Sustainable Roofing Strategies

Lowering Energy Costs, Advancing Environmental Goals
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Advertorial course provided by: American Hydrotech, Inc., Cool Roof Rating Council, Evergreen Slate, NEOGARD®, division of Jones-Blair Company, and Petersen-Dean Roofing Systems
Barbara A. Nadel, FAIA

The urban heat island effect creates an unhealthy environment, with air pollution and smog. Open green spaces within city centers decrease as land values rise, while rooftops, parking lots, and roads in the cityscape are impervious to water. As a result, drainage and sewer systems become overloaded, thereby increasing the risk of flooding following heavy rains. Continued urban development will exacerbate the problem, making the cost of improving existing sewer infrastructure prohibitive in most communities.

Replacing the impervious system of a conventional roof with a green, or garden, roof can substantially reduce storm water run-off and restore the balance with nature. Green roofs combine high-performance waterproof membranes with garden planting roof technology. Many green roof assemblies are lightweight, and can be installed on new and existing buildings. Three types of green roof assemblies are commonly used for urban applications:

• Extensive:These are designed to be lightweight, and support hardy plants, with low-maintenance, and no irrigation in most climates. This is the optimum choice for mitigating urban heat island effects and reducing storm water runoff.

Semi-intensive: Another lightweight assembly that has deeper soil, designed to support sod lawns and perennials. Irrigation depends on plant choices and climate. It is typically accessed by nearby hardscape areas, such as a patio, or other usable space. !Intensive: Greater soil depths are needed to support different plants, shrubs, and trees. This rooftop system can replicate yards and gardens and be designed for recreational use. Irrigation and regular maintenance are necessary.

 

Schwab Rehabilitation Hospital, Chicago, IL.

This rooftop healing garden is used for rehabilitation and horticultural therapy in Chicago.
Schwab Rehabilitation Hospital, Chicago, IL. Architect: Stephen Renkin Associates.
Landscape Architect: Douglas Hills Associates, Inc.

Photo courtesy of American Hydrotech, Inc.

 

Green roofs provide many ecological, technical, and building owner benefits, such as:

• Ecological:Green roofs cool and humidify the surrounding air, creating a microclimate with beneficial effects in the immediate vicinity. They provide natural habitats for plants and animals, such as birds and butterflies. Green roof vegetation reduces dust and smog levels. Nitrates and aerosol contaminants are absorbed out of the air and rainfall, and bound into the soil.
• Technical: Depending on the design, green roofs can reduce storm water run-off by 50 to 90 percent. The peak flow volume is greatly reduced, and peak flow periods are delayed by as much as four hours, minimizing impact on sewer systems. Green roofs reduce temperature extremes on roofs, thereby reducing cooling costs. Typical extensive green roofs reduce reflective sound by up to three decibels (dB), and improve sound insulation by up to eight dB. This is most effective for buildings near airports, factories, or busy freeways.
• Owner incentives: Like a protected membrane roof (PMR) assembly, a green roof protects the roof membrane from climate extremes and physical abuse, and increases roof longevity. Converting usable rooftop space to green roofs increases property values by creating an amenity for building occupants. In hospitals and extended care facilities, the ability to view and access a natural setting provides a therapeutic environment.

Green Roof Assembly

Traditional roofing assemblies consist of three components: the roof structural deck, insulation, and the membrane assembly. The roofing should protect the building and the insulation from moisture. In protected membrane roofs, extruded polystyrene insulation is placed on top of the waterproofing membrane for superior moisture resistance. In a conventional roof, the insulation is placed under the waterproofing membrane. Insulation placement relative to the membrane impacts the membrane's performance.

A green roof assembly is an extension of the PMR. This system protects the membrane from foot traffic, damage and degradation from freeze-thaw cycles, and ultraviolet radiation. It features the membrane adhered to the deck, covered by insulation and a ballast on top. The membrane's service life is extended because it is not exposed to the environment.

Green roofs must keep buildings watertight, and provide Environments conducive to ve getation survival. An eff ctive assembly Includes proven technology, such as a fluid-applied rubberized asphalt Membrane, which has been used for over 40 years. Membranes are Available containing 25 percent recycled materials, and are commonly Used in fountains, pools and applications with wet, submersed Conditions. This membrane system has several advantages: it is seamless And monolithic, bonded directly to the substrate, can be installed on Su bstrates with little or no slope, is easy to detail at penet rations and Terminations, and resistant to fertilizers and mild acids.

Design Issues

Choosing roof membranes is critical to the long-term success of a Watertight green roof. Roof membranes should be able to function in a Wet environment, have a successful performance history, and be In stalled by trained applicators. Other planning and design issues that Should be analyzed are as follows :

•Structural requirements: include reviewing the height, size, slope, and Maximum loadbearing capacity of the roof. The arch i tect should dete rmine The type of roof structure, number and position of drainage outlets And roof penet rations, roof access locations, transport and storage Availability of water and electricity, and loads imposed by green roof Assemblies when fully satura ted with wa ter and plants.

•Wind uplift:Green roofs must remain stable in high wind uplift conditions. Wind pressures can vary across a roof, depending on location. At The center of the roof, a thin growing media (soil) layer of 15 lbs. Per Square foot (psf) may be adequate. At perimeters and corners, high Winds may necessitate multiple rows of precast pavers to prevent uplift . Taller buildings have a greater risk of wind uplift. Ballasting requirements Vary by building height, parapet height, and wind design speed.

• Fire prevention: Green roofs that are regularly irrigated are Considered to be resistant to sparks and radiant heat. Roofs that are Not irrigated are considered fireresistant provided that the growing Media is two inches deep, with no more than twenty percent by weight Of organic matter; gravel or concrete bre a ks occur in the vegetation Eve ry 100 feet, at least four feet wide; vegetation - f ree zones occur at all Roof penet rations and at perimeter walls with openings; and safety Strips are ke pt free of flammable vegetation .

 

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Originally published in GreenSource Magazine.
Originally published in July 2007

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