The rise of the blue roof

by Katie Daniel | May 11, 2017 11:19 am

Photo by Jonathan Riley (

by John Engle
Blue-roof implementation is on the rise in the United States and, indeed, around the world. First conceived in 2008, these ROOF assemblies have proved to be an emerging technology that changes the game in densely populated, urban areas like San Francisco, Philadelphia, and New York—all large cities with high amounts of impervious surface. (For more on this background, see the online article, “One Roof, Two Roofs, Green Roofs, Blue Roofs[2].”) What exactly is a blue roof, however, and why should one be specified?

In many ways, the New York City Department of Environmental Protection (NYDEP) led the advancement of blue roofs through a concerted and widespread implementation of the technology. Recognizing the environmental challenges posed by impervious surfaces, NYDEP began to proactively test blue-roof concepts[3] in 2010. The pilot project continues to test both blue and green[4] (i.e. vegetated) technologies on rooftops such as that of PS 118, an elementary school in Queens.

The initiative takes advantage of the unused area on existing school and municipal rooftops to help collect and distribute excess stormwater through both passive and active rainwater collection techniques. As rooftop hard surfaces account for approximately 10 to 20 percent of Manhattan’s overall built surface area, these projects take underutilized portions of the city rooftops and employ them for a better purpose. In some cases, they use a series of modified recycled plastic trays, held in place with ballast alone, to produce an astonishing 45 percent reduction in roof runoff during rainfall events. (See the 2014 Roofing Magazine article, “From Green to Blue: Making Roof Systems Sustainable in Urban Environments,” by Steven Roy, LEED AP, Marcus Quigley, PE, CPESC, D. WRE, and Chuck Raymond, CPSM. View it online here[5].)

Photo courtesy PHP Systems/Design

Using a variety of active and passive water collection methods and techniques, the work is considered to be at the forefront of blue-roof testing. In 2015, New York City renewed its commitment to sustainability with the One NYC initiative—an extensive measure to create sustainable rooftops all over the city and an example of how blue-roof technology will be a central part of sustainability in cities in the future.

Around the globe, parallel efforts are similarly taking shape to try to offset the host of issues being created by impervious surfaces. In 2012, a British-led project by the Imperial College of London called the Blue Green Dream (BGD)—an initiative part of the wider European Union Climate Knowledge and Innovation Community (KIC)—began an ambitious project to introduce blue and green roofs across London and the European continent.

Challenges with combined sewer systems
In the United States, urban areas prone to stormwater issues often have not only high amounts of impervious surface, but also what is known as a ‘combined sewer’—a single system handling both sewage and stormwater simultaneously. (For more on combined sewer overflows, visit the U.S. Environmental Protection Agency (EPA) page[7].) As even small amounts of rain can overtax these kinds of systems, combined sewers can easily send untreated sewage into streams, rivers, and lakes. New York City, having mostly a combined sewer system, can actually experience stormwater overflow issues with as little as 12 mm (½ in.) of cumulative precipitation.

Combining the two systems makes the cost of building the infrastructure cheaper upfront, but at a high cost to the health of our watersheds. According to some sources, there are more than 800 communities in the United States using this combined system, or about 40 million people (concentrated in the Northeast and Pacific Northwest). New York City offers an attractive grant program[8] for private property owners in the city’s combined sewer areas. The minimum requirement is to manage 50 mm (1 in.) of stormwater runoff from the contributing impervious area. Previous recipients include the Brooklyn Navy Yard and the New York Restoration Project in the Gowanus Canal watershed.

For urban blocks, blue-roof assemblies can take advantage of existing spaces to help collect and distribute excess stormwater through passive and active rain collection techniques.
Photo by Gabriel Santiago (

Active and passive rooftops
Blue roofs often try to mimic natural systems to catch and temporarily store water so they can reduce or slow the effects of rainwater overflow from storms. They attempt to slow the rapid discharge of water runoff bouncing from
hard surfaces that can overflow sewers and create veritable epidemics for natural bodies of water. Instead, rooftop detention ponds, acting more softly and slowly like sponges, can imitate the natural absorption and slow moisture-release mechanisms of soil.

Blue roofs can consist of an array of equipment, including physical tanks, shelves, leaves, pipe systems, valves, catchments, pools, barrels, and trays that function in either an active or passive capacity (or somewhere in between) in order to collect rainwater. Active roofs can use techniques to store, slow, move, and reuse water according to a pre-designed mechanism. Active blue roofs, which are  also sometimes called automated roof runoff management systems, can be anywhere along the gradient from highly sophisticated to more limited in scope.

Some actively controlled systems will use programmable, hydraulically controlled valves to control the retention and release of water for reusable collection, drainage, or subsurface storage into a series of containers. Some systems even use communications or data tools such as forecast integration—
a technology employing sensors and Internet-based data feeds to help estimate rainfall quantities. Projects can use such tools to help schedule and record their real-time rainwater collection to meet user demand.

Whereas passive blue roofs typically need little to no upkeep, one of the primary considerations of active blue roofs is their need for regular maintenance. Passive blue roofs simply catch and hold water, and may also function as temporary holding tanks for storm events by later releasing water via the process of evaporation.

Generally speaking, the volume of water capable of being retained by a given blue roof system depends on several factors, the most significant being roof size, quantity and depth of trays/containers being deployed, and geometry of the overall configuration. The total volume contained can ultimately be calculated using the lengths, widths, and heights of the total collection of all containers.

In addition to the geometry of a blue-roof system, water collection will also ultimately depend on the climate conditions at that particular site at that time. According to the annual precipitation based on weather data collected from 1981 to 2010 for the (NOAA) National Climatic Data Center, the average rainfall in New York City was 1268 mm (50 in.) whereas the average rainfall in San Francisco was only 525 mm (20.7 in.).

Safety is of primary importance for maintenance personnel. Blue roof trays and basins are commonly less than 0.3 m (1 ft) deep, but maintenance crews must still be able to reach components of the system that require regular tuning, repairing, or cleaning. As maintenance can be costly and intensive, systems requiring limited upkeep should generally be prioritized. The NYDEP was one of the first city departments in the country to use a passive and essentially automated tray design incorporating special fabrics to laterally transfer and filter water. Using highly limited supervision to increase efficiency and enhance function, NYDEP made effective choices to decrease maintenance.

Photo by Freddie Marriage (

Benefits of being blue
One true beauty of blue roofs is they can be relatively low in cost when they are carefully and strategically designed. With a price of installation often close to or less than $1/sf, blue roofs provide a relatively inexpensive[11] yet effective tool to help manage rainwater collection. Whereas a vegetated roof can cost from $15 to $20/sf, a blue roof is typically substantially more affordable. Some projects, using a combination of both blue and green roofs, are able to achieve the financial balance necessary to meet a given budget.  (For more, see “Blue Roofs: The Stormwater-Sustainability Link.[12]”)

By any account, existing structure is a primary consideration. As noted, the overall volume of a blue roof system depends on its overall geometry and configuration. When being installed on existing roofs, the installation of materials and new water loads must be carefully planned so as not to exceed the existing roof structure’s load capacity. When the projected loads are designed to be greater than the current capacity of the existing roof structure, structural engineers must carefully calculate and add the additional structure required (e.g. joists, beams, or other structural members) to help carry the newly projected water loads.

Depending on the slope of the existing or proposed roof, ponding can be a major consideration. This occurs when water gathers at low points on the roof, creating excessive concentration of load. For example, the New Jersey Stormwater Best Management Practices Manual specifies that steep roofs, or those greater than two percent in slope, must be equipped with partitions (i.e. low barriers that retain water in a stepwise fashion across a roof, thereby evenly spreading the weight of water).

In addition to stormwater management and rainwater collection, blue roofs can also provide highly beneficial secondary outcomes. They can offer substantial heat island cooling effects by providing the opportunity to use basins with white and/or reflective linings to deflect solar radiation from the building. (For more on this background, see the online article, “One Roof, Two Roofs, Green Roofs, Blue Roofs[2].”) These energy-saving measures can offset the initial installation cost of a blue-roof system[13] or generate significant financial savings for building owners over the long run. They can also create opportunities for temporary water-reuse storage. Rainwater can then be harvested for landscape irrigation purposes or converted to potable and non-potable onsite uses.

As rooftop hard surfaces account for approximately 10 to 20 percent of Manhattan’s overall built surface area, blue roofs take underutilized portions of the city rooftops and employ them for a better purpose.
Photo by Max Ostrozhinskiy (

Designed by Pelli Clarke Pelli and completed in 2003, the Solaire has been billed as the first green residential building in the United States. Certified Platinum under the U.S. Green Building Council’s (USGBC) Leadership in Energy and Environmental Design (LEED) Existing Building (EB) program, it is located in New York’s Battery Park City. The building boasts a 50 percent reduction in potable water usage due to its waste- and stormwater-reuse system, which relies extensively on blue-roof technology. The Solaire roof catches rainwater and reuses it for toilet flushing, landscape irrigation, and HVAC purposes; it significantly reduces water usage in a city where water is costly and rain is cheap.

The lifespan of a typical blue roof is generally estimated at around 35 years. This is based on concepts of normal wear and tear, with which many builders are familiar. After 30 or 40 years of exposure to the sun and the elements, depending
on the details and specifications for the project, synthetic materials (especially plastics) can begin to disintegrate and break down, valves and rubber gaskets can harden, and some metal fasteners or flashings can rust or corrode.

Nevertheless, it is evident a reasonable upfront investment in a blue-roof system can easily yield many years of productive stormwater collection and rainwater diversion down the line. Along with savings in water usage and energy consumption, blue roofs have proven to be highly beneficial for a building owner over the long term.

In addition to those savings, federal programs and city programs can help supplement initial funding for sustainable projects, especially those that might not otherwise get built. For example, LEED can offer certain tax benefits on a national level. There are also many secondary incentives available through city governments. One example is the aforementioned NYDEP grant program, which awarded money to the Brooklyn Navy Yard.

Some projects use a combination of both blue and green roofs to achieve the financial balance necessary to meet a given budget.
Photo ©

Design considerations and challenges
Local codes often set key standards for the successful installation of blue roofs. In New York City, the building code provides various incentives to install blue and green roofs, and even stipulates certain qualitative standards like the fact that jurisdiction’s blue-roof basins must have a secondary membrane.

The goal of the secondary membrane is to provide a materially redundant support layer that can both catch any leaks penetrating the primary membrane and stop the leak from gathering on the roof. Leaks are potentially a serious problem to rooftops because they can eventually lead to ponding, chronic water damage, and infiltration, or even structural failures over the long term.

Other standards provide for basic levels of safety while maintaining blue roofs. In Philadelphia, for example, standards[16] specify safe access to blue roofs for periodic cleaning.

Introduced in 2010, San Francisco’s “Stormwater Management Ordinance” requires any development project that will disturb more than 465 m2 (5000 sf) of ground to meet specific stormwater management standards, including the installation of blue and green rooftop areas. Such guidelines are essential to ensuring new technology is both encouraged and will eventually function properly in a given location and jurisdiction. However, in places where blue roofs have not yet been used, design professionals will need to do some added research as well as take additional steps to help facilitate the work.

While blue roofs can be used in both commercial and residential applications, they are still a relatively new technology in the building world. At present, they seem to be most interesting to the public sector—perhaps because of the presence of larger complexes of buildings that have more significant rooftop areas. Some large housing projects in California are starting to see the advantages of blue roofs, such as rainwater collection and water savings, especially in the state’s warmer, drought-prone portions. They are also indirectly seeing the energy savings that come from using reflective basin surfaces able to modify the heat island effect.

For private and residential building owners, ownership and other legal issues can come into play when making plans for shared building rooftops. Condominium buildings and other facilities owned by multiple people may face greater obstacles to investing in rooftop equipment due to the need for consensus. It is up to each building owner and group to decide collectively whether the potential gains outweigh the initial efforts required. Given the technology is new, there are also those who may resist its introduction outright because of the potential hassles and unknowns. New technology often needs some careful tuning, debugging, and testing to make it work properly and overcome initial hurdles.

Blue roofs, just like any building component, must be properly designed in advance to facilitate their proper function over time. As noted, some critical considerations include the appropriate and thoughtful design of roof structures for considerable additional water weight. Where blue roofs are added to existing building rooftops, the existing roof structure will need to be thoroughly checked and verified to be adequate to carry the new water and equipment loads and respond to climate conditions. Where blue roofs are planned for new buildings, structural engineers must design the roof structure to safely carry all new rooftop loads and to account for climate conditions, especially the weight of the total system under peak load.

Peak load is the point at which the system is carrying its maximum possible quantity of water. The system must also be structured to anticipate this correct volume and weight of water at specific peak load times and during the system’s function. Additionally, materials need to be thoughtfully chosen to reflect the climate conditions in which they are located. In some cases and climates, materials will need to consider the building’s orientation to the sun.

Using custom-designed options like flat-roof support systems can help facilitate the application of blue assemblies onto existing rooftops through better organization and accessibility for maintenance. These kinds of supports rely on metal brackets and structures to route conduits, trays, and generally organize the rooftop equipment to make it neat and accessible. By utilizing zero-penetration roof supports, designers have the ability to carefully place blue-roof equipment on existing rooftops with greater ease and to properly mesh with the existing surface and structure.

In many cases, starting with a relatively flat roof is a prerequisite. Most design/construction professionals are familiar with the widely accepted, code-required minimums of slope (i.e. ¼ in. per 12 in.) for built-up (BUR) or membrane roofs. However, material and technology improvements are slowly changing the game. Equipment can also be elevated and structured above the roof surface to function independently and ultimately limit the importance of rooftop plane geometry in blue-roof design. Tray geometry can also be designed and adjusted to the existing roof geometry in order to accommodate varying rainwater levels in a stable, reliable way.

Blue-roof systems also need to be designed to address water-borne bacteria, insects, and plant material that often grow in standing water. Especially in certain regions, mosquitos and bacteria, like algae, thrive in stagnant water with a high pH. In many parts of the world, certain bacteria and mosquito growth can quickly become significant health issues. (In some parts of the world, designing for locales with mosquito-breeding issues is paramount in order to avoid a true public health crisis. Special additives like biological mosquito controls, sometimes referred to as mosquito dunks, are now widely available and safe for fish, wildlife, and human populations.)

Diseases caused by some bacteria, however, are preventable simply through proper water filtering. Water-carrying organisms like Salmonella typhi, E. Coli, and Legionella must be specially treated by high temperature (or other means) to ensure the water is safe for human handling or even consumption.

Even for non-potable uses, water captured in blue-roof systems must be routinely checked for purity and contamination. Simple algae growth or leaf-debris buildup can cause pesky blockages and also further contaminations by fouling valves and disrupting water flow. Water must be able to consistently turn over, flow, and move. Many roofing materials have long been susceptible to moisture as evidenced by the growth of mold—blue roofs will require professionals to pay even closer attention to these roofing hydrology lessons learned in the past.

Materials will need to incorporate bacteria-, mold, and algae-resistant technologies. Architects and engineers must be savvy about the use of new materials, water flow, and new structural geometries. One example of the necessity for innovation in this new technology is the need to find ways to construct tanks and trays with techniques specially designed to prevent water leakage. Looking to cross-disciplinary practices and exploring the use of fabrics and fibers employed in boat manufacturing, for example, is one of the ways designers can transfer technology from one industry to another to find viable solutions.

New equipment and materials must inevitably be economical, durable, and able to stand up to the elements, particularly in severe climates. In places like Florida and Louisiana, where structures are susceptible to high winds, high rainfall, and storm surge, blue-roof applications will need to be additionally sturdy and adequately structured to withstand such extreme weather conditions. Similarly, owners, engineers, and designers seeking to construct blue-roof projects in seismically active areas like California will need to take additional precautions, including structural dampening, to ensure systems can withstand the maximum shaking that can occur during the most extreme seismic event projected for that region.

In the future, blue-roof technology seems poised to become an integral part of the sustainability approach not only in the hard-surfaced, urban areas of North America and Europe, but also in the rainiest countries of Southeast Asia, Australasia, and Africa—all places where rainfall collection will be of sincere financial interest to future populations. Around the world, all cities will require new ways to deal more responsibly with rainwater-runoff collection.

Particularly as climate change continues to influence and alter precipitation levels in new ways, governments and the global building and business communities will increasingly need to cooperate to address stormwater management and water reuse. Collectively, they must act in greater and more responsible measure to accomplish sustainability goals.

Sustainability is quickly proving to be less ‘a responsible choice’ than it is an obligation. All communities will have to adapt building practices to be successful stewards of this Earth. Blue roofs will simply be another tool in the toolkit.

John Engle is the sales director at PHP Systems/Design, an engineer and manufacturer of innovative, high-performance roof pipe and equipment support systems. With 13 years of experience in roofing, he is extremely knowledgeable about the industry’s best practices and current trends. Previously, Engle worked in the pipe support and roofing industries; he is an active member of the Refrigerating Engineers and Technicians Association (RETA). He can be reached via e-mail at[17].

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