Get to know green roofs: benefits, cost, and more

Image source: Urbanscape

Gianna Cappuccio

By Gianna Cappuccio

August 25, 2021

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There’s a reason cities are known as concrete jungles. With miles upon miles of skyscrapers and apartment buildings, it’s hard to find a patch of green.

Innovative building owners – and homeowners, too – have decided to reclaim their roofs and install an eco-friendly alternative: green roofs.

Green roofs are the up-and-coming sustainable roofing alternative that provides plenty of environmental benefits. Let’s dive in.

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What is a green roof?

A lush green roof installed on a residential home

Image source: Architectural Digest

A green roof, also known as a living roof, is a vegetative layer that is grown atop a roof or building. Typically installed on flat roofs, green roofs are essential to the energy efficiency of commercial and residential buildings. Green roofs provide shade, remove heat from the atmosphere, and reduce the temperature of the roof’s surface. and the surrounding air.

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How green roofs help the environment

Green roofs curb the urban heat island effect

The sun beating down on a city

Image source: Buildings.com

The urban heat island effect occurs when metropolitan or urban areas are significantly warmer than their surrounding rural or suburban areas. Green roofs can combat this effect by reducing the air’s temperature.

How does a green roof work?

Green roofs can reduce the temperature of a building in two different ways:

  • Shading, meaning the roof is shaded by the layer of green on top.
  • Evapotranspiration, the plants on the green roof will absorb rainwater through their roots and use the heat from the surrounding air to evaporate the water.

Using these two methods, green roofs can actually remain cooler than the surrounding air temperature, effectively acting as the city’s air conditioning unit.

How does a green roof work?

Green roofs are made up of several layers, each with its own specific purpose:

A diagram illustrating the layers of a green roof

  • Vegetation – The top layer of a green roof is vegetation. It’s important to choose vegetation that will thrive in the surrounding climate. Taller, heavier plants can be used on flat, commercial rooftops while smaller, lighter vegetation can be used on a slanted or residential roof.
  • Growing medium – The growing medium serves as the foundation of the vegetation layer. The type of soil and depth of the medium depends entirely on the vegetation chosen.
  • Root barrier – If you choose deep-rooted trees and shrubs for your roof, you’ll need a root barrier. This layer protects the roots from growing through your roof.
  • Drainage – In order to keep leaks at bay, a drainage layer is added to dispel excess water from the roof.
  • Insulation – Insulation protects the below layer by preventing the weight of the green roof from crushing and impairing the membrane.
  • Membrane protection – Usually a slab of concrete, insulation, copper foil, or thick plastic, the protection layer prevents deterioration of the membrane below.
  • Roof membrane – The membrane is the very last layer of a green roof and the last line of defense of the structural support below. The membrane must be strong enough to support the often excessive water weight of a green roof and prevent it from seeping through to the structure below. Waterproof membranes are available but will cost you more.

Types of green roofs

Green roofs can be installed on a wide variety of buildings, from residential homes to apartments and offices – provided the building is equipped with a flat or low-slope roof. There are two types of green roofs: extensive and intensive. Each type serves a unique and specific purpose.

An extensive green roof on a city building

Image source: Zinco

Extensive green roofs are usually a bit simpler, with smaller, more manageable plants and a growing medium depth of two to four inches. They are relatively lightweight and low maintenance once they’ve fully grown.

An intensive green roof on a city building

Image source: Urban Green Blue Grids

Intensive green roofs can be a bit more complex. These roofs often feature large plants such as full trees and shrubs and resemble your typical garden or park. They require more structural support than extensive green roofs and tend to cost a bit more. Intensive roofs may also require more maintenance down the road.

With either type of roof, you’re introducing a variety of plants that promote biodiversity in your community.

Cost of installing a green roof

The actual cost of your green roof varies depending on the type of roof you install. Extensive green roofs can run anywhere from $10 to $12 per square foot. Intensive green roofs will cost you $16 to $19 per square foot.

Benefits and disadvantages of green roofs

Green roofs offer many benefits for communities and individual homeowners, but they are not without their drawbacks.

Benefits of green roofs

Improved air quality – A plant’s primary function is to remove carbon dioxide from the air and turn it into oxygen. When you install a green roof, you’re effectively breathing cleaner air and reducing your carbon footprint.

Energy efficiency – In the heat of the summer, green roofs can protect your home or office building from direct solar heat. In the colder months, green roofs can minimize heat loss and save you money on electricity bills.

Long lifespan – A green roof typically lasts between 35 to 50 years. as long as it is well-maintained.

Natural habitat – A green roof can provide a natural habitat for local birds and other wildlife. If you utilize your green roof to plant a pollinator garden, you could be saving the bees as well!

Stormwater management – Heavy rains often result in an abundance of stormwater runoff, which can push excess nutrients from fertilizers, pet waste, and other refuse into rivers and streams, polluting our waters. Acting like a sponge, the green roof will absorb as much stormwater as it can, therefore reducing the amount of runoff directed into the city’s sewer systems.

Disadvantages of green roofs

Expensive – Depending on the type of green roof installed and the proper irrigation, drainage, and waterproofing, a green roof can become costly. An intensive roof is often more expensive, especially when you take into account the types of plants as well as the square footage.

Maintenance – While extensive green roofs are relatively maintenance-free, they’ll require a bit of pruning and attention as they grow. Intensive roofs, however, may require the landscaping expertise of a professional groundskeeper to maintain.

Potential for leaks – With a roof that absorbs stormwater, leaks are possible. It’s important to ensure that when your green roof is installed that there is sufficient drainage and insulation to protect your home below.

Famous green roof systems

The Javits Center, New York City

The Javits Center's green roof

Image source: Untapped Cities

Installed in 2014, the Javits Center’s green roof is a nearly 7-acre oasis that can absorb up to seven million gallons of stormwater run-off, reduce an exceptional amount of heat gain, and has reduced the Center’s energy consumption by 26%. Home to 29 species of birds, 148 seagull nests, five beehives, and the occasional bat, the Javits Center’s green roof is often used as the highest example of sustainable roofing.

City Hall, Chicago

Chicago city hall's green roof

Image source: Geographical

As part of an EPA study and initiative to combat the urban heat island effect and to improve urban air quality, the city of Chicago began to dream up a green roof design. The semi-extensive green roof was installed in April of 2001 and has saved the city up to $5,000 a year on utility bills.

Nanyang Technological University, Singapore

Nanyang Technological University's green roof

Image source: Design Dautore

Nanyang Technological University’s School of Art Design and Media boasts sloping, green roofs that are fully available for students to use as they please. Not only does the roof serve as the coolest quad on campus, but it also harvests several gallons of rainwater for landscaping irrigation.

Whether you own a commercial building in the Big Apple or a home in the suburbs, a green roof is an excellent roofing choice. Albeit expensive, a green roof can reduce cooling costs for buildings as well as homes, all while doing a great service to the environment.

If you’re interested in installing a green roof on your building or home, we recommend getting in touch with a contractor experienced in green roofing today.

Get in touch with a local contractor today.

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