Green Roofs: Their Role in Urban Food Security

Green roofs are a fairly recent addition to urban spaces but have had an astronomical impact. The growing interest in green roofs has sparked due to their promise of mitigating the Urban Heat Island effect (UHI) and providing a solution for excess stormwater. With roofs making up around 50% of urban areas’ impermeable surfaces, it is very important to introduce and create ways to make these spaces more sustainable (Shafique et al., 2018). Green roofs are part of the overarching concept of green infrastructure which has proven to provide numerous benefits such as those already mentioned as well as energy conservation, improved air quality, and enhanced local biodiversity. Due to rapid urban growth with close to 90% of Americans anticipated to be living in urban centers by 2050, there is an increasing demand for green spaces. Some cities – Toronto being the first – have now made green roofs mandatory for new buildings. Toronto has created a green roof bylaw that was implemented in 2009 requiring buildings with a floor area of ≥ 2000 m2 mandate to include a green roof that takes up 20–60% of the total roof area. Other places have since joined this phenomenon. For example, Japan now requires public buildings larger than 250 m2 and private buildings larger than 1000 m2 to green 20% of their rooftop; and Portland, in the United States, requires all new buildings to devote 70% of their roof to green roofs (Shafique et al., 2018).

What are Green Roofs?

These roofs are essentially roofs planted with various different vegetation and plants on top of a growing substrate like soil. The vegetation should be determined based on location, rainfall intensity, humidity, wind, and sun exposure (Shafique et al., 2018).  Green roofs were designed with the intention of encompassing various ecosystem services including environmental, social, and economic benefits with a lifespan of 40-55 years (Shafique et al., 2018). They are made up typically of several layers including a waterproof membrane, insulation, drainage material, filter layer, substrate, and then vegetation which can be seen in the following diagram. 

Figure 1: Components of a green roof
Source: (Shafique et al., 2018)

Green roofs can then be classified into four categories based on substrate thickness including intensive (> 12in), semi-intensive (6-12in), multi-course extensive (4-6 in), and single-course extensive (3-4 in) (Shafique et al., 2018). The thicker the substrate, the higher the water retention rate and the more opportunity for various vegetation options. The thinner the substrate, the less maintenance island irrigation is required, and is often more suitable for buildings with weight restrictions.

Benefits of Green Roofs

As we know, green roofs are very environmentally conscious and help reduce the ecological impact of the buildings they reside on. These spaces not only insulate their buildings but can also reduce the UHI of the surrounding area. A study conducted in Toronto found that green roofs have the capability to reduce the surrounding air temperature by up to 0.4 °C during the day and 0.8 °C at night time (Shafique et al., 2018). Green roofs offer numerous benefits to the local area and have opportunities for individuals who may lack access to other green spaces. These roofs offer the potential for residents or users of the building to engage with nature where they otherwise would not have been able to. Adding green space to a residential building offers a sense of ownership to those who have access and thus tend to personally care for the space compared to a public green space. Creating a sense of ownership also offers the potential to develop a relationship between person and space. This relationship being inherently eco-conscious then offers inspiration for individuals to share the experience and nourish the space. 

Socially, these green roofs also offer the potential to aid in food security through urban agriculture. With deeper substrate, these green roofs can double as an urban farm which can contribute to food diversity and food production. However, it is unrealistic to believe that these farms are the entire solution to feeding cities and ending food insecurity. With the spread of COVID-19, weakness in food supply chains has been revealed and the importance of local food has been highlighted. These rooftop farms have the ability to fill these gaps in the food supply chain when necessary but are not substantial enough to completely take over. A study conducted in Boston found that rooftop farms have the capacity to produce 30% of the city’s fruit and vegetable demand but that is it. What these rooftop farms can offer is food diversity which can offer various culturally significant foods that are not offered or are difficult to find in the standardized grocery. 

Challenges of Green Roofs

Despite these remarkable contributions to urban scapes, green roofs also pose challenges as they still are a very young concept. Numerous studies and reports on the almost magical idea that these roofs have the capability to essentially end world hunger and food insecurity will soon be a distant memory that is unfortunately far from the truth. The foods that can be grown on roofs are not at the capacity to feed entire cities, let alone provide enough nutrients. The fruits and vegetables grown on roofs can not provide a complete diet full of all the required vitamins such as B12 and vitamin D for a person. The cultivation of certain crops is often also very difficult on rooftop farms. Certain solutions exist such as hydroponics which is an excellent option for growing leafy greens such as lettuces and herbs. 

Another area in which rooftop farms face complications is nutrient leachate. A study conducted at the Brooklyn Grange in New York City found that 97% of annual leaching was due to fertilizers used on the farm. The leaching of nutrients such as Nitrogen is responsible for polluting local groundwater which can have detrimental effects on the greater water cycle. Fertilizer leaching is responsible for the contamination of groundwater in places such as the Chesapeake Bay, the Gulf of Mexico, and Long Island Sound now have “dead zones”. Risks of nutrient leachate need to be given heightened consideration in urban areas with combined sewer systems, such as in New York, due to the higher risk of bypassing treatment plants during heavy storms. It is considerations like these that prove the complexity of these urban systems, as well as the necessity for green roofs to be properly monitored.

One final challenge that will be briefly explored here is the threat of heavy metal accumulation on urban farms. With urban farms often being in close proximity to major roadways, it has been proven at locations like Brooklyn Grange that these urban farms do not only act as carbon sinks but also act as a net sinks for heavy metals such as lead and manganese. With limited research, it has been established that this heavy metal accumulation has the potential to pose issues for growing vegetation over time. Though these levels have not exceeded safe operating levels, this does prove a gap in long-term research. To reduce the risk of heavy metal deposition in urban farm soils, it is crucial to cover the soil with mulch and replace it regularly.

References

Shafique, M., Kim, R., & Rafiq, M. (2018). Green roof benefits, opportunities and Challenges – A Review. Renewable and Sustainable Energy Reviews, 90, 757–773. https://doi.org/10.1016/j.rser.2018.04.006 

Sarah Lawless
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