In 2004, Chicago Mayor Richard Daley returned from a European vacation inspired by all of the gardens he saw planted on rooftops. Europe had been utilizing green roofs for years, seeking environmental, economic, and even aesthetic benefits. In a National Geographic News article in November 2004, Daley explained that with “all [of] the flat roofs in Chicago, you could reclaim thousands of acres for the environment and also help buildings with heating and cooling and controlling rainwater going into the sewer system.” Fast forward nearly 10 years and Chicago has created more than seven million square feet of green planted roofs, which was more than any city in the United States at that time and became one of Mayor Daley’s legacies.
Benefits of Green Roofs
The potential benefits of green roofs are numerous. They can absorb 50 to 60 percent of the rainwater that falls on them. The remainder of the rainwater that is not absorbed or released into the atmosphere enters the sewer system in a more controlled manner than in a typical urban high-volume rainwater surge from thunderstorms. In addition, green roofs can lead to energy savings in warmer climates, since they can help reduce air conditioning costs.
Long-term benefits of green roofs can include a longer life-span for a rooftop. Often, green roofs can last twice as long as conventional roofs. The green roofs can protect a roof’s waterproof layer from ultraviolet rays and cracks from extreme day/night temperature fluctuations.
Environmentally, green roofs can help reduce the urban heat-island effect that occurs when densely-packed conventional roofs absorb the sun’s heat energy. On a hot summer day, a city filled with conventional rooftops can be nearly 10 degrees warmer than in nearby rural areas. In addition, companies want to appear environmentally friendly, and people enjoy being exposed to fresh air and greenery in an otherwise sterile urban area.
Cost and Design Problems
Despite all of the benefits of green roofs, there are downsides and pitfalls for owners, developers, contractors, and design professionals. First, it is costlier to design and maintain a green roof. Green roofs typically cost twice as much to initially construct as a conventional roof. The increased expense may, ultimately, be outweighed by the extended life for a green roof, but it may be a costly upfront financial burden for a cash-strapped building project. In addition, irrigation and maintenance costs of the green roof may be more than an owner anticipated. Without proper inspection and maintenance, the considerable loss of money spent on replacing dead plants or removing weeds can result in claims against design professionals.
Additionally, if large amounts of soil, plants, and water are added to the rooftop of a structure that is not designed to withstand the weight, significant damage can occur if the structure fails. This especially can be true with older, historic buildings constructed 50-100 years ago with no anticipation of adding a heavy green roof at a later date. That became evident in 2013 when the roof of a supermarket collapsed in Latvia after topsoil had been added, killing 54 people.
From a practical standpoint, the initial hoisting of the soil and plantings can be extremely difficult once the crane comes down. The scheduling of soil unloading must be carefully coordinated, as each soil load can weigh as much as 8,000 pounds. In addition, the storage of the soil and plants is crucial once they are on the roof. Structures have load limits, which means the soil and plantings cannot all be stored in one place.
Also, green roof construction must prevent added water from leaking and penetrating the building. The leakage can cause major structural and mold damage. Water damage is typically the most common claim against a design professional in the construction of green roofs.
Finally, designers must plan on how to anchor the soil and plants on the roof. In a city like Chicago, wind speeds routinely exceed 70 mph at skyscraper heights. Depending on the locale, the designers must plan for the additional weight from snowfall and how it affects the plant life on the roof.
Managing the Risks
Designers face a variety of risks and potential claims, so it is crucial to have a seasoned project team in place before undertaking a green roof construction. Many claims are made against design professionals inexperienced in the design of a green roof. General contractors, architects, roofing contractors, and landscape architects must work closely together from the start through the end of a green roof project.
Architects who design and renovate buildings must be careful to design the installation of the proper materials, such as a quality water-proofing membrane to prevent water damage. The membrane must be strong enough to avoid punctures and properly adhere evenly to the surface. Landscape architects need to be careful to have the right balance of minerals and fertilizer in the soil to avoid weeds and loss of plant-life. In addition, the management of anticipated storm water runoff is crucial to proper hydration and avoidance of excessive maintenance issues for the building owner.
Designers also must emphasize and provide in writing to the owner and general contractor any necessary maintenance requirements for the roof after completion of the construction. This can include requirements for the irrigation of the plant life, review of the soil content, and inspections for minor membrane damage that can be repaired before structural damage occurs.
Lastly, designers must be wary of engaging in projects with tight timeframes that do not allow for unanticipated construction issues, especially with regard to the retrofit of an older, historic building. Claims for construction project delays have become more common in recent years.
In addition, government entities or non-profit owners may be inexperienced when pursuing such projects, and they may not be properly financed to account for the additional costs for installing a green roof.
A well-constructed green roof can be a great addition to any building or urban environment; however, design professionals should be knowledgeable and cognizant of the risks before entertaining a green roof project. The grass is not always greener when it comes to the design of a green roof.