Green Roofs on Existing Buildings

Green vegetative roofs are becoming increasingly common. We see them popping up in cities big and small. Beyond the beauty they impart, green roofs can play an important design role in our buildings. A vegetative roof can help manage stormwater runoff, reduce the urban heat island effect, provide habitat for birds and insects, and help to increase the life span of the roofing membrane. Installation of green roofs is typically associated with newly constructed buildings; however, there are instances where green roofs can be installed on existing buildings.

Green roofs are classified in two different categories: extensive and intensive.

  • An intensive green roof is any green roof with a soil, or growth medium, depth greater than 6 inches.
  • An extensive green roof has a growth medium depth of 3 to 6 inches.

Because of weight limitations on most existing structures, extensive green roofs are better suited for retrofits. Intensive green roofs provide more options for a larger selection of plantings but typically weigh much more than extensive roofs. Typically, a green roofing system wears like a hat over a traditional roof. Typical roof membranes, roof drains and tapered insulation are a part of a green roofing system.

Manage Stormwater

Stormwater management is becoming an increasingly important issue in our built environment. As we increase the amount of impervious surfaces in our built world, there are fewer places for stormwater to infiltrate back to the water table naturally. With existing infrastructure nearing capacity in our cities and increasing development fees imposed by municipalities, more designers are looking for alternate methods to decrease the impact of stormwater. Most green roofing systems are designed to accommodate at least 1 inch of stormwater before discharging excess water to the storm drains. Instead of immediately getting rid of this valuable asset, we are able to use the stormwater to water the green roof.

Minimize the Urban Heat Island Effect

There are additional benefits to implementing a green roof system, including creating habitat for wild creatures and decreasing the urban heat island effect. Urban heat islands are described as the phenomenon that urban areas have a greater ambient temperature than the surrounding rural areas. This is due in part to the dark, heat-gathering surfaces that make up our urban environment including asphalt and dark roofing membranes. These materials absorb the sun’s energy and re-radiate it back to the environment causing a rise in the ambient temperature. Green roofs can help mitigate this problem and put the sun’s energy to use rather than use it for unwanted heat.

Extend Roof Membrane Lifespan

Green roofs can also help extend the life of a roofing membrane. Left to bake in the sun, a typical roofing membrane has an expected life of approximately 20 years. Most roofing failures are caused by long-term exposure to the sun and UV rays. Roofing materials shrink and crack and become brittle after long-term exposure to the elements. However, if we put 4 inches of soil on top of that membrane and remove the effects of the sun and UV rays, we can extend the life of a roofing membrane well beyond 20 years, possibly even doubling life expectancy.

Green Roof Weight

One of the biggest challenges associated with installing a green roofing system on an existing building is accounting for the extra weight of the roofing system and determining capacity of the existing structure. A typical extensive green roofing system can add as much as 35 pounds per square foot. Existing buildings with steel bar joists and lighter-gauge framing typically cannot handle the additional load. Some of the most successful retrofits of adding a green roof to an existing building have been on older concrete or load-bearing masonry-framed buildings. Some older buildings were built more solidly and able to accommodate the extra structural loading. Before considering any green roof retrofit on an existing building, it is recommended that the structure be reviewed by a structural engineer to confirm loading requirements.

Alternately, companies are developing new solutions for installing green roofs on existing buildings. Tray roofing systems that come pre-grown are a popular solution. Some companies are developing lightweight tray systems that can weigh as little as 17 pounds per square foot. These lighter-weight systems can give options for structures that cannot handle a heavier load but may come with trade-offs, such as the need for an irrigation system. The lighter-weight systems sometimes cannot handle extra water storage capacity and need subsequent watering to keep the plant material alive in times of long-term drought.

Cost of green roofing systems varies but generally run from $15 to $20 per square foot—roughly the same cost as installing premium hardwood flooring. Pricing varies based on geographic location, system design and type of green roofing system selected.

The benefits of green roofs are numerous and can solve development problems that architects and real-estate developers face. With some creative solutions available on the market, retrofitting an existing building to accommodate a green roofing system may be a viable option.

About the Author

Nathan M. Gillette
Nathan M. Gillette, AIA, LEED AP O+M, CEM, is director of Natura Architectural Consulting, Grand Rapids, Mich., and a retrofit editorial advisor. He works with clients to successfully implement and manage energy efficiency and sustainability projects.

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