Active Blue Roof vs Passive Blue Roof

Blue roofs help to manage stormwater runoff by collecting, infiltrating, and slowly releasing excess rainwater.

Although the basic concept of how a blue roof works are straightforward, there are two types of blue roofs in terms of how they handle stormwater runoff.

These systems are known as active and passive blue roofs.

Passive blue roof systems use non-mechanical techniques to control the rate at which water drains from a rooftop.

They use passive methods to slow and manage the flow of water, such as control flow drains, gravel drainage beds, perforated pipes and filters, and modular tray designs.

While active blue roofs use pumps and other mechanical means to move water off the roof.

That being said, the terms active and passive blue roofs don’t refer to a single type of design but rather a spectrum of designs that have common methods of controlling stormwater runoff.

Hence, there are numerous active and passive blue roof solutions on the market that offer a variety of options for stormwater management.

Passive Blue Roof Systems

Passive blue roof systems rely on natural processes to slow down, absorb and infiltrate water through the use of control flow drains, gravel drainage beds, modular tray designs that are filled with an engineered soil mix, and perforated pipes to slow down the movement of water.

Some of the most commonly used passive blue roof strategies include:

Control flow drains

Control flow drains are the type of drainage system that regulates the rate at which water passes into the drainage system from a roof.

These drains control the rate of water infiltration, which in turn controls how fast stormwater runoff occurs.

Perforated pipes and filters

Perforated pipes and filters control the rate at which water flows off a roof.

These components are often used in conjunction with other passive blue roof strategies to provide an engineered balance of flow rates.

Like control flow drains, perforated pipe systems help manage rooftop runoff during storm events by slowing down water drainage rates.

Filter systems provide an additional layer of filtration by trapping sediments and other pollutants before the water enters the drainage system.

Rooftop retention systems

Rooftop retention systems are synthetic swales designed to hold water for extended periods of time.

They have multiple layers and a drainage system that allows the roof to be used as an infiltration site without being overwhelmed by heavy rainfall events.

This system is more effective in areas where high volumes of rainfall are common.

Rooftop rain gardens

Rooftop rain gardens are a form of a retention system that uses plants and soil to help infiltrate water, while also adding the benefits of rooftop vegetation.

They are designed to capture and slow down water, while also providing additional benefits of rooftop vegetation.

Modular Tray Designs

As its name suggests, the system is made up of multiple trays that are usually installed in a series to hold water.

These systems are easiest to implement for both new and existing roofs. They are cost-effective since they can be installed in stages to slowly upgrade a roof.

The trays might be directly fastened to the roof deck or connected by a mesh to prevent movement.

They are typically made of corrugated plastic, which is available in different colors for aesthetic purposes.

Trays can also be held in place by ballast made of coarse stone or other weighted materials. The depth of the ballast or media in the trays can be adjusted to decrease the occurrence of exposed water surfaces.

Depending on the goals of the installation, the trays can be filled with different materials like gravel, soil, sand, or engineered materials that control water flow rates.

Modular tray blue roof designs can be deliberately integrated with green roof components to increase looks and provide some of the green roof’s additional benefits.

Roof-Dams/Roof-Checks

Roofdams or roof checks are low-profile barriers that are designed to help slow down stormwater runoff.

They work by diverting the runoff to a storage area where it can be held and infiltrated over time.

There are various passive blue roof designs that can be utilized to suit particular roofing conditions.

However, since they rely on passive means to slow down water infiltration, they are only effective in climates where it doesn’t rain often or is heavy.

These systems can get easily overwhelmed and flooded by high rates or large volumes of rainfall.

Active blue roof systems

Active blue roof systems use mechanical methods to control how quickly water drains from a rooftop.

Because they take advantage of pumps, electronic controls, and other engineered systems, they are more effective for roofs that receive heavy rainfall.

Active blue roofs can be designed to meet specific environmental or design goals by controlling water infiltration rates, drainage methods, and other variables.

There are numerous types of active blue roof designs all of which use different components to manage water infiltration.

There are active blue roof designs equipped with sensors and weather data collection systems that help designers and building managers track performance and adjust the system accordingly.

Most active blue roof systems work with storage tanks instead of directly pushing water into the drainage system.

These tanks collect and store excess water until it can be slowly released, which helps reduce the risk of flooding.

Depending on the system, the stored water can be used for irrigation, cooling systems, gray water usage, or other purposes.

Can active and passive blue roof systems be used together?

Active and passive blue roof strategies can be combined to create a hybrid system that takes advantage of the benefits of each type.

For example, a gravel bed can be used to help infiltrate water, while an active pump system will send excess runoff into a storm drain.

Do active blue roof systems work during a power outage?

Storm events are also likely to cause temporary or widespread power outages, which can make it difficult to use to manage stormwater runoff.

Since active blue roof systems rely on electricity to operate, they are only effective if the power is working.

However, many active blue roofs have backup generators or battery backup systems that can provide power for pumps and other components.

However, not all active blue roof systems will include generators or battery backup.

So it’s important to plan in advance and might be better to go with an active blue roof system that can function during power outages.

Should you have an active blue roof or a passive blue roof?

Both active and passive blue roof systems are effective at managing stormwater runoff.

However, they each have their own unique benefits that make them the best choice for different applications.

Factors	Active Blue Roof	Passive Blue Roof
Water Infiltration	Mechanical Methods	Passive Techniques
Rainfall Conditions	Heavy Rainfall	Low to Moderate Rainfall
Water Storage	Storage Tanks	Natural Retention
Power Outages	Backup Generator/Battery	Not Affected
Cost	Expensive	Cost-Effective
Maintenance Needs	Regular maintenance	Low Maintenance
Flexibility	Adjustable	Fixed Design
Water Usage	Irrigation, Cooling, etc.	Infiltration Only

In general, active blue roofs are better for managing large amounts of stormwater runoff from a rooftop.

Passive blue roof systems work well in areas where it doesn’t rain often and the water infiltration rate is slow.

However, it’s important to note that there are hybrid systems available that take advantage of the benefits of both active and passive blue roof designs.

In the end, the best choice for a water management system will depend on the specific needs of a building and its location.