Using Green Roofs to Reduce Urban Heat Islands

green roof on a house

When heat absorption happens on a large scale, the urban heat island effect warms up a city’s climate. Fortunately, there are ways to combat heat waves in the city, and homeowners can help by installing green roofs.

If you notice, pavement is usually burning hot in the summer, while grass is much cooler. There’s a difference in surface temperatures because concrete absorbs heat. With a rooftop garden, you can provide shade for your house and add vegetation to the city’s artificial infrastructure.

What is a Green Roof?

top view of a green roof on a house
Photo Credit: Arlington County / Flickr / CC BY-SA 2.0

Literally and figuratively green, this eco-friendly roofing system consists of a layer of vegetation on top of a growing medium. It can add a splash of natural beauty to your home and help improve the environment.

When cities are built, natural vegetation is removed to make way for new buildings. Installing a green roof helps add some greenery back into the area. With its waterproof membrane, this roof can protect and insulate your home. It can be decked out with various types of:

  • Wildflowers
  • Small trees
  • Succulents
  • Grasses
  • Mosses
  • Shrubs

Green roofs, built for flat or low-sloping roofs no steeper than 45 degrees, are a unique type of roofing that stands out in any neighborhood. As long as you choose the right kind of plants for your climate and location, this roof only requires minimal maintenance.

There are three types of green roofs:

  • Extensive. No deeper than 6 inches, extensive roofs support mostly small plant life with shallow root systems. These low-maintenance roofs support 15 to 50 pounds per square foot.
  • Intensive. Built with a deeper growing medium, intensive green roofs are more expensive. But they can function as recreational areas and support larger plants – holding approximately 80 to 150 pounds of weight per square foot. The downside is that they require more maintenance.
  • Semi-Intensive. These roofs are deeper than extensive green roofs but not quite as deep as intensive roofs.

A green roof’s surface temperature can be cooler than the air temperature. This is quite different from a conventional roof, which can sometimes reach hotter temperatures than the air temperature by up to 90 degrees Fahrenheit.

What is the Urban Heat Island Effect?

The urban heat island effect (UHI) is an urban temperature increase when green spaces are displaced by artificial building materials such as concrete, asphalt, glass, and metal. These artificial materials absorb heat, increasing air temperatures.

Contributing to climate change, urban heat islands make air temperatures in urban areas notably higher than in rural areas. This effect is harmful to the environment and causes many problems, including:

  • Air pollution
  • Decrease in productivity
  • Increase in greenhouse gas emissions
  • Hot and uncomfortable living environment
  • Increased risk of heat-related deaths and heat stroke
  • Higher energy costs due to increased energy consumption

The temperature difference can be as drastic as 10 degrees. The United States Environmental Protection Agency (EPA) reports how much urban heat islands typically increase temperatures:

  • Daytime: Temperatures increase by 1 to 7 degrees Fahrenheit
  • Nighttime: Temperatures increase by 2 to 5 degrees Fahrenheit

How Do Green Roofs Reverse the Urban Heat Island Effect?

sloped green roof
Photo Credit: Lamiot / Wikimedia Commons / CC BY-SA 4.0

Due to their ability to resist solar radiation absorption, green roofs are known for their albedo – the amount of solar reflectance of a roofing material. Because green roofs are made of natural vegetation, they don’t absorb as much heat from the sun as artificial materials do.

Although green roofs aren’t as effective at reducing heat waves as cool roofs, they still help with urban heat island mitigation. Plus, they are a more environmentally friendly option than traditional roofing materials like asphalt.


Trees, shrubs, and other plants provide shade for the ground below – helping create cooler spaces in those areas protected from the fierce heat of the sun. Although most green roofs don’t have trees, even thick grasses or wildflowers can shade your roof surface from the harsh rays of the sun.

Shade stops the sun’s rays from reaching the ground – preventing it from absorbing heat. If there are a lot of trees to provide shade, the urban heat island effect can be reduced significantly.


Because cities have so much artificial infrastructure, they displace trees and other vegetation, reducing the amount of air cooled by evapotranspiration. As a result, this displacement of natural greenery increases global warming by causing heat waves in urban environments.

Thanks to evapotranspiration, the vegetation of a green roof can help reduce heat waves and cool down the urban environment. Put simply, the process of evapotranspiration, which combines evaporation and transpiration, is as follows:

  1. Plants absorb water through their roots.
  2. The collected water travels up the plant to the leaves.
  3. Water in the leaves turns into water vapor and is released into the air.
  4. The moisture released in the air cools down the atmosphere.

Benefits of Green Roofs

Through shade and evapotranspiration, green roofing systems help cool off urban heat islands. The Environmental Protection Agency reports that this type of roofing is significantly cooler by around 30 to 40 degrees Fahrenheit than a conventional roof.

This cooling effect has a positive impact on the environment. And that’s not all – green roofs can boost public health and offer the following benefits to both residential and commercial building owners:

Environmental Benefits

Apart from helping prevent global warming by reducing the urban heat island effect, this eco-friendly roofing choice has the following benefits:

  • Cooler living space: Green roofs decrease hot city temperatures and provide a cooler and more comfortable environment to live in.
  • Energy savings: Electrical grids are put under strain during hot summers when energy demand for air conditioning is high. In extreme cases, these grids can even experience blackouts. By helping lower urban temperatures, green roofs reduce energy consumption.
  • Cleaner air: Plants reduce air pollution by providing cleaner air, so the vegetation growing on top of your green roof can help improve the air quality around your house.
  • Reduced greenhouse gas emissions: Natural vegetation reduces the amount of greenhouse gas emissions, so a green roof reduces your carbon footprint.
  • Improved stormwater management: During the summer, green roofs absorb up to 90% of rainfall, which reduces the amount of stormwater runoff and protects local bodies of water from flooding.
  • Reduced water pollution: The reduction in stormwater runoff improves water quality by decreasing the amount of dirt and pollutants in waterways.
  • Increased biodiversity: Green infrastructure provides shade, shelter, and food for local wildlife – creating a more diverse ecosystem in your yard.

Financial Benefits

Saving the planet while saving money is the most convenient way to go green. Green roof installation comes with financial benefits as well, including:

  • Reduced energy bills: Utility bills can be expensive, but the less energy you use for air conditioning, the less you have to pay for electricity to power your HVAC unit.
  • Incentives: The government offers incentives for homes with green roofs. Taking advantage of these tax breaks helps lessen the financial burden of installation.
  • Reduced HVAC unit repairs: Cooler outdoor temperatures can help reduce repair and maintenance costs for your air conditioning units.

Additional Benefits

Did you know that installing a rooftop garden can increase public health? Aside from reducing stress, green roofs have other neat benefits, including:

  • Good insulation. Because of their deep growing medium, green roofs work effectively as an insulator that prevents any heat from entering your home.
  • Noise reduction. An excellent insulator, green roofs are covered with a layer of vegetation that effectively reduces noise. It is especially good for homes near busy streets.
  • Prevents health problems. Urban heat islands cause an increased risk of heat stroke and other heat-related deaths. Air pollution can lead to respiratory problems, especially for people with asthma. With a green roof, you can prevent these health issues from affecting your family.
  • Less stress. By reducing stress, green roofs have a positive impact on human health. Studies show that exposure to nature has a soothing effect on the human mind, resulting in less stress, lowered blood pressure, and improved sleep.
  • Recreational space. Intensive green roofs function as garden spaces where homeowners can relax and hang out while enjoying the natural beauty growing right on their roofs.
  • Long lifespan. Green roofs usually last 40 to 50 years. The vegetative layer protects your roof from the weather, helping it to last longer.
  • Fire-resistant. Because plants contain water, they are fire resistant, which decreases fire risks for your home.

Cost of a Green Roof

green roof with patches of wildflowers
Photo Credit: marcin_szmyd / Canva Pro / License

Adding a new green roof is an intensive project. In some cases, your roof may have to be reinforced so it can support the weight of a growing medium. On average, homeowners can expect green roof installation projects to cost $13,000 to $50,000, or around $15 to $50 per square foot.

Keep in mind that several factors affect costs:

  • Location
  • Labor costs
  • Roof size
  • Pitch or slope
  • Roof shape 
  • Permit costs (not all areas require building permits for a green roof)

Of course, the cost of a green roof depends on how deep the substrate you are installing is. Intensive green roofs are the most expensive, while extensive green roofs are more affordable.

Green Roof TypeCost per 1,000 Square Feet (Including Labor and Materials)
Extensive green roof$10,000 – $20,000
Semi-intensive green roof$15,000 – $30,000
Intensive green roof$21,700 – $38,000

Don’t forget to add plant costs to the overall price of a green roof as well. Plant prices depend on the type and size of the plant since bigger ones cost more.

Before you commit to installing a green roof, consult a roofing professional and ask them to assess if your home is compatible with supporting a green roof.

Also, look for financial incentives in your area. Because these roofs are eco-friendly, the government sometimes offers tax rebates or other incentives for homeowners who install a green roof. Know what rebates or other incentives are available in your area so you can benefit from them.

FAQ About Green Roofs

Can I Walk on a Green Roof?

It depends on the roof. Intensive roofs are good for recreational areas and support some foot traffic. Extensive and semi-intensive roofs aren’t usually designed to be walked on, so it’s better to stay off those types of green roofs.

What are the Best Plants for a Green Roof?

Not all plants can fit your green roof. Various factors come into play when determining good plant choices for a green roof, including:

– Local climate
– Personal preferences
– Type of green roof
– Roof size
– The amount of upkeep a plant requires

Hardy, durable plants with shallow roof systems and low water requirements are the best choices for green roofs. Some examples are:

– Blue sedge
– Common thyme
– Gold moss stonecrop
– Houseleeks
– Oregano

Can I Convert a Traditional Roof to a Green Roof?

A traditional roof can be converted to a green roof, although the roof must be flat or low-sloping. You must have a flat roof or one with a slope no steeper than 45 degrees.

Your roof also will have to be adjusted to accommodate the weight of all those plants, which is an expensive project that will involve boosting the house’s structural integrity to ensure it is capable of holding the weight of a green roof.

What is the Difference Between a Cool Roof and a Green Roof?

Cool roofs and green roofs have different approaches to mitigating the urban heat island effect. To maintain cooler temperatures, a cool roof surface reflects sunlight rather than absorbing it. On the other hand, a green roof relies on shade and evapotranspiration to keep homes cool in the heat of summer.

Do Your Part in Reducing Urban Heat Islands

Apart from mitigating the effects of urban heat islands, green roofs offer many benefits to both homeowners and the environment. Green roofs reduce energy consumption and increase public health by reducing water and air pollution.

Not only do green roofs stand out and add a beautiful touch of nature to homes, but they also help reduce carbon footprint. If you need help making a green roof, find a roofing contractor in your area today.

Main Image Credit: ferrantraite / Canva Pro / License

Danielle Gorski

Danielle Gorski lives with her family in Texas. She has a degree in Professional Studies and a minor in marketing. Her hobbies include reading, drawing, and writing.