Greening The Roof

Roof gardens help ease stormwater management, as well as HVAC system and general maintenance demands.

By Alex Drescher
From the October 2017 Issue

Although roof gardens may seem like a more recent phenomenon in many locations, these building features have actually been in existence since the fourth millennium. While 21st century efforts toward environmental protection and green space cultivation have expanded the use of roof gardens, particularly in urban areas, it is not a new development.

roof gardens
Seaport Hotel in Boston, MA (Photo: Carlisle SynTec Systems)

Many terms used for roof gardens: green roofs, vegetated roofs, and living roofs. But what exactly is a roof garden? By definition, it is a contained green space that sits either above, below, or at grade on top of an existing manmade structure.

There is a reason that roof gardens are not a new phenomenon, and that is the plethora of benefits that these provide. These exterior areas help to facilitate stormwater management, increase a building’s amenity space, reduce the urban heat island effect, extend a rooftop’s service life, and reduce required building maintenance.

Easing Stormwater Management

Stormwater management is a significant challenge in many communities. According to the U.S. Environmental Protection Agency, there are 772 communities across the U.S. that operate with combined sewer systems. This means that rainwater runoff, domestic sewage, and industrial wastewater are collected in the same pipes.

While most modern communities have separate pipe systems to collect and divert stormwater and sewage, many older American cities were built with these combined systems. In these locations, during periods of heavy rainfall or snowmelt the water entering the sewer system often exceeds the capacity of local wastewater treatment facilities. When this occurs, these systems overflow and discharge excess wastewater directly into nearby streams, rivers, and other bodies of water. For obvious reasons, this method of stormwater management is less than ideal, and can negatively impact the surrounding environment and community.

This is where roof gardens can help to ease stormwater management challenges. With the combined sewer systems described above, stormwater is not “managed” but merely flows through the sewer system, overflowing system capacity. Roof gardens help to prevent this overflow by detaining and retaining the rainwater, thus filtering and reducing stormwater runoff. The deeper the roof garden, the more stormwater retention that is possible; studies have shown that a 4″ extensive sedum roof is able to capture 60-75% of annual rainfall in temperate climates.

For the Rice Fruit Company in Gardners, PA, a roof garden provided a solution for the construction of a new facility. Since 1913, this company has distributed fresh fruit throughout the U.S., and as the largest apple packing facility in the eastern half of the country Rice has a reputation for high standards.

When the company’s owners decided to expand operations and construct a new cold storage facility at its Gardners site, they wanted to make the building environmentally friendly and unobtrusive, so it would blend into the surrounding countryside. A roof garden seemed like the perfect solution.

While the aesthetic of the new facility was important, there was another significant benefit to Rice including a new roof garden. Pennsylvania building codes required that a retaining pond be installed with most commercial building construction in order to manage stormwater runoff and prevent flooding. With the new construction, there would be no space on the property for a retaining pond. By installing a roof garden system, however, Rice was able to meet building code requirements for stormwater management without the additional retaining pond.

Aiding Aesthetics, Insulation, And Maintenance

The use of roof gardens has been especially popular in urban areas, partly because stormwater management is a significant concern in these densely populated areas, but also because these enhance building aesthetics, provide occupants with more green space, and can increase property value. Designs vary from simple sedum roofs with plaza areas to more intricate designs that emulate traditional parks.

While improved aesthetics is beneficial in and of itself, studies have also shown that adding a roof garden to a building increases occupant satisfaction, productivity, creativity, and even physiological well-being. Visual and physical access to green space has been shown to restore mental focus and improve job and academic performance.

Another reason why roof gardens are popular in urban areas is their ability to reduce the heat absorbed by a building. Urban areas experience what is known as the urban heat island effect—because of lack of green space, which would typically absorb and diffuse heat, cities are often up to 10°F warmer than surrounding locales.

Roof gardens can help mitigate urban heat island effect by providing shade and removing heat from surrounding air through a process known as evapotranspiration. This effectively reduces the temperature not only of the roof surface but also of the surrounding airspace.

Meanwhile, the aesthetic benefits of a roof garden might seem obvious; but what about its ability to extend the service life of a building and rooftop? The waterproofing membrane underneath a roof garden is not exposed to ultraviolet light, extreme temperature fluctuations, or physical damage. Because of this, the waterproofing membrane and underlying components perform longer than those of a traditional roofing system. In Germany, there are several roof gardens that have been installed for more than 50 years without experiencing a single leak.

Because of the protection these provide to underlying roofing systems, roof gardens can also reduce maintenance demands. And, the gardens themselves require minimal maintenance. During the first 60 to 90 days after installation—often known as the establishment period, these areas require some irrigation to ensure plant life flourishes. After that, most only require annual fertilization, weeding, drain inspection in the spring, and a checkup and removal of debris in the fall.

If the roof garden is more complex—perhaps reminiscent of a city park, then more maintenance (e.g., pruning, weeding, and extensive irrigation) could be required. And, if an irrigation system is in place, it should be disconnected and drained each fall, and tested each spring. Apart from the maintenance expected to care for plant life, however, facility managers should find that roof gardens will thrive with very little upkeep.

roof gardensDrescher is the roof garden product manager for Carlisle SynTec Systems in Carlisle, PA. Passionate and technically oriented, he has contributed to improving Carlisle’s various stormwater management technologies since beginning his work with green roofs in 2015.

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