By MICHAEL TOBIAS
Flat-roofed houses can accommodate sustainable blue roof designs. Photograph: Shutterstock
Engineers have been designing blue roof systems for city buildings for about a decade, in our endeavor to help manage rainwater runoff and control stormwater, preventing it from flooding our streets and combined stormwater-sewer systems. Reasonably simple in concept, they are especially useful in cities and other urban environments where buildings have a large footprint, including high-rise buildings and apartment blocks.
Instead of water being channeled away from the roof via gutters and downpipes that cannot cope in violent storms and heavy downpours, the water is detained on the roof and then released back into the environment at a controlled rate.
So, when it rains, the water is directed into some kind of cistern or rain barrel, or left to pond on the roof surface. It is then released via a flow control device so that it drains slowly without causing sewer back-ups in building basements or any other kind of flooding. While some of the water may be used, for instance, to supply non-potable water for toilet flushing or for irrigation, most is simply kept in the cistern temporarily.
Of course, you don’t have to be a professional offering engineering solutions in Chicago, New York, London, or any other city in the world to realize that storing water on the roof of a building requires the structure to have enough bearing capacity to be able to carry the weight of the water in the cistern. This is why legislation, including local codes in different cities, require blue roof controlled-flow systems to be designed, installed, and engineered for stability, with compliant waterproofing systems forming part of the installation.
In New York, rooftops are generally designed to carry a minimum live load of 30 lbs/square foot. If you add just an inch of water, the load will increase by about 5 lbs/square foot, which is substantial.
In any case, if water is allowed to pond, rather than being detained in a cistern, it shouldn’t be more than 2-4 inches deep and should be drained or moved to a storage container within 24 hours.
Blue Roof Designs
Stainless steel tanks used for water retention on an upper floor of the building. Photograph: Shutterstock
Blue roof designs are 100% sustainable and eco-friendly. Generally, the roof itself should be flat or, if it slopes, the slope should be very slight, no more than 2%. Alternatively, tanks may be located on an open upper floor below ground level as seen in the picture above.
Waterproofing is vital and it must be properly engineered. The New York City (NYC) School Construction Authority (SCA) is a pioneer of blue roof systems, and their specialists recommend a waterproofing membrane formed by applying a hot-fluid rubberized asphalt to reinforced fabric, that is installed over a layer of insulation. Sometimes ballast and geotextile filter fabric are included in the design.
Technical calculations determine exactly how many drains will be needed for blue roof design, as well as the release rate required for each one. The depth of flow and storage availability also have to be calculated. Also, these calculations must be done in accordance with construction codes.
The now renowned NYC blue roof pilot program, launched in 2010, incorporated roof trays with risers designed to collect water that was allowed to pond near drainage outlets. Roofs also incorporated protected dam areas that stored up to 2 inches of water. Results showed that rainwater runoff reduced by 45%!
Benefits of Blue Roof Rooftop Detention Systems
Rainwater gushes off the room of a building during a storm. Photograph: Shutterstock
One of the major problems in dense urban areas is that there are few soft landscapes. Instead, surfaces have been paved and roads and highways constructed with asphalt and concrete respectively, forming hard, impervious surfaces that cannot absorb water of any sort. Blue roof systems provide a solution and offer these benefits:
- Blue roofs are part of sustainable urban drainage systems (SUDs) that enable us to collect and store stormwater and use it or release it back into the environment in a controlled manner.
- Blue roof detention systems are easy to install.
- Blue roof systems are a much cheaper solution than stormwater reservoirs and also reliable, offering a low-impact alternative to compromised combined sewer systems that have to deal with stormwater and sewage sometimes overflowing at the same time.
- They result in much cleaner streets, rivers, and lakes, simply because they limit sewage discharge and flows.
- Maintenance of blue roof systems is minimal provided the roof is kept clean and all debris, including leaves, is removed to prevent drains clogging.
- They can be combined with rainwater harvesting initiatives.
- Depending on design features, they may be used to cool interiors and contribute towards energy savings.
While there is no doubt that blue roof design is both sustainable and eco-friendly, in spite of its potential, it isn’t yet the norm. It is also more financially viable in large buildings rather than smaller residential homes. But there are elements of blue roof design that ordinary homeowners can mimic, especially those relating to rainwater harvesting.
They are definitely a viable part of our future.
Michael Tobias is the founder and principal of New York Engineers, an Inc 5000 Fastest Growing Company in America. He leads a team of more than 30 mechanical, electrical, plumbing, and fire protection engineers from the company headquarters in New York City, and has led numerous projects in New York, New Jersey, Chicago, Pennsylvania, Connecticut, Florida, Maryland, and California, as well as Singapore and Malaysia. He specializes in sustainable building technology, including blue roof design.