Engineer ran study to prove value of green
roofs to communities.
A green roof has the
potential to lower stormwater runoff from 65 to 94%. In cities
with combined sewer systems, that translates to a significant
reduction in the demand placed on wastewater treatment plants
during storms, according to a two-year study of green roofs in
In addition, green roofs allow building owners to reduce
the size of stormwater detention tanks needed for peak
rainfall. That can offset the cost of a green roof by 30 to
60%, according to Magnusson Klemencic Associates, the local
engineer that led the $125,000 green-roof evaluation project.
The study quantified the effectiveness of green-roof
stormwater management, based on rainfall and weather
conditions, from July 2005 through this January.
At best, “only 6% of the rain became runoff,” says Drew A.
Gangnes, MKA’s director of civil engineering, who led the
study. “That’s a huge finding.” The engineer had projected 50%
mitigation based on earlier modeling.
Civil engineer Drew A. Gangnes, who led the
green roof evaluation project, maintains that cities
should give incentives to developers to build green
roofs because they can significantly reduce stormwater
runoff and demand at sewage treatment plants.
MKA also found the “magic number” for soil thickness to be
4 in., not 6 in., as suspected. The thinner soil bed can dry
out more quickly, readying itself to absorb the next rainfall,
Green roofs often get value-engineered out of a project
because even the simplest installations, off limits to the
public, costs $7 to $10 per sq ft. Gangnes wants more cities
to offer developers’ incentives.
“Seattle fully expects to provide drainage rate credits at
some level for green roofs and other low-impact development
stormwater technologies beginning in the 2009 rate year,” says
Dick Lilly, Seattle Public Utilities’ sustainable strategies’
Calling the MKA results “really encouraging” for a
small-scale study, he says the utility has a three-year
project to monitor several full-size green roofs. It will look
at MKA data and others.
The seeds of the MKA study were planted about five years
ago by clients asking whether a green roof could replace a
detention tank. To answer the question, MKA developed a tool
that models water evaporation, percolation and storage in the
soil matrix. Research and a tour of green roofs in Sweden
convinced Gangnes that green roofs were a promising stormwater
management tool. But there were no measurements to prove
it. There were green roofs, but they were not being monitored,
For help in underwriting the costs of the monitoring gear
and the five, 8 x 12-ft building plots at four test sites, MKA
enlisted local developers Urban Visions, Unico Properties and
Vulcan Inc., and one local contractor, Sellen Construction Co.
Additional local contributors include Cedar Grove, for soil;
landscape architect Gustafson Guthrie Nichol for consulting on
planting and reports on plant materials; mechanical
engineer-constructor McKinstry for weather stations and
monitoring solutions; Sunbelt for scaffolding needed to
elevate a test site on a parking lot; and Ness Cranes, for a
hydraulic crane used to lift the dirt onto the roofs.
The $125,000 cost includes in-kind services. About
two-thirds of that was for MKA’s soft costs.
Green-roof cost can be offset 30 to 60% by a
decrease in the size of the stormwater detention tank.
Photo Engineer had predicted that 50% of
stormwater would be mitigated, not 65 to 94%, as
Each plot contained a different green-roof thickness and/or
planting medium. That allowed a range of performance
characteristic data. The four different downtown locations
helped determine whether prevailing winds, building shadows
and other variables affected performance. The weather station
and monitoring equipment, wirelessly linked to MKA’s office,
allowed the collection of data, including amount of
rain, air temperature, humidity, wind speed and soil
moisture. MKA collected more than 1.5 million
At each site, rain was first taken up by the plants. Then,
it dissipated through evapotranspiration. Finally, the water
slowly made its way through the green-roof materials. Water
that made its way to a drain at the corner of the installation
was measured for flow and temperature before it was discharged
to the existing roof. The data quantified the amount of runoff
to both the building and to city infrastructure
Sellen contributed a site for an installation, and built
the five plots. The contractor is now applying lessons learned
on its first green-roof project to its first large-scale,
permanent green-roof project, a 65,000-sq-ft installation that
will top a 1,000-car garage for the 500 Fifth Avenue North
Development Campus. It currently is a hole in the ground.
“The MKA project helped us work with the design team and
with the city to explain the benefits of reducing or deferring
detention,” says Jack Avery, Sellen’s director of
Gangnes says that there is very little structural penalty
for a green roof on a new commercial building. For a 12-story
steel frame, the penalty is less than 0.5% of the steel
tonnage. On a concrete frame, where the roof is a concrete
slab, there is “virtually” no change to the structural design.
The weight of gravel ballasted to the roof to hold the
membrane down is about the same as the weight of the 4 in. of
soil, says the engineer.
Gangnes, who just started building a 150-sq-ft pitched
green roof on an addition to his house, hopes to do more
studies on the topic. “Every gallon of rainfall eliminated at
the building site is a gallon that does not require treatment
with chemicals, processing, etc.,” he says. “Research also
suggests that this local repairing of the hydrologic cycle can
reduce urban temperatures, which saves energy as building
cooling demands are released.”