Photo courtesy of GAF Materials Corp., Wayne, N.J.
Photo courtesy of Johns Manville, Denver
Photo courtesy of Johns Manville, Denver
An oft-quoted definition from a sustainable low-slope roofing workshop held by Oak Ridge National Laboratory (ORNL), Oak Ridge, Tenn., describes a sustainable roof system as one that is "designed, constructed, maintained, rehabilitated and demolished throughout its life cycle with an emphasis on using natural resources efficiently and preserving the global environment." Quality asphalt low-slope roof systems, including built-up and polymer-modified bitumen roof systems, perform well with regard to these sustainability aspects.
Various factors related to built-up and polymer-modified bitumen roofing contribute to sustainability. The Asphalt Roofing Manufacturers Association (ARMA) has formed a sustainability council to examine these factors in greater detail. But much work remains to fully understand and quantify this complex subject.
A valuable byproduct
The asphalt used in roofing products is a byproduct of petroleum refining. This byproduct, known as asphalt flux, is the residue remaining after high-value fuels such as gasoline, kerosene, aviation fuels and furnace oils have been released from crude oil through atmospheric and vacuum distillation or fractionation. Asphalt flux is further processed into materials for the manufacture of asphalt shingles, mopping asphalt, asphalt-saturated roll roofing and polymer-modified bitumen membranes.
From an environmental perspective, using asphalt flux to manufacture built-up and polymer-modified bitumen roofing products is beneficial because this material might otherwise become waste material or additional energy would need to be expended to refine the material into usable fuel products.
Environmental benefits
The U.S. Environmental Protection Agency, as well as regional and state environmental agencies, have prioritized construction material recycling programs and committed resources to investigate and implement such programs. The U.S. Green Building Council's Leadership in Energy and Environmental Design® (LEED) Green Building Rating System™ also encourages use of recycled building products.
This can be beneficial for building owners with APP or SBS polymer-modified bitumen membrane roof systems, which contain recycled content that may contribute to LEED credits. However, the percentage of recycled content varies by product, so it is necessary to contact product manufacturers for specific details.
According to Shaik Mohseen, vice president of technology and innovation for Tarco, Little Rock, Ark., the key raw material to make dry felt is old corrugated cardboard or recycled waste paper. Dry felt makes up about 50 percent of the raw material required for asphalt-saturated organic felt, another reason asphalt-based roof systems can be considered sustainable.
In addition, asphalt recycling has become a topic of great interest during the past decade, and there are various initiatives being developed throughout North America to recycle asphalt roofing products.
ARMA and other industrial and manufacturing organizations, such as the Construction Materials Recycling Association, National Asphalt Pavement Association, Asphalt Emulsions Manufacturers Association, and Asphalt Recycling and Reclaiming Association, aggressively promote waste-reduction and recycling programs.
"The main use for recycled asphalt currently is for pavement," says Reed Hitchcock, ARMA's executive vice president. "In many locales and municipalities, asphalt recycling operations are proving economically feasible and profitable businesses are springing up. The key to recycling asphalt is to keep it local."
Asphalt recycling economics are dictated by local disposal costs, location of recycling centers and demand for paving feedstocks. Recycling feasibility depends on regional tipping fees; transaction costs; transportation drop-off synergies; economies of scale; raw asphalt pricing; and efficiencies in producing clean, consistent feedstock.
As local landfill operations become more interested in recycling, new uses for recycled asphalt are being studied. In Minnesota, for example, recycled asphalt shingles have been used in recreational trail surfaces with good results.
Additionally, asphalt can be used in vegetative roof systems. A vegetative roof system installed on the headquarters of the American Society of Landscape Architects (ASLA) in Washington, D.C., is described in Green Roofs by Christian Werthmann, assistant professor in the landscape architecture department at the Graduate School of Design at Harvard University.
Werthmann writes: "The former ASLA roof consisted of a metal deck overlaid with an insulation layer and covered by a PVC waterproofing membrane. For the retrofit, the design team decided to build up the roof differently. The waterproofing membrane and insulation were stripped off the existing metal decking. The steel deck was then overlaid with a dense decking consisting of gypsum boards. The dense decking was then coated with a new waterproof membrane composed of hot rubberized asphalt with fabric reinforcement. The hot fluid (up to 375 degrees Fahrenheit) was applied in two coats, amounting to a total thickness of about a quarter inch. The new membrane adheres directly to the dense decking and forms a monolithic layer without any seams."
Additionally, The NRCA Green Roof Systems Manual—2007 Edition states hot-applied asphalt and polymer-modified bitumen membranes can be used with vegetative roof systems.
Mineral aggregate surfaces
An important feature of built-up roof (BUR) systems that also contributes to sustainability is the use of mineral aggregates to protect the flood coat and layers of asphalt and ply sheets from physical damage and shield the asphalt from ultraviolet radiation. Aggregate generally is available locally. Therefore, it typically can be mined, processed and distributed without expensive transportation costs.
Aggregate also is versatile. It is available in various colors and types and can be coated to suit a building's aesthetics or increase surface reflectance.
According to Pacific Gas and Electric Co., San Francisco, BUR reflectance can be improved from 0.30 to 0.50 with white gravel and from 0.50 to 0.70 with gravel and a cementitious coating. These values can be compared with 0.75 to 0.85 for a smooth-surfaced BUR with a white coating.
Mineral aggregate is quite stable when applied after a flood coat, and, though it can scour after years of use, it can be replenished. A regular preventive maintenance program is key to maximizing the life of a BUR system with a mineral aggregate surfacing.
Although not all mineral aggregate is appropriate for use, suitable materials are widely available. Roof aggregate performs best when it conforms to ASTM D1863, "Standard Specification for Mineral Aggregate Used on Built-Up Roofs."
Notably, the use of aggregate on roof systems in high-wind zones has been reviewed during model and state code development during recent years. In Florida's high-wind regions, 50 percent of the aggregate is required to be embedded into the bitumen flood coat.
Industry stakeholders, including ARMA, continue to evaluate high winds' effects on aggregate.
Labor considerations
Labor is intensive when constructing BUR systems because of the multiple layers and the special skills required to properly install them.
However, labor requirements may be reduced by using polymer-modified bitumen membranes, mineral-impregnated cap sheets, cold-applied asphalt or self-adhering asphaltic membranes. Depending on a building's size and location and labor considerations, these options may be more practical than a hot-applied BUR system.
There are many choices available to find the right balance between labor and material costs when installing an asphalt roof system. Options include hot-applied BUR, cold-applied BUR, cold-applied polymer-modified bitumen membranes or self-adhering polymer-modified bitumen membranes. Different specifications with different labor requirements are especially evident for the choice of the top layer, such as a reflective cap sheet or flood coat, and this choice affects a building's final appearance.
Different U.S. regions or varying building locations may favor one system over another because of labor and safety considerations.
Insulation and coatings
Many specifiers think of a sustainable roof as a "cool roof," or a light-colored or white reflective roof. Reflective roofs can offer energy savings; however, in some cases, similar energy savings for a building can be achieved through the proper use of insulation.
Insulation and reflectance contribute to energy efficiency and, fortunately, are not mutually exclusive. White coatings and reflective roofs have been praised as an effective means for reducing buildings' heat loads. Insulation does not reduce heat load, but it can prevent heat from escaping a building during winter. In many regions, both are important. Achieving the correct balance is the challenge when it comes to energy efficiency and sustainability.
The use of insulation and reflective surfaces also can extend an asphalt roof system's service life by protecting the asphalt membrane from temperature extremes.
ARMA recently formed a Cool Roof Steering Group that views roof surface reflectance as one component of a whole building envelope approach to regulatory policy and design principles for energy conservation, mitigation of urban heat islands and improved air quality.
ARMA believes the whole building envelope approach is the best option for designing and optimizing performance-based, cost-effective buildings. It allows flexibility and consumer choice in the selection of roof systems.
To that end, ARMA members have invested in technology and equipment to factory produce white cap sheets and provide durable white coatings for polymer-modified bitumen membranes in the field. As an additional benefit, capping a BUR or polymer-modified bitumen roof system with a white surface and properly maintaining the roof system can extend the roof system's service life.
The Roof Coatings Manufacturers Association (RCMA) and ORNL have demonstrated that aluminum and elastomeric coatings increase softening points and reduce penetration values of four-ply BUR systems, decreasing the aging of asphalt.
RCMA has formed a committee to promote the use of coatings to extend the lives of roof systems, including asphalt roof systems. For more information about selecting coatings for use on roof systems, see "Selecting correct coatings—parts 1 and 2," November 2007 and December 2007 issues, pages 32 and 38, respectively.
Redundancy
The ability to withstand severe weather events is vital to durability. One of the main advantages of asphalt multiple-ply roof systems in such conditions is their redundancy. Moreover, because of their multiple plies, such asphalt roof systems tend to be more forgiving of installation shortcomings.
Reports from heavily storm-damaged areas support the claim that properly designed and installed BUR systems perform well in extreme weather.
For example, the case studies found in the Roofing Industry Committee on Weather Issues Inc.'s Hurricanes Charley and Ivan Wind Investigation Report, March 2006, include examples of BUR systems withstanding severe weather. Wind speeds from Hurricane Charley, in particular, were in the 120- to 150-mph range, yet, in many cases, affected BUR systems sustained only minor damage.
Proving themselves
Interest in sustainable buildings continues to grow, as does the need for long-lasting, high-quality, sustainable roof systems. BUR and polymer-modified bitumen roof systems can offer sustainable options.
With a few innovations in structural design, as well as advances in materials, these basic roof system types are proving their merits.
James Baker is director of communications and industry affairs for the Asphalt Roofing Manufacturers Association.
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