your area before you start a green roof project.
  1. Work with experienced contractors. Not all roofing contractors have experience in green roof installation, and it is important to hire a company that can handle the complexity of such a system. There are established industry best practices that contractors should follow, including American National Standards Institute (ANSI)/Single Ply Roofing Industry (SPRI) ANSI/SPRI RP-14, Wind Design Standard for Vegetative Roofing Systems, and ANSI/SPRI VF-1, External Fire Design Standard for Vegetated Roofs.

The nonprofit Green Roofs for Healthy Cities has published a directory you can use to locate contractors and certified green roof professionals in your area.

Pursue Some LEED Points

If you’re installing a vegetated roof on your existing building, you can earn points in the Leadership in Energy and Environmental Design (LEED) certification in the Existing Buildings: Operations & Maintenance (EBOM) rating system. The Heat Island Reduction credit in EBOM allows projects to earn one point for roof designs (a green roof, a high-reflectance roof, or a combination of both strategies) and another point for “nonroof” designs (i.e., solar canopies, vegetated structure, or open-grid pavement).

You can also take advantage of credit synergies. The reduction in energy usage of buildings that have a green roof in conjunction with other energy-saving strategies can help projects achieve points in the Energy Performance credit. Projects with green roofs may also be able to earn points in the Site Management and Rainwater Management credits.