October 17, 2023
Written by: Deirdre Smith, Ox Hill Associate Director of Landscape & Sustainability – LEED GA, WELL AP Our Team – OxHill Companies
“On the roof, it’s peaceful as can be. And there the world below don’t bother me, no, no.” — “Up on the Roof” written by Carole King & Gerry Goffin
With the news that Ox Hill’s development, City Centre West, will be the first building in Fairfax City with a green roof, we wanted to take a deeper dive into the subject.
Why green roofs? And what’s so important about them? The answers are pretty clear. Due to urban density, the only place left to go is up! Green roofs provide the opportunity for new sustainable landscapes as well as places for recreation and beauty.
Let’s take a brief look at the history and benefits of green roofs (also called vegetative roofs) and then examine Ox Hill’s green roof projects, including City Centre West.
Green Roof History
Green roofs are not just a modern concept. In fact, they’ve been in existence for thousands of years, beginning with the Hanging Gardens of Babylon (7th century BC). During the Viking Age (793-1066 AD), traditional structures known as “longhouses” were sod-covered to keep the homes warm in winter and cool in summer.
Modern green roofs originated in Europe and have become more widespread in the U.S. in the last 20 to 30 years.
Green Roof Types and Benefits
Green roofs are divided into two categories: extensive and intensive. Extensive green roofs use small groundcover plants, such as sedums, grown in a shallow medium of less than 6”. Intensive green roofs resemble a more conventional landscape area with trees, shrubs, perennials, and even lawn growing in a much deeper soil base.
These two types of green roofs have vastly different requirements and maintenance needs which should be considered prior to implementation. However, both extensive and intensive green roofs provide the same environmental and sustainability benefits.
Here are the benefits:
In most urban and suburban areas, green roofs reduce the flow of stormwater from a roof by up to 65%. They also delay the runoff by up to three hours, thereby preventing municipal systems from being overwhelmed after significant rainfall events which are now occurring with more frequency. Additionally, green roofs naturally filter out pollutants from rainfall before the stormwater enters the municipal system.
Green roofs reduce a building’s energy use by providing thermal insulating effects, which keep the building cooler in summer and warmer in the winter. According to the EPA, green roofs can reduce building energy use by 0.7% compared to conventional roof, reducing peak energy demand and leading to an annual savings of $0.23 per square foot of the roof’s surface. By lowering air-conditioning demand within the building through the implementation of green roofs, associated air pollution and greenhouse gas emissions by power plants are also reduced – a one-two sustainability punch!
Biodiversity and Habitat
Green roofs provide new habitats for animals, such as birds and insects. With the destruction of so much habitat on the ground level in urban areas, it is becoming critical to provide alternative places for pollinators. In some trials, roof areas are being used for food production: though, the jury is still out on whether there is a viable future for large-scale agricultural green roofs.
Green roofs offer interesting and compelling opportunities for innovative architectural design. Data shows that buildings with green roofs command a higher real estate value. Further, the ability to be connected to nature (called biophilia) through proximity to green roofs has been proven to provide an elevated feeling of calm and well-being.
Ox Hill and Green Roofs
Our City Centre West project—Fairfax City’s first green roof—will provide both extensive and intensive green roofs with an extensive roof system which will cover approximately 7,500 square feet.
This mirrors our earlier completed project (July 2022), The Children’s School in Arlington, VA. There, the extensive green roof covers approximately 6,000 square feet of the roof surface.
The intensive roof system at City Centre West is comprised of planters encircling the 2nd and 7th floors. These planters will contain shrubs, grasses, and flowering plants to be enjoyed by the building’s residents as well as visitors and passerbys.
City Centre West is currently in site planning with an anticipated completion date of Q2 2026.