Green Roofs Reduce Volume, Cost of Storm-water Discharge

An Alcoa-sponsored analysis of green roof applications worldwide conducted by an interdisciplinary team from the University of Michigan concluded these roofs can minimize runoff and associated storm-water discharge issues and thus make a useful contribution to Alcoa’s overall goals for eliminating waste.

Alcoa initiated the study as part of an effort to meet its goal of “near zero” storm-water discharge by 2015—a goal driven by a belief that water availability and water quality will be issues of growing concern throughout the world.

Alcoa recognizes that “end-of-pipe” treatment approaches are neither prudent nor sustainable and can be quite costly. For example, the company spent $25 million over the last decade to collect and treat storm water at just one of its operating facilities.

Green roofs are a common tool for storm-water management in Germany, where approximately 14% of all flat roof surfaces grow plant life. Green roof vegetation—usually a variety of hardy and drought-tolerant species—is planted in a lightweight, specially formulated soil that absorbs and retains water in a controlled manner to nourish the plant life. Beneath this substrate is a drainage layer that releases excess moisture and a special waterproof and root-repellant membrane that protects the building’s standard roof.

Typically, green roofs add about 50% to a conventional roof’s initial costs, but this additional expenditure is more than offset by the increased life of the roof, the reduced energy costs for heating and cooling the building, and the reduction in costs for end-of-pipe treatment of large surges of storm water.

A green roof’s soil and vegetation intercept storm water and can reduce runoff by 80% in many areas. This brings a number of benefits, including the following:

  • Intercepts and evaporates clean rainfall, thus reducing sewer peak flow, contaminant pickup, and discharge through the plant outfall system.
  • Delays the runoff and therefore avoids sewage overflow events.
  • Reduces building energy consumption because it serves as superior insulation.
  • Extends roof life by protecting the waterproof membrane from mechanical damage, UV radiation exposure, and thermal flux.
  • Provides insulation from sound. A three-inch-thick green roof can be expected to reduce sound transmission by five decibels; a 46-decibel reduction has been claimed for thicker green roofs.
  • Provides attractive habitat for birds and butterflies.
  • Improves air quality by sequestering carbon dioxide and filtering ambient dust.
  • Increases building aesthetics.
  • Reduces reflectivity, which can contribute to the “heat-island effect” in cities.

Alcoa has developed a green roof decision tool to assist in assessing the economic costs and benefits associated with the installation of a green roof on a given facility. In addition, the company installed a 446-square-meter (4,800-square-foot) green roof on the Alcoa Technical Center near Pittsburgh to study the benefits of green roofs and gain experience in the technology. Lessons from this and other future pilot green roofs in differing climates will be used with the green roof decision tool to assist facilities with their storm-water management challenges that include regulatory compliance, cost reductions, and sustainability goals.

In addition to researching the potential environmental benefits of green roofs, Alcoa is exploring opportunities to enhance current products and develop innovative aluminum products for green roof applications.

http://www.alcoa.com

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