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The Benefits of Blue Roofs

10/12/2022

Sustainable building strategies that have emerged in the past two decades continue to lead to advancements in material technology, application methods, and design practices that consider the building's relationship with its physical and natural environment. It is now more common for all building components – both interior and exterior – to be examined for sustainable options. Being one of the largest exterior components of the building, the roof is commonly designed with the intent of providing sustainable and energy-efficient materials and practices that benefit the building’s internal natural environment. Facility managers should also take into account sustainable efforts to benefit the building’s external physical surroundings. One consideration that is both sustainable and benefits a building’s physical surroundings is the management of a roof’s water runoff.  

Water management is becoming increasingly more important in today’s climate. A facility manager must now be cognizant of the impact that rainwater discharge from their buildings’ roofs will have on the surrounding buildings and community. In some regions of the country, flood threats have increased due to more frequent extreme weather events. Conversely, other regions of the country are dealing with water droughts that require storage of rainwater for future use. In areas with aged sewer and drain systems (100-plus years old) regulations are in place to reduce the volume of water runoff into the sewer systems.

Water management of roofs can be completed by installing blue or green roof assemblies. The application of the blue and green roof assemblies is not only a sustainable strategy, but it will also assist managers in providing positive work spaces for employees. One way to create positive work spaces is through the use of outdoor areas. This can include the use of select roof areas, as roofs can provide opportunities for use as additional spaces for employee gatherings. 

Blue roof assemblies 

A blue roof uses a rainwater retention system to provide controlled drainage of water from a low-slope roof. Unlike traditional low-slope roof systems that are designed to immediately drain precipitation from a roof’s surface, blue roofs are designed to store the water on the roof and manage its release as needed. The retained water can be slowly released into the sewage system to prevent flooding or overflow, or the water can be stored on the roof and used for other building needs – such as irrigation, cooling the building, or water reserves. This type of assembly eliminates the need for cisterns, detention ponds, or underground storage tanks.  

The primary advantage of these systems is to reduce the flow rate of water from the roof into the sewer system. This is an important criterion in large urban areas or locations with older, fragile infrastructure. Large volumes of water runoff or excessive water runoff in a short span of time can lead to flooding or extensive structural damage to the infrastructure. Flooding can lead to extensive moisture infiltration into the building’s above-grade (openings in exterior building) or below-grade (leaks at floor drains), causing damage to interior contents. The excessive waterflow also contributes to sewer problems, such as water main breaks. Damages can also extend below exterior roadways and parking lots requiring extensive, and expensive repairs. 

In standard low-slope roof applications the rainwater immediately flows to internal roof drains or external gutters and is disposed through drainpipes or downspouts away from the building. Blue roof assemblies collect and store the rainwater away from the roof, preventing the drainage system from becoming overwhelmed. The retention of the rainwater is typically regulated at the drain through the use of specially designed restrictors that control the rate of waterflow. The flow control rate is determined through local code requirements and the buildings water usage. The retention of the rainwater is stored with filter fabrics or fabricated void components that are placed over the roof membrane between the final surfacing. The void spacing is determined based on structural load requirements and water flow rates.  

When considering a blue roof assembly, a structural engineer should be engaged in the design process as standing water has an approximate weight of 5.2 pounds per square foot. Standard void spaces can contain water depth of 3.5 inches. That depth requires structural deck capacity for standing water weight of approximately 18.2 pounds per square foot.  

A few of the benefits of blue roof assemblies are as follows: 

Blue roof assemblies can be used on their own with a ballast surfacing, or in conjunction with pavers or green (living) roof assemblies. 

John D'Annunzio, president of Paragon Consultants and Paragon Roofing Technology, Inc., has more than 20 years' experience as a consultant on projects around the world. D'Annunzio, a construction consultant specializing in roofing and waterproofing materials application and field performance, has written four books about roofing. He can be reached at john_paragon@ameritech.net.