Why Do Cities Need Green Roofs?
By: Sarah Woodams
Tags: Vision, Climate Action, Green Roofs, Science, Sustainability

General History and Overview

Image by Peggy Choucair from Pixabay

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 (substrate) 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.

Chicago City Hall Green Roof by Tony The Tiger on Wikimedia Commons

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.