Testing the Limits
Long-Term TPO Exposure Performance Concerns
by Ken Klein, WSRCA Technical Advisor
(Editor’s Note: Ken Klein leads the West Coast Building Technology division of Simpson Gumpertz and Heger Inc. He is a graduate of UC Berkeley’s Civil Engineering program and is a licensed Professional Engineer in California and several western states. Klein has been with SGH for over 35 years and has gained experience in the design and investigation of roofing and waterproofing of commercial, institutional, and residential buildings. Well known in the industry, he has frequently presented to groups of attorneys, contractors, architects, and other design professionals regarding building-envelope design and remediation. Klein is also a Technical Advisor for the Western States Roofing Contractor Association.)
Western States Roofing Contractors Association (WSRCA) members have reported performance issues such as crazing and cracking in aged Thermoplastic Polyolefin (TPO) membranes. To investigate, a field study and a laboratory analysis were conducted on two roofs in Phoenix, Arizona, supplemented by insights from the WSRCA 2011 TPO Weathering Study.
The WSRCA 2011 TPO Weathering Study examined 60-mil TPO membranes over an 11-year period. This study found surface cracking primarily near field seams at two locations, San Antonio, Texas, after seven years and Las Vegas, Nevada, after ten years. The hypothesis suggested that inherent membrane stiffness contributed to the cracking. However, this study lacked laboratory tests to assess changes in membrane properties over time.
The WSRCA engaged a roofing consultant in 2024 to conduct inspections and collected samples from two buildings in the Phoenix area: four samples from a warehouse building that had a 45-mil TPO roof installed over a built-up roof membrane in 2011 with splitting and multiple leaks observed within ten inches of field seams and five samples from a commercial building that had a 60-mil TPO roof installed over polyisocyanurate insulation in 2010, that had concentrated splitting in one quarter of the roof primarily within 12 inches of seams up to 30 inches.
Laboratory testing was done on exposed samples and unexposed samples, with unexposed samples being TPO membrane taken from the seam overlap. With a visual inspection, crazing was detected in two exposed samples with the naked eye and in all nine exposed samples under 7x magnification. The most severe cases showed cracks extending to the scrim, leading to grooves on the underside of the membrane.
The samples were then put through physical testing, with emphasis on peak loading, elongation, and thickness loss. Exposed samples had a 22-26% increase in peak loading, indicating embrittlement. Exposed membranes had a 91% reduction in elongation (~1.55 inches vs. ~17 inches for unexposed samples), leading to brittle failure. Exposed membranes showed a 20-26% reduction in thickness over the scrim as compared to the unexposed membrane.
The field observations and laboratory indicated several areas of concern. Exposure to UV and weathering causes TPO membranes to experience thickness loss and to become brittle which reduces weatherability characteristics and elongation capacity. Brittle failure mechanisms were observed in exposed samples, whereas unexposed samples exhibited ductile failure characteristics. The laboratory testing revealed a different failure mechanism between the exposed and unexposed TPO membrane samples. TPO membrane deterioration was found to align with previous WSRCA findings, with creased areas showing accelerated surface cracking. The 45-mil TPO membrane exceeded its expected ten-year service life, and its deterioration was not unexpected, but the 60-mil membrane’s premature failure raises concerns.
WSRCA advises members to consider the following performance concerns with some TPO membranes: recognize that 45-mil TPO membranes have a nominal ten-year service life; minimize creasing of TPO membranes during installation to reduce cracking risks; be aware that, over time, exposed TPO membranes become more brittle, leading to decreased elongation and increased peak load; understand that, over time, exposed TPO membranes experience thickness reduction over the scrim; and monitor for surface crazing as an indicator of membrane deterioration.
This study highlights performance variability in TPO membranes and underscores the need for further research to assess how different TPO products age over time. WSRCA members should be aware of some long-term performance limitations with certain TPO membranes exposed in high UV exposure climate zones. You can read the complete technical bulletin on the Western States Roofing Contractors Website here.
