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Blue Roofs: What are they and what are some design strategies for Implementation?

Blue roofs are an attractive idea for cities that have issues with controlling rain water drainage volumes and have combined sewer systems still in service such as New York City (NYC). Blue roofs are used as a strategy for slowing the drainage of water from roofs while avoiding some of the extra costs associated with green roofs. They can also provide evaporated cooling as part of the design. Combined with a grey water system blue roofs can capture water for irrigation and within the building to help with water conservation. Most of the research on this is being done on the government level.  NYC is leading this research as we have major drainage problems, as of 2012 they are part of the green infrastructure plan.

For further reading on NYC’s policy and planning check the following link NYC Green Infrastructure plan.

When combined with other roofing strategies they can be used in creative ways to develop beautiful spaces. Green Infrastructure Feasibility study – Check out this feasibility scan for Bridgeport and New Haven CT for further reading on strategies with cost and cost effective designs.

Blue roofs offer more precise control, are less expensive then green roofs and are less weight on roof so that existing roofs might not need reinforcement. Blue roofs can:

  • Helps control Stormwater runoff
  • May provide evaporated cooling
  • Helps with water conservation.

Blue roofs are used only on flat roofs, where ponding can happen and causes roof leaks. Ponding needs to be a consideration as part of the system design. How do you prevent ponding on blue roofs? The Design team must design the roof to drain in 24hr rain event and need to look at roof membrane erosion. A Blue roof must be designed with the following in mind to prevent full-blown ponding by:

  • Controlling the flow of water
  • Placing the drains at real structure low points
  • Designing the roof to drain within 24hrs of a rain event
  • Not creating sites where the flow of water will damage the roof membrane through erosion

Blue roofs can also be called “roof ponds.” There are existing technology that aids in the design of blue roofs, such items are called flow regulators, roof drain governor, roof drain with weir, or flow control roof drains.

Design will need environmental engineers to estimate water volume and flow rate and structural engineer to make sure that the loads are safe for the  existing or new roofs.

Blue roofs strategies have a lot of possibility for innovation and can be put into practice on new and existing roofs. Some strategies are:

  Using Check Dams, slow water down. A Check Dam a system of barriers that would slow water down. 
  Detention berms, like check dames’ detention berms may require roof structural reinforcement. They are areas where water is detained for a short period of time.   
  Trays for existing roofs, this is the easiest type of blue roof to install and most flexible. They do not collect water from surroundings like the other strategies. They do add weight to the roof so structural considerations will need to be thought of in the design phase. 

Some blue roof design parameters to consider are:

  • Design should be based on Drainage rate, Drainage volume and Climatic data
  • Design should be depending on Structural capacity, Location of water storage vs. structural elements, roof slope, detention volume / overflow volume, rate of evaporation, Drain capacity, Roof geometry.
  • Redundancy of certain elements in the design such as Roof membrane and Drainage capacity.

If projects are going after LEED certification, Blue Roofs might be used for – SS Credit: Stormwater Management where a blue roof is used to control stormwater runoff. This process can also find LEED Credit Synergies in:

WE Credit – Water Efficient Landscaping

EA Credit/ Prerequisite Commissioning

EA Credit Measure and Verification

MR Credits Environmentally Preferable Materials

Blue roofs can be a great addition for any project that is looking for lower cost sustainability measures as well as be implemented in urban areas where there is a high risk of flooding and combined sewer systems.

This article was written by: mayacamou

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