Explore the reasons — beyond curb appeal — for installing a green roof.
People have always grown food in urban spaces — on windowsills or sidewalks, in backyards and neighborhood parks — but today, urban farmers are leading a movement that transforms the national food system. In Breaking Through Concrete (University of California Press, 2012) David Hanson and, experienced urban farmer, Edwin Marty illustrate twelve thriving urban farms. The following excerpt looks at the economic and conservation advantages of the green roof.
The green roof has been around for centuries, as evidenced by the old homes and barns in European towns and countryside. The modern version of the manufactured living roof evolved in Germany in the 1960s, and it has taken a few modern steps in its last half century of engineering. The steps are simple and green roofs will certainly continue to become more efficient and affordable as the young technology matures.
Today, landmark living roofs sit atop the city hall buildings of Chicago, Atlanta, Seattle, and Toronto and on the U.S. Department of Agriculture’s Whitten Building in Washington, D.C. Governments and corporations see long-term dollar savings in the energy-conservation numbers of green roofs, plus there’s the added bonus of good PR when you grow organic food on your building. With the increasing attraction of farm-to-plate food items on many restaurant menus, especially in progressive cities like New York, entrepreneurs are looking at rooftop farms as opportunities for social and economic benefit.
The largest rooftop farm in America was completed in 2010, across town from Eagle Street, in Queens. Ben Flanner, who cofounded Eagle Street with Annie Novak, developed the idea after leaving Eagle Street. Brooklyn Grange occupies an acre, or 40,000 square feet, atop a 1919 building. It holds 1.2 million pounds of soil, or 30 pounds per square foot. In its first summer, it is estimated the farm produced 16,000 pounds of fruits and vegetables for sale at markets and to restaurants. In other words, rooftop farms are not just a feel-good, look-good architectural gimmick. And it’s not too crazy to imagine flying into a city in ten years and seeing a puzzle of green patches with hoop houses and chicken coops coloring in the once-dreary rooftops. Indeed, don’t be surprised if you find yourself falling asleep underneath the roots of your salad in the not-too-distant future.
Sometimes the most challenging part is determining whether the initial cost of constructing the green roof will be worthwhile in the long term, so we include three reasons, economic and conservation based, for taking the plunge.
Green-roof installations are pricey, ranging from ten to fifty dollars per square foot, but there are significant economic and societal benefits that offset the cost.
LONG-TERM SAVINGS. An average roof’s waterproofing membrane must be replaced every fifteen to twenty years because of exposure to ultraviolet rays and the thermal expansion and contraction that result from constant heating and cooling. Green roofs have proven to extend the life of the waterproof membrane by a multiple of four. A well-insulated home or building can save 30 percent on heating and cooling bills by reducing heat loss in winter and heat absorption in summer.
URBAN HEAT REDUCTION. Regular roofs, especially those covered in dark materials, absorb heat from the sun and act as hot stones, emitting that heat into the immediate environment. According to the Environmental Protection Agency, the resulting overall “heat island effect” of a city of one million people can raise daytime temperatures up to 5.4°F and evening temperatures up to a shocking 22°F over temperatures in surrounding areas. Green roofs dissipate that heat and often create microenvironments that are actually cooler than the surrounding air temperatures.
STORM-WATER REDUCTION. As more and more pavement is laid in cities, less water is absorbed into the ground and more enters an often outdated sewage system, resulting in water-treatment plant malfunctions and more costs to the taxpayer in terms of utility services. Green roofs capture up to 70 percent of the rain that falls on them. Many municipalities are offering incentives for the installation of green roofs, largely because of the storm-water management improvements. Receiving approval for a building permit in the city of Chicago generally takes up to two years. But if the building incorporates a green roof, the city guarantees permit evaluation within thirty days. New York offers four dollars per square foot toward the installation of a green roof.
Constructing a green roof is simply a matter of adding the right layers, in the correct order. The first step, of course, is to have an engineer evaluate your building to ensure the structure can support the added weight of soil. Once you get the go-ahead, there are six simple steps, in layers, to a green roof.
MEMBRANE. All roofs have a waterproof membrane. Living roofs build on the original membrane with the following series of geotextile membranes.
ROOT BARRIER. This layer rests atop the waterproof membrane to protect the roof from damage due to penetrating roots.
DRAINAGE LAYER. An absorbent layer, this egg-crate-type plastic material has tiny holes between the cups. Water absorbed from rain and watering either drains through the holes and runs off the waterproof membrane, or collects in the cups and eventually evaporates back into the root layer.
SILT SCREEN. The drainage layer is interwoven with a silt screen to keep the soil from eroding and to suspend the egg-crate system off the roof so water can drain.
WATER RETENTION MAT. This is usually a fleece-type layer that absorbs the initial water seepage from the soil and keeps it in the system, via evaporation, longer.
SOIL. The amount and type of soil depends on the intended use of the green roof. A simple, often residential-style roof of low-maintenance, shallow-root plants, such as decorative succulents and leafy greens (kale, chard, broccoli), can require as little as six inches of soil. Larger-production rooftop projects require about eighteen inches of soil that is finely crushed to make it denser and therefore heavy.
This excerpt has been reprinted with permission from Breaking Through Concrete: Building an Urban Farm Revival by David Hanson and Edwin Marty and published by University of California Press, 2012. Photography by Michael Hanson.