What is a Green Roof and How Can it Benefit Your Building?

A green roof is a "living" roof that incorporates vegetation such as flowers, shrubs, trees, and vines. This type of biophilic design is becoming increasingly popular on rooftop decks. 

What is a green roof, and how can it benefit your building?

Green roofs, also known as vegetative, living, and eco-roofs, can be found on top of both commercial and residential buildings. Living roofs offer a variety of benefits that stretch far beyond the aesthetic.​​​​​​​

What is a green roof system?

A green roof is a rooftop that's either partially or completely covered with vegetation. Green roofs can be installed on both flat and sloping roofs. Fauna is planted in a special growing medium, which is placed on top of a waterproofing membrane and drainage layer. Green roofs with shallow growing mediums are best suited for growing low-growing plants, while green roofs with deeper growing mediums can support a wider variety of greenery, including shrubs and small trees.

Green roofs have long been a fixture in European architecture, and they've become increasingly popular in the United States. The most well-known can be found on top of Chicago's City Hall; however, Washington, D.C., currently boasts the most green roofs in the country.

What types of green roofs are there?

Not all green roofs are the same. The different types of green roofs are as follows:

• Extensive: Extensive green roofs are lightweight and require minimal maintenance, but they are not usually suitable for walking on. They are often referred to as "vegetative blankets" because of their shallow depth and are typically installed solely for the sake of their environmental advantages.
• Intensive: Also known as rooftop gardens, intensive green roofs are deeper than extensive roofs and can support a variety of plant life, including shrubs and small trees. They usually require more maintenance than extensive green roofs but can provide an accessible outdoor space for the building's occupants.
• Semi-intensive: Semi-intensive green roofs are a mix of the two previous types and can offer both environmental and recreational benefits. They are able to host richer biodiversity than extensive green roofs and require less maintenance than intensive green roofs.
• Solar green roofs: Solar green roofs support photovoltaic panels, which convert thermal energy into electricity. They enable the building to benefit from renewable energy production, in addition to the other advantages traditional green roofs offer.
• Blue-green roofs: Blue-green roofs are a type of green roof that also supports rainwater harvesting and storage. They are often used in areas with high rainfall to help mitigate the risk of flooding.

What is the purpose of a green roof?

Green roofs transform ubiquitous black asphalt rooftops into elevated gardens that benefit the environment, the building itself, and the people in and around it. The main benefits of green roofs for healthy cities are:

1. They improve a building's efficiency and reduce energy costs.

Vegetation on roofs is a natural insulator. It helps retain heat during winter and reduces the amount of heat that the roof absorbs during the summer. Green roofs enable energy savings that result in lower utility expenses for heating and air conditioning.

2. They extend the life of a building's roof.

A green roof can add years to the life of the roof, in some cases doubling its expected lifespan. The vegetation provides an extra layer of protection that shields the roofing materials from damaging UV rays, extreme temperatures, and severe weather.

3. They improve air quality and reduce noise pollution.

Green roofs can reduce the levels of air pollution as plants take in carbon dioxide and release oxygen. Green roofs also help reduce noise pollution by absorbing sound waves, which can help create more relaxing homes and more productive workplaces.

4. They provide green space in an urban environment.

Green roofs inject green spaces into urban environments, which has several benefits. The plants attract birds and insects, helping boost biodiversity in cities. Green roofs also provide opportunities for urban agriculture and recreation in otherwise unused spaces.

5. They help the wider environment.

Having vegetation on rooftops mitigates the Urban Heat Island Effect, which often causes higher temperatures in cities. They also aid stormwater management, as less rainwater reaches the ground, reducing the chance of flooding and the number of pollutants that end up in waterways.

6. They add to a building's aesthetic appeal and can add value.

A green roof adds to a building's visual appeal. A well-designed green roof complements the architecture of a building and creates an inviting space for people to enjoy. This can increase the value of the property and make it more attractive to potential tenants or buyers.

Things to consider when creating a green roof

Now that you know the answer to "What is a green roof?" and want to create your own green roof, here are some factors to consider:

• Ensure the building can bear the weight of a green roof: Green roofs weigh significantly more per square foot than conventional roofs, so it's crucial that the structure of the building can support this extra weight.
• Use a frame: If you're installing a green roof on a roof that slopes, you'll need to use a frame to ensure the vegetation stays in place.
• Choose the right plants: Not all plants are suitable for green roofs. When selecting vegetation, consider the climate, the weight of the plants, their root system, and how much maintenance they will require.
• Use lightweight green roof substrate: Green roofs need material to support the vegetation. It's essential to use a lightweight substrate that won't add too much weight to the roof, not regular garden soils.
• Call in the professionals: Installing a green roof requires specialist knowledge and skills — it's best to leave it to experts such as those at Bison, who have helped individuals and organizations across the country create bespoke green roofs.

While there are some initial costs associated with installing a green roof, the inherent long-term economic and environmental benefits make them a wise investment.

If you're thinking of installing a green roof on your building, or if you want to learn more about them, Bison can help. We've been creating environmentally conscious rooftop environments since 1994 and specialize in planning, materials, and education. Contact us today to find out how we can help you incorporate a custom green roof that meets the specific needs of your building.