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By: Marek Patrosz

November 24, 2020

Ever since Local Laws 92 and 94 were passed as part of NYC’s Climate Mobilization Act in 2019, there has been an uptick in inquiries about how to enhance the roof of a building to positively affect the environment, the occupants, and the community, during a roof placement project. As with many design elements, there is not a single stock answer. There are many site-specific parameters that could lead to a variety of environmentally positive enhancements to the roof system ingenuity is often required when working with an existing building in order to best adapt to the site conditions, whereas new buildings can be more easily molded.

The two most common approaches, especially on the northeast side of the United States, are either a vegetativegreen” roof or solar panel array systems that meet the jurisdiction and output requirements of the building. There are benefits to both; however, the vegetative roof is the more visually appealing option. Primarily, solar arrays tend to be attractive on paper, when hard numbers show the output and energy cost reduction in a building, but there certainly are ways to make solar panels pleasant to the eye. Either way, both routes offer long lasting benefits to the building and the environment.

Vegetative roofs are capable of:

  • Adding a layer of thermal protection to the roof assembly
  • Controlling storm water runoff
  • Water filtration
  • Sound absorption
  • Improving the aesthetic environment

Solar Arrays can offer:

  • Reduction of CO2 Emissions
  • Decrease the operating cost of the building

The choice of one approach over the other depends on the condition of the building and the building owner’s desired outcome; however, the biggest driving and deciding factors are not the vegetative roof or solar array, but the capacity of the roof. Although many of the new roof assemblies that Sullivan Engineering designs can handle the installation of both green roofs and solar arrays, we always recommend that a structural peer review be performed prior to the selection a roof assembly design. Once the roof capacity is confirmed, the roof assembly that best fits the building can be determined to ensure ease of installation and longevity. No two buildings are exactly the same, so it is important to treat each roof design as its own project. For example, the bulkhead roof of a building may be a perfect area for solar panels while the main roof has greater potential with a vegetative roof assembly because it will capture storm water in an area that only has morning sun exposure. Each building has its own unique characteristics that should be analyzed by the design professional to determine the capabilities of the site and to make the most of the long-term investment.

There are many existing buildings that use a combination of the two approaches to lower energy bills, cut carbon emissions, reduce the urban heat island effect, and better manage stormwater. The perfect combination that will maximize the positive effects for the building owner, occupants, and community, can be determined through a careful review of the site and the building elements.

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