From Green to Blue: Making Roof Systems Sustainable in Urban Environments

Municipal storm-water managers historically have focused on controlling runoff from ground-level impervious surfaces, such as roadways, sidewalks and parking areas. However, the next frontier in storm-water management is rooftops. In urban storm-water management, roofs are part of the problem and potential solution. An exciting new technology to control rooftop runoff is known as blue roofs. Over the next several years, New York City alone will spend several billion dollars on green infrastructure solutions to address its storm-water-control problem, and blue roofs will be a key part of these efforts.

Blue-roof trays are held in place with stone ballast and hold up to 2 inches of water. The tray systems resulted in a 45 percent reduction in roof runoff during rainfall events in a New York pilot project.

Blue-roof trays are held in place with stone ballast and hold up to 2 inches of water. The tray systems resulted in a 45 percent reduction in roof runoff during rainfall events in a New York pilot project.

Blue Roofs

The roofing industry has become very familiar with the use of vegetated, or green, roofs. The vegetative layer grown on a rooftop provides shade and removes heat from the air through evapotranspiration, ultimately reducing temperatures of the roof surface and the surrounding air. By reducing the heat-island effect, these buildings require less energy to cool in the summer and use fewer natural resources (oil or other fuel) in the process.

However, an even newer and less-well-known sustainable technology applicable to roofs is the blue roof. A blue roof temporarily stores rainwater in any of a number of types of detention systems on the roof. They are most applicable and provide the most benefit in highly urbanized cities that are serviced by combined sewers. Combined sewers handle sewage and rainwater runoff from roofs, streets and other impervious surfaces. On dry days, these combined sewers can easily handle the amount of sewage flowing through them to the local treatment plant. However, on days with heavy rain, these combined systems can easily overflow with rainwater and raw, untreated sewage. This combined sewer overflow, or CSO, can flow into local sensitive receptors, like streams, ponds and oceans, contaminating the natural resources and killing fish and other wildlife dependent on them.

The beauty of blue roofs is they can store much of this rainwater during and immediately after a rainstorm, temporarily preventing it from reaching the sewer system. In this way, CSOs are minimized and local natural resources are protected. When the storm is over and the sewer system has the capacity to handle it, the blue-roof retention materials are designed to slowly release the stored rainwater back into the storm-drain system.

This blue roof in New York uses a check dam to retain storm water.

This blue roof in New York uses a check dam to retain storm water.

NYC Pilot Program

Our firm, Geosyntec Consultants, along with environmental engineers Hazen and Sawyer and HydroQual and water-management firm Biohabitats, designed and implemented a groundbreaking blue-roof system in New York. The New York City Department of Environmental Protection (NYCDEP) retained the team to implement a sustainable green infrastructure retrofit pilot program to demonstrate how rooftops can reduce the frequency and volume of CSOs in the city. The objective was to design and install storm-water controls to quantify the benefits of sustainable approaches as a viable solution to reduce storm-water flows to the city’s CSO system. Rainfall of less than 1/2 inch can overload the system and result in untreated discharges. The use of sustainable green infrastructure, like blue roofs, to reduce storm-water inputs to the combined system is one of many approaches New York City is considering to help solve this problem.

Geosyntec’s role on the team was to design several storm-water pilot studies, including blue roofs. Our blue-roof designs included installing risers on rooftop outlets that would result in ponding of water around the outlets, small dams on the roof surface using check dams of angle-iron to create ponding and the most successful technique—blue-roof trays. We developed specially designed trays, held in-place with stone ballast, to hold up to 2 inches of water. The tray systems resulted in a 45 percent reduction in roof runoff during rainfall events. If blue-roof trays were installed on all roofs in an entire drainage area to a CSO, the results would be significant in solving the CSO problem. In addition, trays are more practical because they can be spaced around existing equipment on roofs and moved during repairs and maintenance of other rooftop systems.

Geosyntec Consultants designed a blue roof that included installing risers on rooftop outlets that would result in ponding of water around the outlets.

Geosyntec Consultants designed a blue roof
that included installing risers on rooftop outlets that would result in ponding of water around the outlets.

Roof-system Protection

Protecting the integrity of a roof membrane is an important consideration for roofing and building contractors that are considering installing a blue roof. Blue-roof-tray systems offer the best protection because they rest on top of existing membranes and ballast systems and do not result in any membrane perforations that require additional waterproofing. Other blue-roof systems, like check dams or new drain inserts, may require additional waterproofing. The bottom line is if the roof membrane is old, compromised or currently leaking, any type of blue roof would be problematic until a new membrane is installed.

In addition, during the pilot projects, we took great care to inspect and test the roofs for load-bearing support—a step that should be conducted for all blue and green roof systems.

As we look to the future, roofs in urban areas will most definitely become a major part of the storm-water solution, and blue-roof technologies will evolve to become a common practice.

Learn More

NYCDEP has posted information about blue roofs and other urban green infrastructure for CSO control on its website.
The U.S. Green Building Council offers an online course about blue roofs for storm-water management.

PHOTOS: Geosyntec Consultants

ASTM International and Sustainable Roofing

ASTM International is a well-known standard-writing organization for the construction industry and other industries. As the building design and construction industries have moved toward more sustainable methods and products, it follows that more standardization of sustainability is necessary. This is certainly true for the roofing and waterproofing industry. ASTM now has a group devoted to developing standards for sustainable roofing.

Organizationally, ASTM is divided into numerous committees, each having a specific focus. The ASTM D08 committee is responsible for roofing and waterproofing standards. Within the D08 committee, there are multiple subcommittees that focus on a segment of the roofing/waterproofing industry—from asphalt shingles to spray polyurethane foam to modified-bitumen membranes to single-ply membranes and more. D08.24 is the subcommittee that is developing standards specifically related to sustainable roofing.

Further division of each subcommittee into Task Groups allows narrowly focused groups to develop standards for very specific topics. There are currently four Task Groups within D08.24:

  • D08.24.01 Guidelines for Sustainable Design
  • D08.24.03 Recycling Practices and Reporting Methodology
  • D08.24.04 Durability
  • D08.24.05 Selection Criteria Vegetative Roof Membranes


Standards are developed by Task Groups with active participation by attendees at the semi-annual meetings. Typically, a draft standard (called a work item until it is an approved standard) is initially sent out for ballot to the Task Group to obtain comments that will improve the draft standard. After balloting to the Task Group, the draft standard is balloted to the full D08 membership. At times, a standard is simultaneously balloted to the Task Group and the full membership. During the balloting process, comments and negative votes are reviewed and dealt with according to ASTM protocols. Standards development is a very linear process that works well to achieve a consensus in the D08 committee. Once a consensus is reached, the standard is published for use.

The background on the process is necessary to understand the activities of the D08.24 subcommittee. Because the subcommittee was only recently established, all standards are still in the development stage.

Task Group Specifics

The D08.24.01 Task Group is developing a new standard, work item WK26599, which is currently titled “New Guide for Design of Sustainable, Low-Slope Roofing Systems”. The current scope is:

  • This Standard provides guidance for designing sustainable low-sloped roofing systems, including exposed membrane roofs, membranes covered with vegetative (green) overburden systems, ballasted roofs and protected membrane roofing assemblies. A sustainable roofing system minimizes environmental impact, conserves energy, and has maximized service life.

The scope recognizes the roof’s primary function is to weatherproof the building’s top surface.

The document provides a sequential process for designing sustainable roof systems. The document does not provide a prescriptive approach, but “attempts to help the user define and consider roofing system demands and environmental life cycle impacts, and integrate these with features that contribute environmental, energy conservation, or other benefit in service” through a number of considerations, which include roofing demands, functional expectations, end-user requirements and site restraints. The document is big-picture, technology-neutral and process-based.

The D08.24.03 Task Group is developing a new standard, work item 24614, currently titled “New Guide for Recycling Practices & Reporting Methodology”. The intent of the document is to unify common practices and develop an industry-accepted reporting format for recycling common roofing materials, such as asphalt shingles. The document is in its infancy and has not been balloted to date.

The D08.24.04 task group is developing a new standard, work item 26595, currently titled “New Guide for Roof System Durability”. The task group is still evaluating the specific scope but will focus on PVC and EPDM membranes in two separate documents. The intent is to provide methodology to evaluate the variables that lead to increased durability of PVC and EPDM roof systems. Neither document has been balloted yet.

The D08.24.05 Task Group is developing a new standard, work item 29304, currently titled “New Guide for Selection of Roofing/Waterproofing Membrane Systems for Vegetative (Green) Roof Systems”. The document will provide technology-neutral considerations for selection of appropriate membranes for vegetative roofs. The Task Group is expected to begin the balloting process soon.


The ASTM D08.24 group works with other groups, like E60 on Sustainability and the Built Environment Advisory Committee, to ensure continuity of ideas with all ASTM committees. Specifically, E60.01 on Buildings and Construction and E60.80 on General Sustainability Standards are in the focus of the D08.24 liaison efforts.

The ASTM sustainability standards are intended to be used by the roofing, construction and design industries to formalize the efforts toward more sustainable roofs and roofing. It is hoped that other roofing groups, such as ARMA, CEIR, ERA, NRCA and SPRI, will reference ASTM’s sustainability standards in their documents.

I encourage everyone in the roofing industry to not only join ASTM, but to participate in the development of the standards our industry uses each and every day. ASTM D08.24 needs your input as the roofing industry moves further toward sustainable products and activities.