How Green Roofs Help Beat the Heat

Illustration of skyscrapers with green gardens on their roofs

Green roofs – roofs with soil and plants placed on top of conventional roofs – are growing in popularity. And that’s a good thing. 

Urban sprawl and growing population density contribute to climate change in a way that is often overlooked: heat islands – urban areas that are hotter than surrounding rural areas.

According to the EPA, “Heat islands can affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and mortality, and water pollution.” That’s a significant amount of harm!

Green roofs may alleviate some of the harm that heat islands create.

The benefits of green roofs

Plants on green roofs capture airborne pollutants and atmospheric disposition and filter noxious gases. By helping to moderate temperatures inside the buildings beneath them, green roofs also reduce the stress on power plants, which can reduce the amount of CO2 released into the air. And the benefits don’t stop there. Here is a list of the personal and community benefits of green roofs.

Personal benefits

Fire retardation

In California alone, 14 of the most massive 20 wildfires occurred in the last 15 years. When correctly built, green roofs can aid in the protection of homes because the common plants used in extensive green roofs are sedums and succulents, which retain much more water than other plants making them less likely to ignite or catch fire. 

Increased roofing membrane durability

Green rooftops act as an additional barrier between the sun’s harsh ultraviolet rays and the waterproofing membranes on your roof that can be damaged by drastic temperature fluctuations. 

Energy efficiency

The extra layer of protection from the sun that green roofs provide helps to reduce energy costs by acting as natural insulation. The National Research Council in Canada found that green roofs reduced the need for air conditioning by nearly 75%. This process of lowering the temperature of the building is called evapotranspiration, whereby the plants absorb water through their roots and use the heat in the surrounding air so the water evaporates. 

Improved productivity

A recent study conducted at Stanford Medicine concluded that green roofs aren’t just good for the environment, they actually boost productivity. The study further showed that contact with nature not only lowered stress, but improved concentration and mood. 

Community benefits

Reduces the urban heat island (UHI) effect. 

As discussed earlier, UHI is caused by the heat created by energy from people, cars, buses, and trains in big cities. Areas affected by UHI can have worse air and quality than their rural neighbors. Green roofs help reduce the UHI effect because the plants absorb carbon dioxide and leading pollutants. 

Better water quality

Green roofs absorb and retain 70–90% of rainwater during the summer and 25–40% during the winter, thereby acting as a natural filter for stormwater runoff. 

Types of green roofs

Now that you know about the benefits of green roofs, let’s talk about the different types of green roofs, of which there are mainly two: intensive and extensive, which is determined by the depth of planting soil and the amount of maintenance they need. 

Intensive green roofs

Intensive green roofs are basically elevated parks. They’re recreational areas on rooftops that can support gardens, trees, playgrounds, parks, walkways, benches and other activities that require complex structural support. 

Chicago’s City Hall is a great example of an intensive green roof. 

Aerial view of Chicago City Hall
Chicago City Hall Green Roof by TonytheTiger CC by 3.0

Because of installation and maintenance costs and requirements, intensive green roofs are best suited for commercial or government buildings. 

If your town or city is proposing new construction, attend the town hall or meeting to advocate for the addition of a green roof. You can also organize to introduce a green roof initiative like the one recently passed in Denver

Extensive green roofs 

Extensive green roofs are a shallow covering of greenery on the roof. Not designed for recreational use, the extensive garden is simpler, costs less and has many of the same benefits. 

Because of the lighter nature of extensive green roofs, they require a weight capacity limit of only 15–50 pounds per square foot, which means they need less structural support. And, once established, they also require less maintenance. They are a great option for homes and condos.

What you need to build an extensive green roof

The first thing you should do is check with a professional installer to be sure your roof is a good extensive green roof candidate. 

A green roof can be built using a variety of materials, but the basic features of a green roof include: 

  • Membrane layer: This layer is the bottom-most layer that separates the green roof above from the structural supports below. 
  • Membrane protection: This layer can be made of lightweight concrete, insulation, thick plastic, copper foil or a combination of these materials. 
  • Insulation: This layer protects the membrane below by preventing the weight of the green roof from crushing the membrane below. 
  • Drainage: This layer is designed to absorb and remove the excess water and sometimes store the water to be used by top plants over extended periods of time. 
  • Root barrier: The root barrier is for deep-rooted plants like trees or shrubs, so it is only required for Intensive gardens. Usually, foil or plastic is used for this layer. 
  • Growing medium: Essentially, the soil or foundation for the vegetation that will sprout on top. The type, mixture and amount needed will depend on the kind of plants you choose. To prevent the wind from blowing topsoil, it’s a good idea to also add burlap jute blankets to this layer. 
  • Vegetation: This is the fun part. Once you have the layered structure, it’s time to choose your plants. A few things to keep in mind here are climate and the amount of sun your rooftop receives. For extensive gardens, sedums and succulents are best. 

This step-by-step Instructables Workshop will give you insight into one person’s experience. Or if you need additional inspiration, these aerial views of green roof solutions capture of the artistry and beauty of green roofs. 

Rooftop gardens

You might love the idea of green roofs, but if you’re not quite ready to create one for your building, you might want to consider a rooftop container garden. But, before you start planning the garden, make sure you have both the legal permission and the structural integrity required for the task. 

Our most vulnerable communities are affected by extreme heat first – and most. The EPA warns that cities like Chicago could see 30 more days per year of 100+ degree weather. If green roofs can help reduce the urban heat island effect, then let’s work to ensure municipal governments – and the real estate developers they approve – commit to the best kind of urban jungle: green roofs.