The name green roof doesn’t refer only to the color of plants and other growing things that define a green roof installation. Instead, these roofs functionally support many green ideas like renewable energy, sustainable development and environmental stewardship. In fact, their contribution to these important goals is so significant that support for living roofs has moved beyond the land of Hobbiton and is spreading from Europe, across the US and other parts of the world.
So, what are these amazing contributions? We’ve learned in the last article about how green roofs can mitigate or even eliminate urban heat islands and their negative effects. Now, let’s look at some of the important mechanisms that plants and soil use to support that green moniker.
Green Energy
Clearly, when a green roof deflects solar energy or uses it for other purposes (photosynthesis), it keeps a building cooler which reduces electricity use. This saves money for the building owners, but also produces a host of benefits related to lower energy demands.
Green roofs, however, can contribute directly to sustainable energy goals. One popular feature of roofs all over the place, whether homes, hospitals, or corporate office buildings, is rows of solar panels. Roofs are great for placement of solar panels since they usually enjoy full exposure to the sun, and power produced by those panels can be used immediately in the building below.
It might seem that it’s not clear how to use roofs to their best advantage: is it more important to produce sustainable energy and reduce our reliance on fossil fuels or to promote green community spaces, support habitats for critical wildlife populations, or grow garden-fresh produce immediately where it will be consumed?
Fortunately, there’s not really a conflict between those goals. In fact, pairing solar energy panels and green roofs (called rooftop agrivoltaics) establishes a symbiotic relationship where both sides contribute to the other’s success and the overall result is greater than the sum of its parts.
Obviously, solar panels need to be placed in direct sunlight where shade won’t interfere, such as on a rooftop, but the use of roof materials like tar and gravel are known to produce increased temperatures compared to surrounding air. This is the same mechanism that creates an urban heat island. The real problem in this case is that the performance of photovoltaic (PV) panels is reduced as the temperature around them rises. So, to get maximum efficiency from the panels, there needs to be some way of eliminating the heat island and keeping the air cool.
Enter the symbiotic relationship between solar panels and green roofs. The nature of a green roof reduces the temperature of areas exposed to direct sun. Evaporation and plant transpiration both actively reduce the temperature of the surrounding air because heat energy is absorbed as water molecules go from a liquid to a gaseous state.
That’s great for the solar panels, but what about the plants that make up a green roof? In a typical setup, a green roof is established first and slightly elevated solar panels are added shortly thereafter. Solar panels standing a few feet above the green roof provide partial shade to the plants below. This not only permits greater biodiversity for plants which are sensitive to direct sun, but it reduces their exposure to UV radiation and further reduces extreme temperatures. These effects reduce the water use of the partially shaded plants and reduces drought stress. Relief from temperature extremes, protection from intensive UV radiation, and better use of water are valuable benefits, especially for food crops.
So not only do green roofs reduce consumption of energy, but they’re also able to directly contribute to the efficient production of renewable energy. That’s a pretty good partnership.
Stormwater Management
The presence of green roofs in an urban area have proven to enhance both stormwater management and water quality, which are critical issues in most urban areas.
During heavy storm events, high levels of precipitation may overwhelm the municipal storm drains, causing flooding and even sewer overflow. The resulting mixture of rainwater and partially treated or untreated sewage may back up into streets, presenting significant health hazards, or it may continue on the stormwater route and be discharged directly into nearby streams, rivers, and lakes. Even if no sewage gets intermixed, the scouring effect of high-speed stormwater can trigger erosion. The flowing floodwaters pick up contaminants from streets and other paved surfaces, as well as silt and sediment that can produce turbidity in surface waters that directly impact the health of the aquatic ecosystem. Effective management of stormwater is high on the priority of most municipalities, and the benefits of green roofs are increasingly well known.
Green roofs can both slow and reduce significant amounts of stormwater runoff in urban areas. Urban areas are particularly vulnerable to intense precipitation events because impermeable surfaces do not allow water to naturally seep into the ground. Instead, water stays above the surface and flows together in whatever downhill direction presents the least resistance. This means that the accumulated rainfall from an area measured in dozens of square miles is retained and directed to a single system. It’s not hard to imagine that the system can be overwhelmed.
It varies over seasons and specific rainfall patterns, but green roofs can typically absorb and retain upwards of 80% of storm-related precipitation during summer months, just like soil and vegetation does in nature. Green roofs in winter months typically have a lower retention rate, though it is still valuable. Sloped roofs may reduce the amount of water that’s retained, particularly if it flows off the roof before the growth medium has a chance to absorb it. Extensive roofs typically have a relatively shallow layer of growth medium, and they usually capture between 50% and 100% of precipitation. Intensive green roofs have a substantially deeper layer of growth medium, and they will retain more rainfall.
As a bonus, the roots and growing medium of a green roof effectively filter out many pollutants that come in with the rain. As rain falls through the atmosphere, it picks up particles from smoke, smog, dust and various types of industrial pollution, so when it hits the ground it can carry seriously harmful substances like lead, cadmium, and some types of hydrocarbons. The vegetation in green roofs, however, can bind and retain much of those pollutants, so that water passing through is higher quality water than what fell from the sky.
Filtering Air Pollution
Green roofs, just like green spaces within cities and planted medians on city streets, are also valuable for their ability to reduce not only dust levels but smog production within a city. Plants consume carbon dioxide during the process of photosynthesis, but they, along with certain soil microorganisms, can purge volatile organic compounds (VOCs) like formaldehyde and benzene, which are known to cause cancer. Vegetation growing on green roofs also passively absorb pollutants that settle on leaf surfaces and the soil, or directly absorb toxins through their pores (stomata) during transpiration.
