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March 11, 2004
Image courtesy of MKA
An eco-roof can keep up to 50 percent of the rainfall that lands on it from ever touching the ground. Eco-roofs also absorb carbon dioxide, give off oxygen and reduce the burden on local infrastructure.
Green roofs, also known as eco-roofs, are capturing the attention of designers internationally.
For the neophyte, an eco-roof is a vegetated rooftop achieved through planting carefully selected vegetation in specially designed soil layered over drainage material, all atop a conventional roofing membrane.
While benefits over conventional roofs are widely recognized among their boosters, eco-roofs are still struggling to enter the mainstream for one simple reason: they cost more than conventional roofs. After all, they are a conventional roof plus additional stuff. While conventional roofs run $6 to $10 per square foot, eco-roofs can cost as much as $15 to $18 per square foot.
Yet the benefit statistics of eco-roofs are cited mantra-like in design meetings: “Equivalent to R20 insulation!” “Triples roof life!” “Can reduce runoff by up to 70 percent!”
So why aren't they used more? Because little backup is available to translate these benefits into cost offsets for building developers (in other words, items the developer can cut because of the presence of the eco-roof).
Since eco-roofs cost more, demonstrating cost offsets is essential to their wider acceptance. Toward that end, Magnusson Klemencic Associates has developed a new runoff modeling technique that helps quantify the stormwater management cost offset an eco-roof can bring to a project.
What's so important about stormwater? There are three main approaches that can be used to offset eco-roof construction cost premiums:
The stormwater management benefits of eco-roofs figure significantly in each of these:
Installation incentives are enticements doled out by public agencies to recognize development components included “for the greater good.”
No incentives for eco-roof construction currently exist in Seattle or surrounding communities, but the undeniable ecological and social benefits have many agencies considering such incentives. At the center of the discussion is the fact that eco-roofs ease the burden on public storm drainage infrastructure.
Life cycle cost savings, while appealing in premise, are voodoo in development circles.
Since many building developers sell their buildings soon after construction (so much for the benefit of fewer roof change-outs over time) or have owner-tenant agreements that do not allow life cycle cost savings to be realized by the developer (good-bye to energy savings due to increased thermal insulation), life cycle cost savings are a hard sell as a first-cost offset.
Yet conventional stormwater detention and filtration systems CAN have a large life cycle cost that COULD contribute to the financial analysis.
Construction cost offsets are possible in a few areas: soil-insulating properties can offset conventional insulation, eco-roof thermal benefits can enable reduced HVAC equipment costs, and eco-roof stormwater management properties can replace conventional systems. Of these, stormwater management cost offsets can yield the most construction dollars.
To assign value, whether in the form of incentives, cost savings, or offsets, one must first understand the stormwater benefits brought by eco-roofs:
But how much, how much, and how much? While these benefits have been quantified for some climate areas, there is little information available on the performance of these systems in Seattle. A few cities are currently collecting monitoring data, but none with Seattle's precise climatic conditions.
Much is riding on quantifying the stormwater management benefit of eco-roofs in Seattle.
This could be done by building an eco-roof and monitoring its performance (How much rain fell last night? How much made it to the base of the building? How clean was it?). However, monitoring seems to be in a Catch-22: “Permit me to build an eco-roof, and I'll prove that it handles rainfall in this manner” versus “Prove to me that an eco-roof reduces rainfall in this manner, and I'll give you a permit to build one.”
The solution is to use an analytical model to predict how an eco-roof manages rainfall utilizing agency-accepted climatic and hydrologic science. Since standard analytical runoff models used in the Seattle area are not calibrated to the unique characteristics of an eco-roof, MKA has developed just such a tool.
New modeling tool
The methodology developed by MKA combines accepted modeling practices with a physical-based model of eco-roof moisture-retention processes.
Using this tool to support stormwater management designs will help assure agencies that projects comply with codes, provide owners with the opportunity to offset costs from conventional stormwater management components to the eco-roof, and hasten the wider acceptance of eco-roof installation.
The eco-roof modeling tool tracks rainfall and moisture as it flows through the various rooftop components, which are modeled as reservoirs where water is retained or temporarily stored.
The components modeled are evapotranspiration, canopy interception, depression or drainage layer retention, soil media retention and transient storage. Surface runoff and seepage flow are tracked separately.
The eco-roof modeling tool generates an eco-roof runoff profile that describes its discharge over time. This profile is then used to size stormwater detention facilities using a conventional approach.
The bottom line
MKA's eco-roof modeling tool has been used to assess several Seattle case studies.
Each was carried out on a previously completed project for which conventional detention facilities had already been designed, so that a comparison to the baseline could be easily made.
The modeling determined that stormwater detention facilities could be reduced by up to 50 percent depending, of course, on the level of eco-roof coverage.
Discussions are under way between MKA and the city of Seattle about using this modeling technique to reap real cost offsets on real projects. Even though the city recently built eco-roofs on two new buildings in the City Hall complex, neither project took advantage of a stormwater management cost offset.
Further, while the city's drainage manual allows eco-roofs as a means of stormwater management, it leaves open the issue of how to prove code compliance. MKA is working with both the Department of Planning and Development and Seattle Public Utilities to ensure that as eco-roof-outfitted projects begin showing up for permitting, everyone's expectations are met.
The case-study work is also fueling discussions on incentives, a concept that already exists in other cities. In this area, incentives might take the form of a zoning bonus, utility rate relief or stormwater detention exceptions.
This modeling work has shown that on an average annual basis for Seattle, eco-roofs would keep up to 50 percent of the rainfall that lands on the roof from ever touching the ground.
Eco-roofs also absorb carbon dioxide, give off oxygen, reduce the urban heat-island effect, provide habitat for birds and insects, and Â“green upÂ” vast planes of gray. This clearly has implications Â“for the greater good.Â”
Drew A. Gangnes is a principal at Magnusson Klemencic Associates and the firm's director of civil engineering.