CONTACT INFORMATION
You can contact the following City agen- Rooftop Detention
cies by calling 311, the City’s non-emergen-
cy government information and services
hotline, or by calling the agency numbers A Low-Cost Alternative for
listed below: Complying with New York
NYC Department of Environmental Protection (DEP) City’s Stormwater Detention
718-595-5223
Requirements and Reducing
NYC Department of Buildings (DOB) Urban Runoff
718-566-5280
NYC School Construction Authority (SCA)
718-472-8332
WHAT IS ROOFTOP DETENTION?
Rooftop detention, also known as a
”Blue Roof” or controlled-flow system,
is an easily installed, cost-effective
alternative to temporarily store and
gradually drain rainwater off a build-
ing’s rooftop. Detaining rainwater helps
to slow its rate of release into the sewer
system for a maximum period of 24
hours. It also helps to reduce runoff
during peak rainstorms so that more
combined sewer flows can be treated at
wastewater treatment plants and risks
of street and driveway flooding and
sewer back-ups in basements are
lowered.
Rooftop detention can play a role in
ensuring that our sewer systems are
not overwhelmed during rainstorms 59-17 Junction Boulevard
and our waterways are protected from Flushing, NY 11373
pollutants in urban runoff.
Printed on recycled paper 3/12
HOW ROOFTOP DETENTION WORKS • Is easy to install and maintain, similar to a • Water Depth: Water depth on the rooftop
Controlled-flow roof drains regulate the standard roof. should not exceed two-to-four inches and
flow of rooftop drainage. Small weirs • Extends the life of the roof through a should not be stored for longer than 24
(openings), as shown in the drawing secondary membrane, or other waterproof- hours. Depths much shallower than three
below, are placed inside the inlets of roof ing roofing system. inches are typically required to satisfy
detention requirements.
drains to capture rainwater and slowly • Is an approved practice under New York
release it. Stormwater builds up to a • Scuppers and Overflow Drains: Roof should
City’s construction and sewer connection
predetermined depth and drains at a be designed with multiple outlets and
codes.
unrestricted overflows. Scuppers (wall
desired maximum rate. Rooftop detention • Costs considerably less than other ap- outlets) should be placed in the parapet.
requires that a secondary waterproofing proved detention measures, such as
membrane, or other waterproofing roof- • Number of Drains: A minimum of two
subsurface detention. Costs are estimated
ing system, be applied. drains in roof areas of 10,000 sq. ft. or less;
at an additional $5/sq. ft. over a standard
and a minimum of four drains in roof areas
roof.
larger than 10,000 sq. ft.
• Provides similar stormwater benefits as
• Load Bearing Capacity: Although rooftops
Multi-Weir Barrier green roofs (although green roofs provide
Flashing Clamp are generally built to accommodate rainfall
and Gravel Stop additional environmental benefits) at a
build-up and snowpack, rooftop detention
fraction of the cost – approximately
may require additional structural supports.
$5/sq. ft. for blue roofs vs. $30/sq. ft. for
This will depend on the depth of water
green roofs.
detained, HVAC and other rooftop equip-
ment, as well as further considerations.
DESIGN CONSIDERATIONS
The following are some design consider-
Deck Plate CASE STUDIES
Trapezoidal Weir ations for installing a rooftop detention
Opening Limiting The New York City School Construction
system. The architect and/or engineer on
Flow Rate to Leader Authority (SCA) is a pioneer of Blue Roofs
the project will make site-specific determi-
citywide. Over the last three years, SCA
HOW CAN ROOFTOP DETENTION nations of structural or other design needs
has installed rooftop detention in 14 new
BENEFIT YOU? for the project.
schools. For SCA, the decision was initially
In addition to providing benefits for the • Application on Flat Roofs of New Structures. an economic one. The costs of rooftop
City’s waterways and sewer systems, Following good design practices and the detention were far lower than for subsur-
rooftop detention offers many advantag- requirements of the New York City con- face detention. The systems installed have
es to the developer, building owner, and struction codes will ensure optimal perfor-
required essentially no maintenance and
tenants. A Blue Roof: mance and low maintenance of a rooftop
detention system.
there have been no problems with leak-
• Requires no additional land area, so it is age. “We have been extremely pleased with
well-suited to high density urban areas • Waterproofing: Rooftop detention requires the performance of our rooftop detention
where land is limited. the installation of a secondary waterproof-
systems. The engineer should consider this
ing membrane, or other waterproofing
innovative roof drainage design strategy.”
roofing system.
—Harry Cethoute, P.E., SCA.
