October 20, 2014

Blue Roof

Section of The Jefferson with roof drain detail.

OVERVIEW
A blue roof is a roof explicitly designed to capture and slowly release rainwater, in order to slow the rate of runoff and reduce the potential for related flooding.

BENEFITS
+ Temporary storage of rainfall mitigates runoff impacts
+ Stored rainfall can be reused for irrigation purposes, as makeup water for cooling, or in recreational contexts
+ Less costly than green roofs
+ Can be combined with light-colored roofing materials to further mitigate urban heat island effects
+ A viable retrofit for existing buildings
+ Maintenance required is equal to that of a conventional roof

CHALLENGES AND OPPORTUNITIES
+ Best suited to large flat roofs
+ Structural capacity should be carefully considered
+ Blue roofs are eligible for funding from the Green Infrastructure Grant Program run by the NYC Department of Environmental Protection (renewed annually, applications accepted twice a year)

PROCESS
During and after rainfall, weirs at roof drain inlets restrict the flow rate of the captured water, creating temporary ponding before gradually releasing stormwater to the municipal sewer system. Most weirs allow for adjustment of the flow rate.

The ratio of captured runoff is typically designed to closely mimic the pre-construction hydrology of the site. A blue roof acts as a temporary sponge, replacing the prior capacity of undeveloped, pervious ground to absorb rainwater.

 

CASE STUDIES
The Jefferson
BKSK Architects
New York, NY
115,000 square feet

PS 118
Blue Roof study by NYC DEP
Queens Village, NY

The Osborne Association
Hazen and Sawyer
Bronx, NY
75,000 square feet

 

ADDITIONAL RESOURCES
Green Infrastructure Grant Program [PDF]
NYC Department of Environmental Protection