Otherwise know as a controlled flow roof drainage system
City jurisdictions are going to continue to update and revise storm water management requirements in the wake of high-volume rain events. Building owners have a responsibility to properly drain or otherwise manage the rain water that falls on their site can and should make use of blue roof systems. Strategic implementation of this building system helps to achieve the goals of increasing capacity to hold or detain water on site and then slowly release it into the city’s system. Discussion on how to use rain water in grey water cycles and serve as a starting point for green roof systems will be explored in future posts.
When considering whether your project is right for a blue roof system, there are municipal guidelines established by your local Department of Buildings and the state with in which you are building the project. For example, in NYC the local laws 92/94 of 2019, requires all new roof decks or assemblies being replace to be a ‘sustainable roofing zone’. It goes as far as to say projects of a certain size or type will require a green roof, photovoltaic systems, a combination of those two, or very carefully utilize the exemptions that would otherwise exclude your project from the requirements.
NYC’s motivation for this law is simple, stormwater runoff is generated by impervious surfaces or land that water cannot percolate into. Urban areas, with lots of covered surfaces and plenty of impervious areas experience an increase in the flow to drain and sewer systems. We need only look back to September 2023, for what high volume rain events can do to a city wide system that is configured like New York’s is.
Blue roofs require particular attention to waterproofing systems, ones that are capable of accepting long term ponding should be carefully chosen by your design professional. In addition, structural assessments are required, whether we are retrofitting an old roof system or if we are building a new one, it is important that a structural engineer’s calculation be taken under advisement. It is imperative that the structure be sufficient to accommodate the maximum load of this system. Blue roofs can be installed on varying types of roofs, after determination of adequate capacity and waterproofing considerations.
The roof system can be combined with secondary on-site storage systems, drywalls, retention ponds, etc., serving as a post-construction stormwater control that helps to mimic pre-development drainage patterns and hydrologic processes by increasing detention at the point of rainfall. It is worth noting guidelines vary regarding maximum ponding depth and length of time detained, local municipalities should be consulted.
Water proofing selection is between two main assembly types. There are unprotected conventional roof and there are protected inverted roofs. A key difference in these systems is where in the assembly the insulation and the roof membrane are located. Each system has particular strengths and weakness, and should be selected to suite the particular roofing requirements of your project. The water proofing membrane must be able to tolerate frequent ponding, monolithic membranes or welded seams with a proven history in similar applications are preferred. See technical & design considerations with respect to blue roof for more information.
There is immense value in proper design and due diligence during construction. Steps can be taken to detect issues (via electric field vector mapping), inspections should be done regularly throughout the installation and adhesion tests can bring piece of mind to all parties involved in the new roof system.
There are several roofing details worth paying close attention to; all edges, seems, corners. Penetrations or other discontinuities should be installed according to the specific requirements of the manufacturer. During the installation the contractor should take several precautions to ensure all waterproofing and associated flashing should be protected from damage and they are of a similar service life to the field membrane. Equipment should be mounted on curbs and penetrations should be extended/protected to account for the additional height of ponding anticipated.
Structural considerations include, but are not limited to, assessing roof members for rain loads in combination with ponding, deal, live, wind and snow loads as appropriate (ASCE/SEI 7-10 Chapter 8). 6 inches of water weight about 32 lbs/sqft, Coordination with the plumbing systems achieved during hydraulic analysis will determine the particular controlled drainage systems required that structural system will have to accommodate. Seek guidance from your design professionals in selecting roof member with sufficient strength and effective stiffness to remain elastic under loading.
Reducing stormwater pressure on existing structure, is something architects and engineers have to keep in mind for every project, no matter how large or how small. The intent of the laws and regulations that design professional have to follow is to minimize inundation of the storm drain and/or combined sewer systems, there by protection waterways from urban runoff. The challenges and special considerations we have covered here emphasize the importance of incorporating improved design and detailing throughout the process.
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