Article reprinted from Western Roofing magazine July/August 2007.

 

Size Matters

When it Comes to TPO Membranes, Thicker is Better

by Randy Ober, thermoplastics product manager, Carlisle Syntec Inc.

 

(Editor’s Note: Randy Ober is the thermoplastics product manager for Carlisle SynTec. Ober’s responsibilities include marketing and product development for both the Sure-Weld TPO and Sure-Flex PVC product lines. Starting in 1983 as a Research and Development Engineer with Carlisle’s EPDM line, he assumed responsibility for TPO product development and code testing in 1995. Randy attended Penn State University and graduated with a degree in mechanical engineering. He currently participates in ASTM and serves as the secretary of subcommittee D08.18 (non-bituminous organic roof coverings) and acts as chairperson for several task groups. Ober may be reached at (800) 453-2554.)

 

 

You all know the old adage, “bigger is better.” When it comes to TPO roofing membrane, “thicker is better” as well. For the first ten years or so of its existence in the commercial roofing landscape, 45-mil TPO sheeting was king. This “good” membrane (in the scheme of “good,” “better,” “best”) comprised the majority of TPO roofing membrane sales. As the years progressed, more and more of the thicker TPO membranes (60-, 72- and 80-mil) have been specified and used on rooftop applications. This trend has been so strong that some TPO manufacturers have seen sales of the thicker membrane exceed that of the standard 45-mil product. This column will explain why this trend began, and why it will likely continue unabated for years to come.

                  With a little common sense, it is simple to rationalize that a thicker material will be more resistant to puncture than a thinner material. Sources of puncture on a roof can vary, but the most common are hail, abuse by other construction trades, and foreign objects being dropped by maintenance personnel. Over the years, TPO membrane has shown to be resistant to hail. Reports of TPO membrane surviving substantial hailstorms are common and simulated hail testing over a variety of substrates and temperatures shows excellent resistance to damage, even with ice balls up to 2 1/2” in diameter. The thicker the membrane, the more resistance is provided against hail damage.

                  Another primary explanation for the trend to thicker TPO membranes is the increased performance against abuse from other trades on new construction projects. Most of us have witnessed wannabe circus knife throwers practicing their craft by sticking roofing fasteners into the membrane, while others drag sheet metal across the membrane surface. We’ve even seen welders watch as molten metal falls from welding rods above the membrane, thinking somehow that it will cool sufficiently as to not affect the waterproofing ability of the roofing membrane. Using a thicker membrane will not resist all these types of actions but some of the damage can be minimized and the callbacks to roofing applicators for repairs can be reduced as well. Laboratory testing shows a 50% increase in puncture resistance for an 80-mil thick membrane versus a membrane with a thickness of 45 mils.

                  In addition to providing protection from puncture, thicker membranes are also more scuff resistant. With some early generation TPO membranes, excessive foot traffic could cause areas of the top ply to be damaged, which exposed the scrim. Thicker membranes possess a thicker top ply over scrim and will resist this sort of damage. Measuring the top-ply thickness over scrim using a microscope shows that 80-mil thick membrane provides an increase of over 100% versus the 45-mil membrane.

                  Just like the properties discussed earlier, thicker TPO membranes provide additional resistance to ultraviolet light (UV) exposure. TPO membrane is enhanced with a weathering package that protects the integrity of the sheet and prolongs its service life. As the sun beats down onto a rooftop that is waterproofed with TPO membrane, the weathering package contained in the formulation will protect the surface of the membrane from degradation due to exposure to UV rays. As the weathering package on the surface of the membrane is depleted, the weathering package within the remainder of the membrane will continue to migrate upward. The thicker the membrane, the more weathering package is present and the longer the surface will retain its smooth, glossy appearance. The scientific types like to refer to this phenomenon as the “reservoir effect.”

                  To combat the negative effect of the UV rays associated with exposure to the sun, particularly for buildings in hot, southern climates, TPO manufacturers offer a variety of membrane thicknesses ranging from 45 mils up to 80 mils. Because it is thicker, the 80-mil membrane inherently features more weathering package than 45-mil membranes, thus providing a potentially longer service life.

                  The reservoir effect on a thermoplastic membrane can be compared to sunscreen, which features SPF values ranging from 2 to 50. Sunscreen with an SPF value of 2 is a good sunscreen, but it will not protect your skin for as long as a sunscreen with an SPF value of 50. This is especially important in climates with high levels of radiation from the sun (i.e. while a SPF value of 15 may be fine in Bangor, Maine, that same level of protection may not be adequate in Phoenix, Arizona).

                  Therefore, a thicker TPO membrane will offer longer protection against the dangers of solar UV rays. This is not to say that thinner TPO membranes are not a solid investment, just that their weathering package is likely to dissipate more quickly than a thicker membrane if they are exposed to excessive solar UV rays.

                  To further combat the effects of solar UV rays, the primary color of TPO utilized in the roofing market is a highly reflective white. Some manufacturers have also begun to use special pigments in their membranes that provide even higher reflective characteristics. These pigments can significantly increase the reflectivity of the membrane while lowering the surface temperature. Lower surface temperatures not only extend the life of the membrane, but can also reduce a building’s energy demands and decrease the Urban Heat Island Effect. White and some tan TPO membranes also have Energy Star® approval and comply with energy-efficient building codes such as California’s Title 24.

                  To move to a thicker TPO membrane, the only up-charge is the material cost, since labor costs associated with installation are essentially unchanged. Thicker membrane can be welded without changing the welder speed or temperature settings. Detail flashings do not change based on the membrane thickness. The only real difference in application with a thicker membrane is when the membrane is turned up a vertical wall or over the edge of the roof. Due to the increased stiffness of the thicker membrane, slightly more effort will be required to complete these tasks. The stiffer membrane, although a negative at angle changes, does provide a flatter finished appearance on the roof.

                  With the overall installed cost of the roof system only minimally affected by an increase in membrane thickness and the plethora of advantages brought to the table, it makes perfect sense to specify a thicker TPO roofing membrane for your next project. Remember, “Bigger is Better,” I mean, “Thicker is Better.” •••