The discovery of water on the moon has led to much discussion reminding us that water is an essential element for all known forms of life. In our modern-day world of hurricanes, flooding and stormwater-polluted water bodies, we can sometimes forget that rainwater has been worshipped for being a life-supporting resource since the beginnings of human existence.

Over the past half-century, much of our existing drainage infrastructure is designed and built to dispose of stormwater runoff as quickly as possible, causing increasing flooding, erosion and depletion of groundwater. However, there is a growing belief that we need to remember what a gift rainwater actually is. Accordingly, our drainage infrastructure should be designed to harness stormwater as a resource.

Blue Roof Defined

Green roofs are easily recognizable examples of an increasingly common tool used to slow stormwater runoff. Falling rainwater is captured within the vegetation, reducing the volume of stormwater runoff from the roof. The captured water can then be discharged or reused on site. Other types of designs, such as a “blue roof” design, can be used to achieve the same goal when a green roof is not a viable option. Blue roofs also help designers and engineers find a solution to a sustainable stormwater drainage design that does not drastically increase capital construction costs and maintenance requirements.

The “blue roof” -- the lesser-known cousin of the green roof -- is another roofing design that harnesses stormwater on the roof. Blue roofs are predicated on nature’s hydrology of stormwater runoff, where there is little overall runoff from the site because much of the rain is returned to the ground while some is captured on site. Instead of using vegetation, a blue roof design utilizes simple flow controls atop downspouts in order to regulate the flow of runoff from the roof. Some stormwater will be temporarily stored on the roof, particularly in roof gutters.The discharge can be released to a stormwater harvesting system for later reuse as well as an infiltration system that recharges groundwater. And a small portion can be discharged to the drainage system. The ratio of captured/infiltrated/discharged runoff is designed to closely mimic the pre-construction hydrology of the site.

The most common application for a blue roof occurs in a long, flat roofing style normally seen in commercial development. These roofs typically have wide gutters and a hefty watertight liner. Blue roofs work especially well in highly urbanized areas where there is less space onsite for stormwater detention or where large, highly impervious areas generate a rapid rate of stormwater runoff that can overwhelm drainage systems and cause flooding. Oftentimes, these sites experience flooding problems due to the high degree of impervious area found along drains within the drainage system. Regulating the flows from the roof prevents the drainage system from surcharging and flooding. This effectively mitigates the effects of stormwater runoff, and it also allows for alternative uses of water for low impact development (LID), such as infiltrating it to recharge groundwater.

Blue Roofs and LID

Since water flows from blue roofs at a controlled rate, this allows designers to disconnect the downspouts from the collection system altogether. From there, flowing water can be routed to an infiltration system, such as a rain garden or underground soak-away. This effectively reduces the overall volume of water flowing offsite to an acceptable controlled rate, which can be worth 1 LEED credit under the U.S. Green Building Council’s guidelines. In many cases, the effluent from blue roofs does not contain high levels of nutrient pollution or involve any scope for adverse anthropological inputs and impacts such as fertilization.  

If a site isn’t conducive to infiltration, roof runoff can be collected, stored underground and used for nonpotable needs such as irrigation. Reusing captured stormwater for nonpotable use can be worth 3 or 4 LEED credits within the “Water Efficiency” guidelines.

Blue Roofs and Sustainability

In addition to their LID and LEED benefits, blue roofs also contribute to the overall strategy of sustainable design. One key pillar of sustainability is that the alternative must be cost-effective. If the design method costs too much to implement and maintain, then it is not economically feasible enough to be considered truly sustainable. In many circumstances, blue roofs are installed to actually control costs due to the expensive consequences of stormwater flooding, especially when a collection system fails and damages other aspects of the structure. Adding a “blue roof” flow control drainage design to a flat roof adds less than $1 per square foot in cost.   

Blue roofs are also a viable retrofit to an existing design. Many retrofit design alternatives require increasing structural supports in order to implement a heavy green roof. However, blue roofs do not require a large span of flat roof to be covered with soil and vegetation. Instead, wide gullies can be constructed to hold stormwater closer to the perimeter of the roof to make use of existing structural supports.

Blue roofs are also an attractive design option for those wary of maintaining a green roof. A well-designed green roof with careful plant selection should require little maintenance; however some people are put off by the fear of a costly but unlikely problem such as roots puncturing the roof membrane. Others are wary of the cost of “establishing” the green roof in the first year, when careful watering and fertilizing is required to stimulate plant growth. Other than occasional inspection and removal of leaf litter from gutters, blue roofs require no maintenance over and above a conventional roof.    

Developers installed 71 Reg-U-Flo Vortex Valves from Hydro International in 3- to 4-foot wide channels along the perimeter of the roof to store the rainwater.

Case Study

One prominent blue roof success story comes from overseas. Specifically, a shopping mall in the U.K. experienced significant summer thunderstorms that overflowed the mall’s stormwater drainage system. For the geographical area of this particular shopping mall, a deluge of rainfall over a short period of time was a common occurrence.

The flooded drainage system backed up into the inside of the building, requiring developers to take action. Rather than undertake major reconstruction to install larger stormwater collection pipes at the lower end of the system where the flooding occurred, engineers relied on making efficient use of existing space. They discovered that if they slowed down the roof runoff entering the drainage system and temporarily stored the excess water on the roof itself, the existing collection pipes were able to run to capacity without overloading and flooding. Developers installed 71 Reg-U-Flo Vortex Valves from Hydro International in 3- to 4-foot wide channels along the perimeter of the roof to store the rainwater. By storing the rainwater at source (on the roof), the mall was spared extensive reconstruction costs and, with the valves operating exactly as designed, saved further considerable costs in flood damage.

Conclusion

While we now know that water exists on the moon, we believe we can more efficiently handle the Earth’s most-important resource. Blue roofs take a problem of excessive stormwater discharge and create an opportunity for sustainable water-efficiency practices. By constricting the flows running off a site and dealing with it at its source, developers gain an opportunity to build a low-impact site, gain LEED credits and reduce project costs in the process.

Sustainable site design isn’t about which method is better than the other -- it’s about how we consider all the alternatives in a certain situation. There are thousands of projects in America where a blue roof design could benefit both the environment and the development team itself.

For more information on blue roofs and flow control devices, visit www.hydro-international.biz.