Thermoplastic Polyolefin (TPO) Roofing Systems
01/08/2007 | BY RICHARD L. FRICKLAS
TPO-based products have been available since the 1980s; the first sheets for membrane roofing were introduced in 1989. By 1993, the earlier non-reinforced membranes were replaced by fabric-reinforced sheets, the version available today. To understand why TPO membranes are the fastest-growing segment of the membrane market, a little background is in order.
Elastomers and Thermoplastics
Low-slope commercial roof systems were dominated by bituminous multiple-ply systems from the 1800s through the 1990s. For a variety of reasons, including economic, social, and performance considerations, the conventional built-up roof system represents less than 30 percent of the roofs now being installed. Certainly, the cost of petroleum and the difficulty of finding skilled laborers willing to work with hot asphalt are major factors.
Polymer-modified bituminous roofs have the advantage of using both less material and labor, and, in many regards, are tougher and more flexible than the asphalt and pitch roofs that preceded them.
Elastomeric sheets, particularly EPDM, use just a single ply of membrane. Advances in production technology, combined with the use of ballast for wind resistance (rather than adhesives) and the ability to use the thermally efficient polystyrene foam insulation have proven to be extremely cost effective. In addition, both the ballast and styrene foam may be recycled.
EPDM roofs become less competitive when they must be adhered or mechanically fastened, but not all buildings can handle the weight of the ballast. In addition, EPDM is most durable when compounded with carbon black for ultraviolet (UV) resistance, and this is not a particularly attractive color for those roofs that can be seen from the ground. Also, EPDM roofs are vulcanized, and seams must be carefully fabricated with tapes or adhesives in order to resist weather attack.
Weldable Thermoplastic Roof Membranes
Once the seams on a weldable thermoplastic roof membrane are properly welded together, they will be stronger than the membrane itself and unlikely to give problems in the future. However, it is important that the membrane retain its ability to be welded later on, when patches or new penetrations will affect the roof system.
Sheets of polyvinyl chloride (PVC) have the advantages of being easy to process, used in single-ply membranes, and are available in lighter colors (usually white, off-white, or beige). This is a strong advantage in hot climates where ENERGY STAR® or urban heat island considerations must be met.
Many early PVC sheets were non-reinforced, but as with the TPOs mentioned earlier, all are now reinforced for dimensional stability and toughness.
In addition to PVC, other polymerics used in single-ply roofing have included chlorinated polyethylene (CPE), polyvinylidene chloride, chlorosulfonated Polyethylene (CSPE), ketone-ethylene ester (KEE), and various copolymer blends.
Cost considerations, ease of fabrication, and potential for improved performance come to mind. As compared to vulcanized EPDM rubber, TPOs can be welded and reprocessed like other thermoplastics. Yet by containing EP rubber within the TPO, low-temperature flexibility is very good. By not needing plasticizers (required by PVC and CPE), shrinkage and aging through plasticizer loss is less of a factor. The absence of chlorine in the polymer chain suggests that these membranes may be “greener” than those that contain chlorine.
ASTM Specification D6878-Specification for Thermoplastic Polyolefin Based Sheet Roofing has finally been published after years of trying. It is notable that ASTM recognized Paul Oliveira of Firestone with a Distinguished Leadership Award in December 2006 for his patience and guidance in resolving the many issues related to introducing this new roof membrane system specification.
Some of the TPO manufacturing facilities are brand new and are capable of producing sheet with widths up to 12 feet (this reduces the number of seams that must be field welded).
Common thicknesses for TPO are 45 and 60 mils, but 80 mils is also available when increased impact and puncture resistance is needed. As compared to PVC, the increased thickness does not necessarily relate to durability, whereas plasticizer loss in PVC is a surface-related phenomenon, and thicker is better. What is more important with TPO is the thickness of polymer film on top of the scrim. If the sheet has “hills and valleys” due to the texture of the reinforcement, abrasion (such as from rooftop traffic) will expose the fabric to the weather, resulting in water wicking into the fabric and possible delamination of the sheet.
Installation Options with TPO
Most frequently, TPO systems are installed using fasteners installed in the seam area. Ballast defeats the purpose of white sheets, and full adhesion is slower and may be restricted to fewer substrate options. However, a very recent alternative option may be to use self-adhering sheets. These may be more weather sensitive than hot-air welding and mechanical attachment, and some details for selvage edges, T-joints, and the like may still have to be worked out. Detailed information on TPO may be found at the ASTM website.
As with any new roofing system, there are some cautionary remarks that should be mentioned. Richard Baxter, NRCA director and roof consultant, touched on several in his January 2002 article featured in Professional Roofing magazine. Because the consumer cannot perceive product differentiation among several TPOs in the marketplace, Baxter warns that the only variable will be price, and this does not bode well for quality products. There is also limited performance history, especially with the newest peel-and-stick systems.
Mark Graham, technical director at NRCA, commented on the pros and cons of the TPO Standard in Professional Roofing magazine: “The development and publication of ASTM D6878 are steps forward in improving the market credibility of TPO single-ply membrane roof systems. At the same time, however, understand some requirements contained in ASTM D6878 are relatively lax.
“Comparison of the physical property requirements in ASTM D6878 to the results of NRCA’s 2001 testing of TPO single-ply membranes reveals persuasive data for increasing the standard’s minimum overall sheet thickness and thickness-over-scrim requirements and decreasing the maximum linear dimensional change value. NRCA’s testing also provides a strong argument for increasing the standard’s minimum breaking-strength, tearing-strength, and brittleness-point values.”
(For additional information regarding NRCA’s 2001 testing of TPOs, see the “Testing the Differences” article from the November 2001 issue of Professional Roofing magazine.)
An excellent reference on in-service TPO performance was published in ASTM Special Technical Publication 1349, dated 1999, available at (www.astm.org). Field observations included:
- Hot-air weld seams are easier and cleaner than adhesive-based seams.
- Material is not as heavy and is easier to handle than multi-ply membranes.
- Lower cost than some other hot-air welded membranes.
- Mechanically fastened systems work well in re-cover applications.
- Non-reinforced membrane is easy to form for detailing.
- Stiff membrane that does not relax well.
- Noticeable changes in color and texture of membrane over time.
- Cold welds frequently occur; the start and stop positions of the robotic welder are especially critical, as well as T-seams.
- Narrow welding window between cold welds and scorch/burn-through.
- Failure of bonding adhesive leading to poor adhesion to the membrane.
- Membrane, at times, requires solvent wipe before welding.
- Problems with re-welding membrane after exposure to sun to make repairs.
- Black membranes are more difficult to weld than white membranes.
- Black membranes are hot to the touch after exposure to the sun.
- Membrane caulk can be runny on a hot day.
- Cigarettes burn holes in membrane more easily than for other thermoplastic membranes.
According to ASTM: “Some of these items may be attributed to the natural learning curve associated with new products; however, for some applicators, ease and satisfaction with TPO membranes has yet to reach a comfort level. Some consider TPO membranes as not being ‘contractor-friendly.’”
The roofing industry has made a huge commitment to TPO roofing systems with new TPO production facilities. Their growth has been at the expense of Hypalon®(CSPE) and chlorinated polyethylene. Meanwhile, the highly successful PVC and copolymer roofing systems have proven their durability both in the United States and Canada, and satisfied customers would do well to go slow with the newer kid on the block.