Why Do Cities Need Green Roofs?
General History and Overview
Green roofs are not a new invention. Variations of green roofs are seen throughout history in the Hanging Gardens of Babylon in 500 BCE and sod covered houses in Nordic countries and the prairie settlers of the American Midwest.
The rise of modern green roofs began in Germany in the early 1960s within an effort to reduce energy consumption during an energy crisis and have since spread across the world.
While green roofs can vary in size, shape, makeup, and name, the core elements remain the same. They are roofs that have planted vegetation on top of a growing medium which can range from a simple grass in dirt set-up to an elaborate garden. Oftentimes, there are more specialized layers to provide drainage, filtration, insulation, and waterproofing to ensure that the green roof can provide the most benefits to the building.
There are two main categories of green roof: intensive and extensive. They are separated by the depth of the substrate, with intensive green roofs having >6 inches of substrate and extensive green roofs with around 3 to 6 inches of substrate. Extensive green roofs are lighter and require less irrigation than extensive ones so they are the most popular variety.
The Benefits
Improving Stormwater Management: During any rainfall events, rainwater is either absorbed by permeable surfaces or becomes runoff and can end up in the sewers and nearby water systems or create floods. Green roofs are permeable surfaces that absorb rainwater, which decreases the amount of stormwater runoff.
Stormwater runoff can decrease nearby water quality because it can carry urban pollutants into nearby water sources, eroding river banks, and cause flooding.
Additionally, any American city whose sewage system was built before the 1930s often has its storm drains connected to the sanitary sewers that transport wastewater to treatment plants. Too much stormwater can cause a combined sewer overflow, which causes untreated sewage mixed with stormwater discharge into rivers and lakes.
Green roofs can reduce peak flood rates during rainfall events and increase the amount of time it takes for rainwater to reach the sewers. This can decrease the occurrence of urban flash floods and combined sewage overflow events.
Reducing the Effects of an Urban Heat Island: A heat island is an area that experiences higher temperatures than the surrounding land. Urban heat islands are formed due to an abundance of structures such as buildings, roads, and other infrastructure absorbing heat and then re-emitting it more than areas with more greenery and water sources.
Urban areas are around 1 to 7 degrees Fahrenheit higher during the day and about 2 to 5 degrees higher at night compared to the surrounding area. Green roofs can help decrease this temperature difference.
Green roof temperatures are shown to be about 30 to 40 degrees Fahrenheit lower than black surface temperatures during the summer.
How is that possible?
Green roofs absorb less sunlight than traditional roofs and in the summer can cool buildings and the surrounding air through evapotranspiration where water from the soil is released as vapor by the pores of plant leaves. The energy (heat) used to turn liquid water into vapor cools the plant and leads to a cooling effect on and around the building.
Increasing Insulation and Energy Savings: Green roofs can provide increased insulation for buildings. In the summer, green roofs can reduce the transfer of heat from outside the building to the inside by up to 72%. In the winter, they can help keep heat indoors. This can reduce energy costs by decreasing the amount of cooling or heating needed in a building.
This increased insulation is not as important as the cooling effects provided by the evapotranspiration effect mentioned above.
Increased insulation can also reduce the amount of outside noise heard inside the building if the building doesn’t have ceiling insulation.
Improving Air Quality: The plants on green roofs can help improve air quality by removing air pollutants such as carbon dioxide, smog-forming compounds, and particulate matter. This greatly depends on the type of plants chosen as some absorb pollutants better than others and the substrate depth.
Plants that are hyperaccumulators can absorb high levels of pollutants without suffering, and those with larger leaf surface area can absorb more particulate matter.
The Challenges
Strict Building Requirements: Since many green roofs are retrofitted onto buildings, the current roof must be inspected to make sure that all pre-existing problems are identified and fixed and to ensure that the roof can handle the weight of the green roof.
The roof must be accessible for any maintenance, such as structural upkeep, plant pruning, fertilization, plant replacement, or new planting.
The maximum roof pitch for retrofitting extensive green roofs is thought to be either 35 degrees or 32 degrees, while intensive roofs require relatively flat surfaces. The main concern is that the substrate will shift unevenly over time if the roof is titled too much.
Additionally, the orientation of the roof and any shade caused by nearby buildings need to be considered to determine which plants are best for the green roof. While some plants are fine in most sun conditions, others only thrive under high or low sun exposure. Also, if the roof is too high, the higher wind can be unsuitable for certain plants.
Pollution From Production: Low-density polyethylene and polypropylene polymers are used in many green roofs to reduce their weight. Unfortunately, the production of these polymers releases air pollutants such as NO2, SO2, O3, and PM10.
A life-cycle analysis of the net pollution of green roofs shows that it takes green roofs with non-recycled materials 27 to 37 years to reach net-zero pollution, while green roofs with recycled materials take only 13 to 18 years.
However, this study doesn’t appear to consider the varying effects of different air pollutants, as some are worse than others. The pollutants that a green roof absorbs over its lifetime may not be all the same pollutants that were emitted to create it.
Green roofs are also generally thought to not require regular irrigation or fertilizer, but if it’s located in a dry climate, those actions are necessary for it to thrive. The plants, drainage system, and substrate, should also regularly be checked to make sure the roof lasts as long as possible. These maintenance requirements add up to create additional costs for the owner(s).
Pollution in Runoff: While traditional roof runoff is full of its own pollutants such as polycyclic aromatic hydrocarbons (PAHs), organic halogens, and heavy metals such as lead, copper, zinc, and cadmium, green roofs struggle with pollutants from fertilizer.
Studies have found that green roof runoff has elevated levels of phosphorous, whose source was most likely the fertilizer. One study found that despite green roof runoff containing triple the levels of phosphorous compared to traditional roof runoff, the concentration of phosphorous decreased over the first few years of operation. However, if the green roof required further fertilization, this whole process could start all over again.
Nitrogen and phosphorous from fertilizer are a water pollutant that can cause an increase in algae growth and even algal blooms which can disrupt and kill aquatic life.
Green roofs are not the perfect solution to any of the above issues, but they are certainly a part of a holistic approach to improving cities and combatting the negative effects of urbanization. Green infrastructure, such as green roofs, must be integrated into cities across the world to improve the lives of residents.
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