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Traditional living roofs (green roofs) help control stormwater runoff by retaining water in the plants and growing medium. This water is released back into the atmosphere through the process of evapotranspiration, which significantly reduces the pressure on stormwater infrastructures and sewer systems over the course of a year.
Detention roofs such as blue-green roofs or the Purple-Roof concept also offer detention, which means slowing down the water that the roof cannot retain; the water that becomes runoff. This runoff delay of the peak storm runoff helps to prevent combined sewage overflow, flooding, and erosion. These types of green roofs are unique in that they can replace, or reduce, other types of gray stormwater infrastructure such as stormwater tanks or cisterns.
Ultimately, the combination of retention and detention saves costs related to the renovation of buildings and the expansion of drainage, and other stormwater infrastructure systems, on both a city and a project scale.
Traditional roofs experience an abundant amount of heat loss in the winter and heat gain in the summer. Vegetated roofs provide an extra layer of insulation that blocks solar radiation from reaching the building. However, the main energy-saving effect of vegetated roofs comes directly from the plants. As the plants evapotranspire and release water vapor into the air, heat is lost in the process, and this leads to effective cooling. This helps reduce energy consumption, thus reducing energy costs. As an additional bonus, more efficient energy conservation means fewer greenhouse gas emissions released into the atmosphere. This helps reduce energy consumption, thus reducing energy costs for both heating and cooling. As an additional bonus, more efficient energy conservation means fewer greenhouse gas emissions released into the atmosphere.
It should be noted that the energy savings made on a particular building are dependent on the building structure and level of insulation. A low building with a badly insulated roof will generate more savings than a tall building with a well-insulated roof.
Also, the energy savings tend to have more impact during summer than in the winter, so the biggest benefit for the energy savings will be in climates with lots of solar radiation (sunshine).
Reversal of Urban Heat Island Effect
The Urban Heat Island Effect is a problem found in cities around the world. As developments in urban areas increase, there is less green space. Hard surfaces, such as concrete and asphalt heat up much faster than trees, grass, and other greenery. These hard surfaces absorb the heat during the day and radiate it at night. This results in a big zone of hot air around urban environments. Green roofs create an adverse effect by cooling infrastructures and the surrounding air by evapotranspiration. To put this into perspective, in New York City, a mean temperature difference of 2°C could be measured between the most and least vegetated areas of the city (Susca et al. 2011). This might not sound a lot, but a 2°C increase from 35-37°C significantly increases the heat mortality risk of the population!
Reduction in Ambient Noise
The many layers of a vegetated roof reduce sound exposure. This natural insulation barrier absorbs and deflects sound waves. The growing mediums block lower sound frequencies, and the plants block higher frequencies. The level of sound insulation is dependent on the type of green roof used. For example, extensive green roofs, which have a thinner growing medium, can reduce high-frequency noise by up to 3 dB and low-frequency noise by as much as 8 dB. For those needing quieter office spaces, a green roof is a good solution (Renterghem, 2018).
Green roofs provide new opportunities for wildlife to feed and shelter. They also provide conservation for those species affected by habitat loss. A variety of ants, spiders, flies, beetles, and other insects have been spotted on these eco-roofs. It is extremely beneficial to urban beekeeping and endangered species, particularly wild bees, if these intensive green roofs flower in the summer and during dry periods.
Migrating birds use these roofs as a rest stop. New York City is located along a migration route known as the Atlantic Flyway. One can see over 200 different species flying through spring and fall. The addition of these pollinators in urban areas also help propagate seeds.
However, it is still important to keep in mind that sometimes, the best strategy to increase biodiversity is to aim to go from a bare roof to any kind of green roof simply because all types of green roofs are better than bare roofs. Desiring only intensive green roofs in order to achieve the ultimate biodiversity means a higher construction cost due to increased load-bearing weight, increased cost in assembly and maintenance, and substantial irrigation needs. This can reduce the implementation rates of green roofs.
Extensive green roofs still support a surprising amount of biodiversity, and because they are lightweight, more affordable to install, and do not require regular irrigation, they are much more likely to be built.
If you are interested to read more on this topic, please take a look at this article:
Green Roof Biodiversity Challenges
Increased Longevity of Roofing Membrane
Green roofs can increase the lifespan of a roofing system by protecting it from natural elements, such as direct ultra-violet radiation and extreme temperature fluctuations. Minimizing temperature fluctuations means the roofing materials' expansion and contraction peaks are less severe, which extends the life cycle. This protection extends the lifespan of the waterproofing 2 or 3 times as long as conventional roofing, according to several studies, most notably the SOPREMA study from 2020. This decreases the need for maintenance and repairs saving the owner money.
Also, in hail-prone areas, the green roof protects the membrane from hail damage. Regions like Colorado experience lots of extreme hail each year, and a green roof is a real cost saver.
Many of our air pollutants, caused by traffic or other industry, can have a negative impact on one’s health. Some of the effects are minimal, such as allergy-like symptoms, others are more serious, such as heart and lung disease. Research has shown that the plant matter on green roofs helps filter out a significant portion of these pollutants from the surrounding air.
Living roofs utilize the once unused space in many ways. Rooftops can be developed into social and recreational spaces. Some examples of ways this new green space can be used for our office meetings, outdoor gyms, solar panels, rooftop gardening, and soccer stadiums such as the one in Tokyo, Japan. All these uses are beneficial to the health and well-being of citizens. City life is known to keep people inside these buildings, but this newfound space can be an escape from the concrete jungle.
Tax Credit/ Abatement
In the US, in order to reduce the Urban Heat Island Effect or to improve the sewer’s stormwater capacity of a city, incentives are rewarded to property owners that will install green roofs. The state you reside in and the size of your green roof will be the deciding factors on the amount you receive. In the city of Philadelphia, one would receive a credit against the Business Privilege Tax of 20% of all costs associated with the construction of the green roof (as long as it does not exceed $100,000). To be eligible, the green roof must cover 50% of the rooftop or 75% of the eligible rooftop space. Other cities like NYC, Chicago, Washington DC, and many more cities offer rebates, subsidies, or other incentives to help offset or even pay 100% for the green roof.
Similar incentives have been set up in several of the major European cities.
In conclusion, it pays to install a green roof.
Susca, Tiziana, Stuart R. Gaffin, and G. R. Dell’Osso. "Positive effects of vegetation: Urban heat island and green roofs." Environmental pollution 159.8-9 (2011): 2119-2126.
Van Renterghem, Timothy. "Green roofs for acoustic insulation and noise reduction." Nature Based Strategies for Urban and Building Sustainability. Butterworth-Heinemann, 2018. 167-179.
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