Green Roof vs Blue Roof: The Differences and Similarities

The roof is an essential component of any building and covers more than a quarter of an urban area in North America.

Despite the fact that flat roofs cover a large portion of city roofs, they are largely overlooked in city planning. In fact, most flat roofs are left bare of asphalt, tar, and gravel, leaving their potential benefits and advantages largely untapped.

However, roofs can be designed to improve a building’s energy efficiency and overall environmental performance. A well-designed roof may help fulfill insulation needs, control stormwater runoff, provide wildlife habitat and green space, and act as a recreational environment.

Roofs can also reduce a building’s energy needs and carbon footprint by keeping the sun off during peak heat hours and blocking the wind during winter months.

The best type of roof to have will depend on a variety of factors including climate, geography, and building orientation. With so many options available, it can be hard to decide which type of roof is best for your building.

Although there are many different types of roofs, there are two terms that are often used to describe roofs with certain features. They are green roofs and blue roofs.

In this post, we will take a closer look at green roofs and blue roofs to determine the differences and similarities between them.

What is a Green Roof?

A green roof is a roof that is partially or completely covered in vegetation and growing medium, which can be made of soil, compost, gravel, or other material.

It’s designed to reduce the heat island effect, absorb stormwater runoff, provide insulation to the building below it, and reduce energy costs.

small house with green roof

Green roofs also help with reducing noise pollution, creating habitat for wildlife, and improving air quality by providing a space for plants to absorb carbon dioxide and release oxygen.

Green roofs that are planted with native and adaptive plants create habitat for wildlife by providing food, water, and shelter for insects, birds, and other animals.

Nowadays, a lot of building projects use green roof spaces to grow herbs, vegetables, and fruits. This perk of green roofs contributes to building sustainable urban food systems and creates a garden space that can be used to relax or entertain guests.

It is also important to mention not all green roofs are created equal. The depth and type of growing medium, the plant species that are used, and whether or not irrigation is available will all contribute to how well a green roof performs.

Green roofs are generally classified as extensive green roofs which have a shallow depth and require little or no maintenance, and intensive green roofs which have a deep soil medium and require regular maintenance or a larger initial investment.

Advantages of Green Roofs

Green roofs have many direct and indirect advantages that don’t just benefit the buildings, but also individuals, the environment, and communities. Here are some of the benefits of green roofs:

Reduces Heat Island Effect

A green roof can reduce the urban heat island effect by providing insulation to buildings below it. The combination of growing medium, vegetation and building membrane can drastically increase the R-value (insulation) on a roof.

Green roofs also help to cool the roofing surface through evapotranspiration. Evapotranspiration is the process by which moisture is transferred from land to air due to evaporation and transpiration.

The vegetation on the green roof will help to lower surface temperature, reduce heat gain for building interiors which keeps them cooler. This in turn reduces HVAC needs and saves energy.

Improves Stormwater Management

Green roofs can also be used to attenuate stormwater runoff by reducing the volume of water that flows off of a rooftop and into the sewer system.

This is especially helpful in areas with combined sewer systems where stormwater runoff can overwhelm public treatment plants and cause untreated sewage to be released into local waterways.

Reduced stormwater volume entering municipal sewer systems is especially essential in jurisdictions with integrated storm and sanitary mains. Because excess stormwater overwhelms these systems, causing untreated sewage to bypass treatment plants and flow directly into surface waters.

Greenroofs also protect wetlands and water bodies by slowing the rate of runoff to streams and rivers. A green roof surface will absorb some of the rain that falls on it, decreasing runoff volume into local waterways.

Reduces Noise

Noise pollution is not only distracting and annoying, but it can also be harmful to your health. Green roofs can help mitigate noise by providing an insulating barrier between a building and outside noise.

Although this is not a major benefit, it can be helpful if the building in question has noise pollution from traffic or other outside sources. A rooftop completely covered with an intensive green roof can be very helpful in blocking noise.

Extends Roof Life

A green roof can extend the life of a roofing system by shielding the roofing materials from direct UV radiation and high temperatures. This can help to prevent the materials from drying out and cracking which extends their lifespan.

A green roof also provides a protective barrier for all of the rooftop components below it, preventing water intrusion into these structures. This helps to protect against leaks that could cause significant damage or loss of property value if left untreated.

Reduces Greenhouse Gases (GHG)

Plants and trees absorb carbon dioxide (CO2) from the air and release oxygen (O) in its place. A green roof can help to reduce greenhouse gases and support the goal of stabilizing greenhouse gases (GHG) in the atmosphere.

Projects that prefer intensive green roofs can plant a variety of larger plant species can remove significant amounts of CO2 from the air.

Improves Efficiency of Mechanical Equipment

A green roof helps increase the efficiency of rooftop mechanical equipment by making the air on the roof more humid and cooler. This helps to reduce stress on these systems and lower operating costs.

The outside air must be pre-cooled to the desired temperature before being cooled by HVAC equipment. A green roof helps cool the air on the roof, making the process easier and less energy-intensive. Because heat-rejecting rooftop HVAC equipment operates at a lower ambient temperature, cooler roof air temperatures improve efficiency.

Aesthetically pleasing

Green roofs are aesthetically pleasing and eye-catching for building occupants, passersby, or even neighbors to enjoy! The vegetation and other components of a green roof create an environment that is unique, calming, and attractive.

Building owners will have a plethora of options for the design, layout, and type of vegetation used on a green roof. Green roofs can be designed to suit their surrounding environment while adding beauty, color, and a calming atmosphere.

Encourages Outdoor Activity

Not all roofs will allow visitor access, but for the roofs that do, green roofs are a great opportunity for visitors to enjoy outdoor activities on the roof! People can relax, take in views of the skyline, eat lunch, or play games on a green roof.

In addition, green roofs offer a space for employees to enjoy outdoor activities during their lunch break. This can improve employee morale and encourage creativity.

Increases Biodiversity

Green roofs offer a great opportunity for biodiversity to take place throughout the roofing system. They support insects, birds, and other small animals whose habitats are being destroyed by human development.

More diversified plants will attract many more insects, which will boost the level of birdlife in the roof garden.

Providing areas where these species have shelter is beneficial for their continued survival as well as encouraging them to spread out across different regions of an ecosystem or landscape.

Increases Building Value

Depending on the type of green roof, installing a green roof can increase your property value. People find green roofs aesthetically pleasing and comforting, which makes them more likely to purchase a property once they learn that it has a green roof.

Green roofs are generally not a very large investment and can be done at a low cost to the building owner. However, they do provide an aesthetic value that allows for property appreciation over time as well.

Disadvantages of Green Roofs

Green roofs have been shown to have a number of disadvantages that can affect building owners. At times, green roofs are not feasible or practical for every property situation due to the needed design and engineering work required.

Needs ongoing maintenance

Green roofs require ongoing maintenance in order to keep plants healthy and thriving on the roof. This requires effort from an onsite team as well as costly materials.

Most green roofs require a certain amount of water and sunlight for plants to be healthy and grow properly. If either is not available, then it becomes more difficult for plant life on the green roof to survive.

Requires unique design elements

Before getting started with a green roof project, your structural engineer must consider how much weight the roof is able to hold.

Green roofs require unique design elements such as water drainage and waterproofing in order for them to function properly, so this needs to be taken into account before installing a green roof on top of an existing building.

Although green roofs help with managing stormwater runoff, they will increase the water activity on the rooftop even when the roof is dry. Because most green roofs need irrigation in order to be healthy and full of vegetation.

In order to maintain a healthy green roof, it is necessary to have insulation underneath the green roof in order to prevent excess water travel through the roof. If not planned properly green roofs can cause mold growth in the building.

May not clean contribute environment as intended

Some green roofs are not as efficient at removing CO₂ from the atmosphere. This is more true for extensive green roofs that allow only plants with shallow root systems to grow.

So far we have talked about green roofs. Now let’s take a look at what blue roofs are and what makes them different from green roofs.

What is a Blue Roof?

A blue roof is a type of roof that is specifically intended to temporarily hold stormwater and then gradually release it. It collects rainwater and stores it till the storm passes. This helps reduce the strain on storm drains and sewers.

Their main purpose is to collect water and then gradually drain that water off of the roof rather than allowing heavy rainfall to drain off immediately. This is much better for the environment because it prevents stormwater from overwhelming sewer systems and polluting nearby bodies of water, such as rivers, lakes, and oceans.

Blue roofs are often classified as “active” or “passive” blue roofs depending on how they are designed and installed. Active blue roofs use mechanical ways to manage the rate at which water drains from a rooftop. While passive blue roofs use non-mechanical techniques to manage the rate at which water drains from a rooftop.

Depending on the design, blue roofs might have many advantages. It can be used for irrigation, cooling, and even recreation.

Blue roofs can also be constructed as a stormwater retrofit on existing structures with flat, slightly inclined, or terraced roofs provided acceptable structural loading capability, waterproofing, and rooftop utility protection have been confirmed.

Because blue roofs can be installed on almost all roofs, and they are compatible with other sustainable roof technologies. They can be used in conjunction with green roofs to manage water runoff and channel it into a cistern or other storage device.

Advantages of Blue Roofs

Blue roofs have several advantages if they are designed and installed correctly. Here are just a few of the benefits they offer:

Flood mitigation

Flooding is a major problem in many cities. Blue roofs can capture and hold stormwater, reducing the risk of flooding in a building or surrounding area.

They reduce pressure on sewer systems by catching roof run-off. This helps prevent flooding and reduces the chance of raw sewage overflowing into local waterways during heavy rainstorms.

Require no excavation

Blue roofs are constructed on the existing roof by adding layers of waterproofing and drainage materials. They do not require excavation and can be built over existing roofs, which means that you don’t have to remove any tiles or other materials.

Pollution reduction

While blue roofs do not clean water by temporarily retaining it, they do reduce the load on storm sewers, preventing untreated wastewater from entering rivers, streams, and coastal waters.

By reducing the amount of water that runs off of a roof, blue roofs prevent pollutants from being washed into nearby waterways.

Offsets Potable Water Use

Water is a scarce resource. Blue roofs can be used to offset potable water use from the municipal supply by collecting rainwater for later use or infiltration into groundwater systems after storm events have passed and drainage has been restored. This reduces water demand on municipal resources especially during dry periods when demands are high.

Design compatibility

Another significant advantage of blue roofs is their capacity to coexist with various rooftop systems such as solar panels (both solar thermal and photovoltaic) and HVAC mechanical equipment.

Some recreational blue roofs have rooftop waterplay areas that can also be utilized by children and adults.

Cheaper than green roofs

Blue roofs are cheaper than green roofs since they are less site-specific, require no irrigation system or growing medium, and don’t have to be maintained as often.

Disadvantages of Blue Roofs

Blue roofs have several disadvantages and disadvantages and here are just a few of them:

Flooding mitigation is only temporary

While blue roofs can help reduce flooding, they will not protect against all sources of flooding as studies have shown that most cities require more than one solution to fully address their needs for mitigating floods from heavy rainfall events.

The water collected by the roof is an asset that should be treated as such. However, blue roofs can provide relief during rainstorm events while more permanent solutions are designed and constructed to withstand future conditions with climate change in mind.

Requires frequent maintenance

Blue roofs should be inspected regularly to ensure that the roof drains and diverters are clear of debris and that the waterproofing membrane is intact. Maintenance should also be performed before, during, and after a rainstorm or snowmelt event to ensure proper drainage.

This maintenance can be performed by qualified personnel or through an inspection process where a third-party organization monitors the proper functioning of blue roofs.

Standing water can attract insects

Standing water on a blue roof can attract insects such as mosquitoes. This can be a particular problem unless the blue roof is designed to drain quickly or has an effective means of insect control.

Additional water weight

Water on a roof can add significant weight and stress to the structure of the building. This increased stress must be taken into consideration during the design, construction, and maintenance of blue roofs.

In most cases, building owners should hire a structural engineer to determine the additional weight and stress blue roofs will place on building structures.

Sensitivity to freezing conditions

Blue roofs can become ineffective when exposed to freezing conditions. If freezing is a major concern where the blue roof is installed, projects should choose designs with features such as insulation and electric heating to prevent freezing.

There are various electric heating systems that can be used to prevent water from freezing by using a heating cable to warm the water and keep it from freezing.

Differences between Green and Blue roofs?

Although green and blue roofs share many similarities, they also have some distinct differences:

Stormwater Management

Both green and blue roofs provide stormwater management benefits.

However, green roofs are designed to reduce stormwater runoff by allowing rain and melted snow to soak into the growing medium over time. Whereas blue roofs work as temporary management of stormwater through collection and storage during heavy rainfall events.

If the primary concern of a building is stormwater management, blue roofs will offer a quick and cost-effective solution. However, green roofs provide a more sophisticated solution and stormwater management is just one of the benefits.

Maintenance requirements

Blue roofs require less maintenance than green roofs. This is because blue roofs don’t need irrigation, fertilization, pest control, plant monitoring which are valid concerns for green roofs.

Blue roofs’ maintenance is more limited to inspecting and maintaining the waterproofing membrane and removing debris that may clog the drainage system.

Cost

A green roof typically has a higher initial and ongoing cost than a blue roof. Because having a green roof means having a rooftop garden, which requires a lot of maintenance to ensure the plants are healthy. Green roofs require a growing medium, irrigation system, plant selection, plant monitoring, soil fertility management which will increase the initial cost and ongoing cost.

Versatility

Green roofs function as a rooftop terrace, outdoor event space, and rooftop garden. A green roof can be used for recreation or even farming on some larger projects.

lettuce farming - on rooftop

However, blue roofs are not designed to provide any recreational, aesthetic, or farming functions. Blue roofs are designed to provide a temporary solution for water management. If you have a rooftop that needs to have a temporary solution for water management, blue roofs can be a great choice. Else if you have a rooftop that you want to maximize the benefits you can get from a green roof.

Conclusion

There is no “one size fits all” option when it comes to blue or green roofs. The decision should be based on your building’s specific needs and requirements for stormwater management as well as the overall benefits that each type of roof provides.

However, green roofs are a more complex solution that can offer far more benefits than a blue roof if it is planned and implemented properly. If you’re looking to install a roof that will provide more benefits than just the management of stormwater, green roofs are what you should be aiming for.