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TPO Roofing Membranes
ASTM Specification Adds Market Credibility to New Roofing Material
by Randy Ober and Paul Oliveira
Thermoplastic polyolefin membrane is based on polymers defined by ASTM D 5538, Practice for Thermoplastic Elastomers — Terminology and Abbreviations, as a thermoplastic elastomer - olefinic. A thermoplastic elastomer is a diverse family of rubber-like materials that, unlike conventional thermoset rubbers, can be reprocessed and recycled like thermoplastic materials. Thermoplastic polyolefins are in the thermoplastic elastomer family and are commonly referred to as TPO in the single-ply roofing industry.
TPO-based products have been used in various applications, including the automobile industry since the 1980s, and have seen their usage continue to expand. In 1989, TPO-based membrane moved into the roofing industry as a non-reinforced sheet. In 1993, the original non-reinforced TPO membrane was replaced with membranes containing reinforcing fabric. Since that time, the TPO single-ply roofing market has grown to hundreds of millions of square feet annually with TPO membrane comprising the fastest growing segment of the U.S. single-ply roofing industry.
Currently, there are at least six manufacturers of TPO roofing membrane in the United States as well as several marketers. Until recently, there has been no ASTM standard established for TPO reinforced roofing membrane. The approval and release of an ASTM TPO standard has defined minimum physical properties of the membrane, providing needed standardization to the industry.
The task group challenged with the development of this ASTM specification, D 6878, Specification for Thermoplastic Polyolefin Based Sheet Roofing, is under the jurisdiction of Subcommittee D08.18 on Polymeric Materials, part of Committee D08 on Roofing, Waterproofing and Bituminous Materials. The task group is made up of individuals representing roofing material manufacturers, consultants, architects, specifiers, roofing contractors, governmental agencies and other organizations. The task group meetings for the development of the TPO standard have always been popular and well attended and many members travel to committee week specifically for this event. With all of the input from various interests that has been incorporated into this specification, D 6878 will serve the roofing industry well.
TPO Composition
TPO roofing membrane is typically based on polypropylene and EP (ethylene-propylene) rubber polymerized together using state-of-the-art polymer manufacturing technology. This technology enables the production of TPO membranes that are flexible at low temperatures without the use of polymeric or liquid plasticizers. Unlike some other popular thermoplastic roofing membranes, the TPO polymer does not contain chlorine and no chlorine-containing ingredients are added during sheet production. This lack of chlorine has allowed TPO marketers to tout their membrane as an environmentally safe, “green” product.
The TPO resin is compounded with other components including a weathering package, fire retardants and pigments for color to create a product that can withstand the elements associated with rooftop exposure. The membrane is comprised of TPO based top and bottom plies encapsulating a reinforcing fabric that enhances the physical properties of the sheet (see Figure 1). The combination of the fabric and TPO plies provide reinforced membranes with high breaking and tearing strength and puncture resistance. Sheet widths are available up to 3.66 m (12 ft) and membrane thickness is typically 1.1 or 1.5 mm (0.045 and 0.060 in.), however, membrane up to 2.0 mm (0.080 in.) is available.
Installation/System Types
TPO roofing membrane is typically installed using mechanical fasteners and plates placed along the edge of the sheet and fastened through the membrane and into the roof decking (see Figure 2). Adjoining sheets of TPO membrane are overlapped, covering the fasteners and plates, and joined together with a minimum 40 mm (1 1/2 in.) wide hot air weld.
EPDM Roofs
The EPDM (ethylene propylene diene terpolymer) rubber roofing membrane has been an appealing choice of the low-slope commercial roofing industry for over 40 years, with more than 500,000 warranted roof installations on 20 billion-plus square feet. Annually, EPDM accounts for over 1 billion square feet of new roof coverings in the United States, and is the most frequently used roofing material in the marketplace. Building Design and Construction magazine and the National Roofing Contractors Association (NRCA) confirmed in their 2002-2003 market surveys that EPDM continues to be the number one roofing choice of architects, roof consultants and contractors for both new construction and replacement roofing projects.
Superior Performance
The greatest test of any construction material is how it performs under actual field conditions. Forty years of empirical experience in field applications has shown EPDM to have the roofing industry's longest average service life. Characteristics that contribute to this superior overall system performance include:
Cyclical membrane fatigue resistance
Proven hail resistance
High resistance to ozone, weathering and abrasion
Flexibility in low temperatures
Superior resistance to extreme heat and fire
Thermal shock durability
Ultraviolet radiation resistance
EPDM's high resistance to wind damage has also proven to be an increasingly desirable attribute. These roof systems can be designed to meet a variety of wind uplift criteria from Factory Mutual, including 1-60, 1-90, and 1-120 ratings and greater, and the stringent code of Dade County, Florida.
Architects, roof consultants and contractors have come to depend on EPDM's time tested, long-term performance.
For more information about the attributes of EPDM, please click here to view/download a PDF version of the EPDM Attributes Brochure.
Please note that EPDM is a commercial roofing product, designed to be used in non-residential roof systems.
