Up on the Roof with Urban Agriculture

By Kisha Lewellyn Schlegel, 6-12-07

	Caption: The Portland Building

While much of the Rocky Mountain West is rural, our urban spaces are the areas that are most inhabited and quickly growing. As these cities expand though, we have the fortunate ability to look at other examples of urban growth and the effects that growth has had on the environment. While urban sprawl is the commonly discussed disruptive and destructive form of growth, urban densification (the concentration of buildings in a small area) presents its own problems.

Many of issues of densification are directly related to the flat roofs that become the ceiling to a city, and raised surface of the earth. The space on flat roofs is often a hot wasteland of blacktop and brick, which absorb and retain heat. Roof tops with little space between them form an island of heat, creating the heat island affect and causing city temperatures to be six to eight degrees hotter than surrounding rural areas. Hard roof surfaces are also meant to be impermeable and rainwater runs off into streets, often draining pollutants into the watershed. In the process, habitat loss has resulted in the decrease in biodiversity in urban centers.

These issues are seen worldwide as dense cities become the place where most people live. (In 1994, 45 percent of the world’s population lived in cities. By 2025, it is thought that 65 percent of people will inhabit the urban spaces.) But the rooftops that seem so problematic remain underused spaces that could be the very source of greenspace that urban centers (residents and wildlife) desperately need. In order to increase urban biodiversity, and mitigate the effects of habitat destruction, the flat, heat seeking black top roofs can actually become gardens.

Countries in Asia and Europe have long traditions of urban agriculture, stemming from an era when transportation depended on a cart and buggy. Currently there are over 200 million urban farmers in the world, providing food to twelve percent of the population. Fifty percent of Asian urban households farm. Most Latin American urban families spend 1 day a week in the urban garden.

While urban agriculture exists on the ground level in community gardens, and while many grow food in window boxes, the rooftop is yet another resource for the urban farmer. Many rooftop projects focus on creating greenspaces more akin to parks, but a growing number are being used for urban agriculture. These spaces can provide access to much needed space and much needed food security. (Link opens pdf)

Rooftop vegetation cools buildings by insulating and shading the roof. The heat from the building can actually extend the growing season. In turn, the vegetation cools the air that the building’s intake system uses (especially for air conditioning units in the summer) meaning that less energy is used to lower the air temperature in the building. Roof vegetation can actually reduce heating in winter as well by 25 percent, since vegetation protects buildings from wind.

Rather than release all of the storm water, green roofs can also retain 70 to 100 percent of precipitation in summer and half that amount in winter. This diminishes the water runoff that usually dilutes and transports urban pollution into watersheds. In turn, less water enters the storm water drain system, meaning that cities might be able to pay less for infrastructure costs. Plants also remove pollution, and improve air quality, breathing in nitrous oxides and airborne particles and exhaling cleaner air.

But this water absorption makes builders and building owners nervous. The basic premise of a roof is to provide protection from water, not a sponge for it. However, the technology of roof membranes actually allows for green roofs to absorb water without it seeping into the structure.

The cost can also seem prohibitive as a normal roof costs ten to fifteen dollars a square foot while green roofs are fifteen to thirty-five dollars a square foot. However, like most green efforts, the initial investment pays off with time. While a typical roof membrane is replaced every ten to twelve years, green roofs can last two to three times as long if properly installed.

By far, the most significant issues are those of access and roof load capacity. In countries obsessed with the liability of such roofs (where people will walk and play and grow) these safety concerns, while they might be valid, can become excuses for not taking on such projects. However, many cities such as Atlanta, Chicago and Toronto have taken on the challenge to cool and green their roofline. Portland, Oregon, even has incentives for green roof building. (a list of projects: http://www.greenroofs.com/projects/plist.php )

In time, these rooftop gardens might also be connected by rooftop bridges and could be considered an extension of national parkland and protected as such. (Wong, Nyuk Hien, Su Fen Tay, Raymond Wong, Chui Leng Ong, and Angelia Sia. 2002. “Life cycle cost analysis of rooftop gardens in Singapore.” Building and Environment 38 (3): 499-509.)

According to Michelle Nowak’s “Urban Agriculture on the Rooftop” (opens pdf) there are three main types of urban rooftop gardening:

Container Gardening
With this technique few changes are made to the structure of the roof. Instead, containers of any sort are place on the rooftop and filled with soil and plants. Soil should be amended each year to keep it healthy and special attention should be paid to the drainage system of each container. It is essential that water doesn’t collect under pots where it can damage the roof structure. For larger containers such as large, raised beds, layers of special membranes (sheets of plastic or other material) are placed between the soil and the roof in order to protect the roof. By far, this method is best for urban agriculture on the roof. Containers provide deeper soil, which can hold more water and nutrients.

Rooftop Garden
Here the rooftop actually becomes the base that holds the soil and plants. Again, layers of special membranes are used to protect the roof. These gardens are most successful with succulents and other low growing vegetation. Growing vegetables in this way can be complicated by the amount of soil needed and the amount of water, which can cause drainage issues.

Hydroponic Garden
With hydroponics, plants are grown in a soil-less medium and given a nutrient solution. This option weighs the least (which can be an important factor depending on the structure of the roof) but also takes specialized knowledge and an initial investment in equipment.

Regardless of the technique, green roofs provide a medium for changing not only the way we use space, but the way we envision our city skylines. The top story view has long been the desire of builders and architects who compete for the distinction of building the tallest building. Now these buildings can harbor a little earth on that flat space up there, a reminder of our roots…a reminder that our high ambitions come from and go some place else.

Each week in the NewWest.Net ”Spade & Spoon” section, writer Kisha Lewellyn Schlegel discusses the localization of the food system in the Rocky Mountain West by profiling organizations and individuals who are attending to the issues and possibilities of eating closer to home. Bookmark Spade & Spoon at www.newwest.net/spadeandspoon.