Green roofs, also known as vegetative roofs, can protect buildings and result in higher rents and real estate values than conventional roofs.
AS DESCRIBED ELSEWHERE in this issue (see “Up on the Roof, Cities Grow Green”), urban rooftop farming is taking many forms these days, from innovative hydroponic greenhouse systems to open-air, soil-based farms operating atop a wide array of commercial, government and residential buildings.
This trend in urban agriculture is firmly rooted in the practice of “green roofs,” also known as vegetative roofs. Germany began making great strides in green roofing during the 1970s. Today, investment in these roofs is getting serious attention in North America. More than 5.5 million square feet of green roofs were installed throughout the U.S. and Canada in 2014, according to the latest “Green Roof Industry Survey,” conducted annually by Green Roofs for Healthy Cities.
Green roofs, which have been known to last three times as long as conventional roofs, protect the underlying structure from the ravages of ultraviolet (UV) radiation, freeze and thaw cycles, stormwater runoff and physical damage, thereby lowering a building’s operating costs and improving its resale value. Research also shows that green roofs improve air quality by removing pollutants, lower ambient air temperature, provide habitat for many species and offer an aesthetic respite from the concrete hardscape of urban areas.
Green roofing systems are typically classified by the depth and components of their growing media as extensive, semi-intensive or intensive. A simple, extensive green roof containing less than six inches of soil and vegetation is well-suited to a roof with light load-bearing capacity and little or no foot traffic. Intensive green roof systems are more complex, with deeper soil supporting a wider variety of plants, people and even trees or ornamental ponds. Semi-intensive green roofs are a combination of both extensive and intensive systems.
Generally, the more biodiversity on a green roof, the deeper the growing medium needs to be and the heavier the load on the roof. Most commercial buildings today are not designed to withstand the extra weight that an intensive green roof requires. One of the most important measurements is “wet weight,” when saturated soil, plants and materials are at their heaviest. With six inches of fully irrigated topsoil and vegetation, an intensive green roof can weigh up to 150 pounds per square foot.
According to the U.S. EPA, the cost of installing a green roof can start at $10 per square foot for simple extensive roofing and $25 per square foot for intensive roofs. Annual maintenance costs for either type of roof can range from $0.75 to $1.50 per square foot.
In 2011, the U.S. General Services Administration analyzed the costs and benefits of green roofs for public and commercial buildings. Compared to a black roof, the GSA estimates that, based on a 50-year average of annual savings, a three- to six-inch green roof covering 10,000 feet has a net present value of $2.70 per square foot per year, a cost payback of 6.2 years and an internal rate of return of 5.2 percent nationally.
The study also estimated the potential real estate value of a green roof to be about $13 per square foot, taking into account rents, expenses, vacancy, discount rates, absorption and lease lengths. In addition, the report found that buildings with green roofs realized 5.6 percent more in rent than conventional buildings nationwide.
One of the highest paybacks of a green roof comes from the increased longevity of the roof membrane, with planted roofs lasting 40 years or more. In cities with combined sewer and rainwater systems, the GSA report states that planted roofs significantly mitigate the runoff effects from densely populated areas, reducing the flow of stormwater from a roof by up to 65 percent and delaying the flow rate by up to three hours.