What Is a Green Roof?

Sod Roof, Turf Roof, Green Roof

It's not just grass on a roof. What's underneath can make all the difference in the world. This overview explores some of what you need to know about green roof layers, sod roof construction, and considerations to ease your move toward going green from the top down.

For thousands of years, roof vegetation was used as an insulator against the hard climates of Iceland and Scandinavia. The Icelandic turf church shown here is not that ancient. Built in 1884, the Hofskirkja Turf Church in Öræfi has walls made of rock and a roof of stone slabs, covered with turf.

Modern green roofs are much different. Today's Green Roof Systems grew out of the ecology movement of the 1970s, mixing new technology with environmental awareness. For decades, the US government has been a proponent of green roofing systems on federal buildings. They offer this definition of green roofs in context with alternatives:

Green roofs—consist of a waterproofing membrane, growing medium (soil) and vegetation (plants) overlying a traditional roof....Conventional roofs are often known as black roofs, their traditional color. They are descended from the "tar beach" roofs once common in urban areas, and are still petroleum-based....—US General Services Administration Report, May 2011

Other names for Green Roofs include vegetative roof, eco-roof, sod roof, turf roof, organic roof, planted roof, and living roof.

Types of Green Roofs:

The vocabulary of green roof types is constantly changing. Types of vegetation and their specific needs (e.g., irrigation, drainage, maintenance) can vary immensely with the latitude and climate of the installation. Green Roof systems should be thought of as a continuum of choices between these two extremes:

• Extensive, also known as Low Profile: Most common, with low growth vegetation such as moss, sedum, herbs, and grasses
• Intensive, also known as High Profile or Deep Profile: Most expensive, so usually commercial, with landscaped lawn, shrubs, and even trees.

Structural Engineering Considerations:

• Roof Pitch or Slope: The International Green Roof Association (IGRA) suggests that roof slopes be less than 45 degrees and more than 2 degrees. "Even though it is possible to build pitched Green Roofs with a slope of 45° it is not recommended to exceed 30° due to significant limited accessibility for upkeep and maintenance."
• Load: Most water-saturated Extensive Systems will carry the same weight as a typical gravel roof. "Simple Extensive Green Roofs weigh between 60-150 kg/m² (13.0-30.0 lb/sq.ft.) depending on the thickness of the Green Roof system build-up," claims the IGRA. Intensive green roofs will have more "high point loads" to consider (i.e., trees, walkways).

Challenges Most Often Cited:

• Design is unsuitable for the building (e.g., load calculations) and building site / climate (e.g., unsuitable plant selections that lack diversity)
• A problem in any construction is improper installation by inexperienced contractors
• Need for specialized maintenance
• Fear of creating an uncontrolled organic architecture
• Extreme weather from ongoing climate change may make a harsh, rooftop environment even more difficult for living organisms to survive wind, rain, and temperature fluctuations
• Initial cost and untested (long-term) technologies

Green Roofs on Historic Buildings:

Like solar panel technologies, green roofs are acceptable on historic structures, but the "historic character of a property shall be retained and preserved" according to the Standards for Rehabilitation. This means that as long as you can't see the vegetation, the Standards have been met. Plantings should be low and not visible above the roofline; plantings that show above historic parapets are incompatible with the Standards. ITS Number 54 Guidance also warns that "...as with any rehabilitation treatment there are specific issues, including increased structural loads, added moisture, and root penetrations through waterproofing layers, that must be addressed before considering installing this feature on a historic building."

But just because you CAN, should you? "Green roofs are expensive and many of the benefits can be achieved through more cost-effective strategies," says Ric Cochrane of Preservation Green Lab. "The takeaway here is that green roofs are viable strategies for improving the quality of the urban environment, but the preservation community should carefully consider alternatives that might achieve greater benefits for lower costs, with less risk to historic buildings."

^{Sources: The Benefits and Challenges of Green Roofs on Public and Commercial Buildings, A Report of the United States General Services Administration (GSA), May 2011 (PDF); Vegetation technology, Construction Engineering, and Special constructions, International Green Roof Association; ITS Number 54, "Installing Green Roofs on Historic Buildings," Interpreting the Secretary of the Interior’s Standards for Rehabilitation (PDF), September 2009, Liz Petrella, Technical Preservation Services, National Park Service; "Green Roofs and Historic Buildings: A Matter of Context" by Ric Cochrane, September 13, 2013. [accessed April 21, 2014]}

Why a Green Roof?

Anyone (or any community of people) who is concerned with the effects of climate change will understand the value of more vegetation in the world. Admittedly, the benefits of installing a green roof are more pronounced for urban areas than rural areas. These reasons for installing Green Roofing systems are most often cited:

• Mitigates rainwater runoff as water is absorbed and drained slowly
• Mitigates roof deck damage from temperature extremes
• A natural habitat promotes biodiversity, including birds and insects
• Mitigates the "urban heat island" effect. Green roofs do not hold heat like building materials do, thus lowering the temperature of the air and the city overall.
• Building insulator for heat, cold, and sound
• Improved air quality in neighborhood
• Lower energy costs largely from the cooling effect of plant respiration
• Increase roof life
• Increase aesthetics for nearby buildings overlooking flat roofs with mechanical equipment

Green roofs have several layers. Oftentimes, blocks or cells of soil are placed on a prepared roof deck. The cells are not seen after the vegetation takes hold. These interlocking squares give the whole system stability, just like a retaining wall may support the undesired movement of ground soil.

Green Roof Layers

A green roof typically will include many layers for different functions. The vegetation will always appear to be at the top, but a root system cannot be forgotten. Layers may include:

• Vegetation (variety of plant species)
• Growth media
• Filter fabric
• Geocomposite drainage layer
• Moisture management fabric
• Insulation layer
• Drainage layer (if insulation not slotted)
• Root-barrier membrane
• Waterproofing membrane
• Roof deck

Cells Being Placed on Prepared Roof

California Academy of Sciences in San Francisco, California celebrates the environment inside and outside. What they have called a living roof was created with 50,000 biodegradable vegetation trays. The roof is held in place not by the trays, but by the massive root system created when 1.7 million plants mature. Pritzker Laureate Renzo Piano designed the roof to be an extension of the museum's environment at Golden Gate Park.

^{Source: Living Roof, California Academy of Sciences website [accessed January 28, 2017]}

Are Green Roofs Ugly?

The Living Roof at the California Academy of Sciences is not an ordinary flat roof. It's not a gable roof like a church in Iceland. With seven hills, complete with operable portholes, the roof is a meadow habitat in the urban environs of San Francisco. Architect Renzo Piano designed the building, showing the aesthetics possible with Green Roof technology.

US Coast Guard HQ, St. Elizabeths Campus

The US Government has long been committed to building green, sustainable office buildings. The US Coast Guard Headquarters in Washington, DC is no exception. Both extensive and intensive types of roofing at a 2% slope cover over a half million square feet of green roof area at the campus that was once a contaminated brownfield.

The Layered Look of Green Roofs

A Green Roof will fail if not properly engineered. You can't simply throw dirt onto a sturdy roof. Building codes must be followed. Construction standards must be met. Germany has been the acknowledged leader in developing guidelines and standards. The FLL Guidelines for the Planning, Execution and Upkeep of Green-roof sites is the go-to resource to help communities better understand Green Roof Basics. Throughout the world, more and more communities are developing their own guidelines specific for their own climate. Here are a few examples:

• Green Roof Toolkit for the District of Columbia (PDF)
• Green Roofs: Best Management Practices, City of Chicago, Illinois
• A New York City Guide to Green Roofs, Cooper Union
• Minnesota Green Roofs Council
• Green Roofs and Walls, Sydney, Australia

Summary:

"Green roofs — also known as ‘vegetated roofs’ or ‘living roofs’ — are ballasted roofs consisting of a waterproofing membrane, growing medium (soil) and vegetation (plants) overlying a traditional roof. Well- designed, engineered and maintained green roofs provide multiple environmental, social, economic and aesthetic benefits." - US General Services Administration

^{Source: Green Roofs at GSA, US General Services Administration [accessed January 28, 2017]}