Project Profile: Green Roof Designed to Reduce Stormwater Runoff into the Mississippi Sets Sustainable Design Example

The five-story Central Public
Library of Minneapolis, Minnesota,
completed in the spring of
2006, is an architectural
benchmark for Midwestern
urban landscapes and sets an
example for sustainable building design.
The new library features an energy-efficient
design complete with three green roofs and
is being billed by the city as an architectural
jewel that will support a vibrant downtown
culture.
The landscaping
architectural
company, Kestrel
Design Group,
was faced with the
challenges of 1) creating
an extensive green
roof that would retain stormwater runoff
while supporting the high-tech needs of a
modern information hub and 2) preserving
the library’s fragile historical collections.
For help on the project’s green roof, The
Kestrel Design Group selected Henry
Company, based on its track record with
green roofing systems.
The green roof infrastructure meets
both the Environmental Protection Agency
(EPA) and the Mississippi Watershed Management
Organization’s stormwater mandates.
The importance of managing stormwater
runoff into the Mississippi River from
the downtown Minneapolis development
was a critical design component in the specification
of the green roof for the new building.
The Central Public Library’s green roof
system employs two water management
systems working together to retain the
stormwater that reaches the property for
capture and reuse. First, directly beneath
the vegetation across the entire growing
platform, the green roof uses a Henry® DBR
Rootbloc System. This water management
system captures a portion of stormwater in
small plastic cups, which then re-hydrates
the green roof by evaporating upward
through the soil. Water in excess of what
the subterranean plastic cups can hold is
channeled into a secondary water management
system.
This secondary water system collects
excess stormwater in below-grade water
storage tanks. The water collected in these
tanks is then pumped back onto the roof,
where the rooftop gardens and streetscape
are automatically watered with a slow-drip
watering system.
The Central Public Library’s three green
roof systems cover more than 18,560
square feet, with an extensive vegetation
design that includes prairie plants and
hardy indigenous vegetation that thrives in
Minnesota’s harsh climate. The green roof
landscaping grows in a specially designed
“light” growing medium, and with two water
management systems, very little mainte-
Project in progress, before green roof installation.
Green roof planting in progress.
34 • I N T E R FA C E J A N U A RY 2007
Left: The secondary drip watering system is
integrated into the waterproof membrane, over
the green roof underlayment.
nance is required. The landscape
design is intended to connect the culture
of the community with nature. A
variety of plant species (totaling more
than 19,000 individual plants) was chosen
to form a changing wave of colors across the
roof during different seasons, thus reflecting
the Mississippi River’s strong influence
on Minneapolis street grid and building
design.
On a 90-degree day in July, Central
Library officials measured noontime temperatures
on the building’s roof. The black
asphalt portion was 170 degrees, while the
green roof was 92 degrees. A few days
before the library opened, building
officials tested the AC equipment.
The intake air readings on the green
roof were so much cooler than what
they were used to that “they thought
their equipment was broken,” noted
Peter MacDonagh, founder of The
Kestrel Design Group.
Not Just a Reduced Watershed Project
The benefits anticipated for the
green roofs on the new library go
beyond stormwater management and
include:
• Reduced building energy consumption.
• New green space in downtown
Minneapolis.
• Reduction of urban heat island signature.
• Improved internal air quality.
• Increased longevity of the library’s
roof system.
• A model for clean and sustainable
building design in Minneapolis.
The waterproofing product used was
Henry® 790-11, a hot-applied, rubberized
asphalt waterproofing membrane that currently
protects more than 1 million square
feet of green roofs, plazas, foundation walls,
and parking garages throughout North
America and Europe. The system is backed
by ISO registration, UL Class A fire rating,
CCMC approval, and LARR listing. The
Henry® 790-11 system is applied by a network
of Henry-trained and certified contractors
to ensure proper installation and peace
of mind for building owners.
Green Roofing Contractor
Rosenquist Construction Inc.
Greg Reiser, PM
Minneapolis, MN
Green Roof Design
The Kestrel Design Group
Edina, MN
Green Roof Waterproofing
Manufacturer
Henry® Company
El Segundo, CA
Minneapolis Central Public Library
Minneapolis, MN
A rendering of the green roof design.
Above left: The green roof basin is flooded to test the waterproof seal.
Installation of a coconut
windscreen over the
growing medium.
J A N U A RY 2007 I N T E R FA C E • 3 5
Left to right:
waterproofing, dirt, and soil.