Green Roofs: A Comprehensive Guide

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. Despite being one of the most expensive roofing types, costing anywhere from $10 to $35 per square foot, green roofs seem to be the up-and-coming sustainable roofing alternative that provides plenty of environmental and financial benefits. Let’s dive in.

On this page

• What is a green roof?
• How does a green roof work?
• Types of green roofs
• Pros and cons of green roofs
• Government incentives for green roofs
• Is a green roof right for you?

What is a green roof?

A green roof, also known as a living roof, is a vegetative layer grown atop a roof or building. Typically installed on flat roofs, green roofs can contribute to the energy efficiency of commercial and residential buildings by providing shade, removing heat from the air surrounding the building, and reducing the temperature of the roof’s surface.

How does a green roof work?

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

• 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. 
• Filter  – The filter fabric keeps the media from clogging up the drainage while allowing water to flow freely. It contains granular materials such as pozzolana, lapilli, pumice, perlite, expanded clay, slate, and broken bricks, alongside a substrate containing mineral materials like zeolite.
• Drainage – In order to keep leaks at bay, a drainage layer is added to dispel excess water from the roof. 
• 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.
• 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.

Although these are the typical layers for a green roof, the number and arrangement of different layers are subject to variation based on the specific type and design of the green roof system you opt for.

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 three main types of green roofs: extensive, semi-intensive, and intensive.

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.

Image source: Zinco

Semi-intensive green roofs, also known as rooftop gardens, are a little heavier than an extensive roof and typically consist of modular containers. Despite needing more maintenance and irrigation, they can offer a more sculpted look than other types of green roofs. 

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 or semi-intensive green roofs and tend to cost a bit more. Intensive roofs may also require more maintenance down the road.

Pros and cons of green roofs

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

Pros of green roofs

Energy savings – The natural insulation provided by the soil and vegetation of a green roof is very effective in maintaining a building's internal temperature, which reduces the need for excessive heating, leading to lower energy consumption and savings on electricity bills.

Cooler urban temperatures – Buildings with green roofs can also contribute to energy savings at a broader, community level by mitigating the urban heat island effect. This phenomenon, common in densely built-up areas, occurs when traditional building materials like concrete and asphalt absorb and radiate heat, significantly raising urban temperatures. Green roofs could help to cool these urban areas, reducing the overall demand for energy across the community.

Improved air quality – Plants engage in the process of photosynthesis, where they absorb carbon dioxide (CO2) and release oxygen. This biological process is what makes green roofs capable of purifying the air around them. Green roofs can also host a variety of plants that can trap airborne particles such as dust and pollutants. This can not only clean the air but can also reduce the prevalence of smog, leading to a healthier urban environment.

Stormwater management – Urbanization has led to an increase in impervious surfaces like concrete and asphalt, which prevent water from naturally seeping into the ground. This leads to an increase in stormwater runoff, carrying pollutants into waterways and increasing the risk of flooding issues. The layers of vegetation and soil used in green roofs act as a sponge, absorbing and retaining rainwater. This process significantly reduces the amount of runoff flowing into storm drains and lowers the risk of sewer system overflows during heavy rainfalls.

Contribution to rooftop solar energy – Green roofs have a natural cooling effect on buildings. This is particularly beneficial for solar panels, as they tend to lose efficiency when overheated. Studies have shown that cooler rooftop environments can boost solar energy production by 5 to 15%. This increase in efficiency means that buildings with both green roofs and solar panels can generate more electricity from the same number of solar panels, compared to those with traditional roofs.

Less material waste – The durable nature of green roofing leads to fewer replacements, contributing to a reduction in material waste in landfills. The protective layer of soil and vegetation prevents the typical wear and tear seen in conventional roofs, such as cracking and degradation due to sun exposure or thermal shock from rapid temperature changes. Regular maintenance of a green roof can also extend its lifespan by over twice that of a conventional roof, somewhat compensating for the initial installation costs and reducing the need for more materials.

Long lifespan – A green roof can last more than 40 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!

Cons 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. A basic extensive roof can cost from around $10 to $20 a square foot. An intensive roof is often more expensive, reaching around $25 to $35 a sq.ft.).

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 there is sufficient drainage and insulation to protect your home below.

Government incentives for green roofs

Green roof incentive programs can take various forms, including tax credits, funding (grants, rebates, or subsidies), stormwater fee credits, density or floor area ratio bonuses, and residential stewardship programs.

Portland and Chicago were among the early adopters of green roofs in the U.S. Both cities implemented financial incentives, regulations, and policies to encourage green roof installations. In fact, Chicago leads U.S. vegetated roofing incentives with programs like the Green Roof Improvement Fund, Green Roof Grant Program, and Cool Roofs Grant Program, all providing grants for vegetated roofing projects.

More recently, cities like Portland (Oregon) and Cambridge (Massachusetts) have made green roofs mandatory for certain new buildings. Other U.S. cities, including New York, Seattle, San Francisco, Gatineau, and Washington D.C., have also established green roofs to meet green building goals and stormwater management objectives.

Is a green roof right for you?

The decision to install a green roof is not one-size-fits-all. It’s influenced by a variety of factors such as the local climate, specific building design, and existing local regulations. While green roofs present a highly sustainable option with multiple environmental benefits, it’s important to acknowledge their drawbacks, including higher upfront installation costs, ongoing maintenance, and structural considerations.

Despite these challenges, the growing interest in green roofs is a positive sign towards more sustainable urban development. With increasing governmental support through incentives and regulations, along with advancements in technology, the implementation of green roofs could become more widespread, contributing to healthier and more resilient urban environments.