A close look at TPO
Cost, heat-welded seams and energy efficiency are among the factors driving interest in TPO membranes
THERMOPLASTICS REPRESENT a large and growing piece of the commercial roofing market. In fact, thermoplastics now comprise 21 percent of the commercial roofing market.
The thermoplastic market is composed mostly of polyvinyl chlorides (PVC), vinyl roofing and thermoplastic polyolefins (TPO). Having been on the market for more than 30 years, PVC roofing systems are familiar to many facility executives. By contrast, TPO, which has been around less than a decade in roofing applications, is unfamiliar to many facility executives.
Not that TPO is new. TPO has been used in underground cabling and other below grade waterproofing applications for more than 20 years. Early formulations, produced in black, also appeared on several test roofs where they stood up to weather conditions but had problems because the lap seams used in those tests tended to develop leaks.
TPO roofing membranes have come a long way from those early days. Today, TPO roofing systems combine attributes of two established options in the single-ply marketplace: EPDM and PVC.
According to the Single-Ply Roofing Institute, TPOs offer weathering resistance, flexibility, tear resistance, puncture resistance, chemical resistance and heat-seaming capability.
As a thermoplastic product, TPO shares similarities with PVC and vinyl roofing. All are produced in white and light colors, offering rooftop reflectivity to reduce air conditioning loads. And like their thermoplastic predecessors, today’s TPOs feature heat-welded seams.
Thermoplastic roofing membrane manufacturers say that heat-welded seams are one reason that thermoplastic systems are popular. “Heat-welded seams mean fewer errors are made on the roof both in the short term and the long term,” says Steve Ruth, director of sales for Duro-Last.
TPO membranes are also flexible. As a result, they can be attached to almost any type of roofing design, from relatively flat or low-sloped roofs to those with steeper slopes. Domes, sawtooth roofing and barrel roofs also can have TPO membranes installed. What’s more, TPO membranes are flexible enough to accommodate the normal structural movement of a building without splitting or cracking.
A fabric reinforcing layer – known as a scrim – improves strength, puncture resistance and durability. TPO has no plasticizers, which can break down, causing the roof to become brittle or shrink. If minor repairs are needed, they can be accomplished easily and economically.
Like all single plies, TPO membranes are light weight. They can be installed over most types of existing roof – if permitted by code – reducing costs in reroofing. Those features have fueled interest in TPO roofing and helped drive up sales of the membrane. “Even though commercial III, roofing is down somewhat this year, thermoplastic membranes continue to show positive numbers, and that’s been driven by sales of TPO products,” says Tom Gallivan, marketing manager for Stevens Roofing Systems.
The commercial roofing market is embracing TPOs for several reasons. One big motivator is cost. When it comes to price, TPOs compare favorably with EPDM membranes. “On the membrane material, EPDM is lower priced in general than TPO,” says Dave Harrison, senior vice president of marketing and technical services for GAF. “But the second part of the roofing equation is labor. TPO installs faster and with less labor than EPDM does.”
“Price is driving the polyolefins,” says Jerry Beall, FiberTite sales and technical manager for Seaman Corporation. “They are gobbling up market share primarily because of pricing wars between manufacturers. History repeats itself in our business. The EPDMs went through this same scenario in the 1970s. They beat each other up until just a few companies were left.”
Improved membrane formulations are another factor in TPO’s popularity. “Initially, TPOs had a few growing pains because of some early formulation issues,” admits Joe Stassi, market manager for single ply for Johns Manville. “But the TPO formulations today are very sound.”
Often installed in reflective white, TPO membranes can save energy. Other colors, of course, are available from manufacturers. For example, the recently completed Air Canada Center in downtown Toronto has two giant red maple leaves on a gray TPO membrane that can be seen from neighboring buildings and airplanes using the nearby Toronto Island Airport.
“The idea was to create the logos on the roof using custom– colored TPO roofing membrane,” explains Bruce Merstoft of Bothwell-Accurate Co. Ltd. “Once the idea was approved, Air Canada gave us the exact color red they wanted, and we ordered specially manufactured sheets.”
Installation versatility is another benefit. The membranes may be installed ballasted, mechanically attached or fully adhered. TPO membranes can be incinerated, meet landfill requirements and can even be recycled.
Not All TPOs Created Equal
Many manufacturers use the same supplier for the polyolefin used in producing TPOs. But the way they assemble the end membrane’s bottom scrim layer and top layer varies. In addition, there are different formulations for making the membrane fire retardant and stable to ultraviolet light.
Typically, TPOs are formulated using either a Union Carbide/Dow supplied polyethylene base polymer or a Basell (formerly Montell) polypropylene base polymer.
Generally, polypropylene-based TPOs are stiffer than their polyethylene counterparts, which means the polyethylene sheets are somewhat easier to lay and provide better conformance to rooftop variations. They also may be better suited to applications in colder temperatures than polypropylene-based membranes.
Conversely, polypropylene-based membranes, though stiffer, may have better mechanical properties, particularly in higher-temperature areas.
Both TPO types are manufactured with a reinforcing scrim encapsulated between two layers of compounded membrane. This multi-layer construction provides TPOs with strength, puncture resistance and durability.
Another difference in composition among TPOs is the method used to protect against fires, says Jim Burkett, national product and marketing manager for GenFlex. “Some are made with halogenated fire retardants, and some use nonhalogenated fire retardants, for example.”
Some studies indicate that halogenated fire retardants may reduce the membrane’s ability to stand up to the sun’s damaging ultraviolet rays over long periods of time. Essentially, halogenated fire-retardant additives such as bromine compounds tend to chemically attack the UV stabilizers.
In practical terms, that means the roofing membrane can begin cracking and degrading after just a few years of exposure. In addition, halogenated TPOs are not as environmentally friendly as their nonhalogenated counterparts.
Currently, the American Society for Testing and Materials (ASTM) is trying to develop a TPO standard. ASTM D 08.18 “Standard Specification for Thermoplastic Polyolefin Based Sheet Roofing” is under debate because, among other elements, it would allow for sheet thicknesses of 39 mils with a minimum coating over the scrim of 9 mils.
Some industry sources say that 39 mils is too thin and will result in disappointing performance in both application and roof longevity. “It’s important to make sure the top layer of a TPO is as thick as possible,” suggests Ron Head, marketing manager of thermoplastic/FleeceBACK systems for Carlisle SynTec Incorporated. “That thick upper layer is where maintenance and servicing people will be walking. And it takes the abuses of weather.”
Membrane thickness also is an issue for Sarnafil, which offers a polyolefin membrane in Europe, but prefers to provide PVC membranes in the United States.
“European flexible polyolefin membranes, called FPOs, are different than American TPOs in formulation, amount of polymer used, construction and reinforcement,” maintains Brian Whelan, vice president of sales and marketing for Sarnafil. “The average thickness of FPO in Europe is about 60 mils, and no product sold there is less than 48 mils. TPO membranes in the U.S. have 31 to 45 mils of polymer. In Europe, FPOs are usually more expensive than PVC sheets of the same thickness. But in the U.S., TPOs are sold at a lower price.”
Many TPO membranes are available in 6-foot wide rolls.
Now, some manufacturers are offering 8-foot, 10-foot and even 12-foot widths, which reduce costs because installation is faster and less labor is used in the process.
Stassi with Johns Manville agrees that the wider sheets offer labor and installation economies but worries that, even though TPOs are reinforced, the additional expanse on mechanically attached roofs could put too much stress on fasteners, particularly in high wind conditions.
Some manufacturers also are offering fleece-backed TPO membranes that can be used in adhered applications. Many adhesives do not bond effectively to TPO sheets. Even when they do bond, the adhesives may not be fully set when the effects of expansion and contraction occur. But with fleece-backed TPOs, the fabric or other backer interfaces between the TPO sheet and the adhesive layer, allowing long-term adhesion of the roof membrane.
Another variation among TPOs is how they are heat welded. The National Research Council of Canada recently tested the wind fatigue on single-welded and double-welded mechanically attached TPO systems on its Dynamic Roofing Facility. The one-side weld system passed 60 psf, while the double-side passed 90 psf.
The double-side weld reduced both the wind’s effect on each seamed area and the membrane’s tendency to tear at the fastener.
Good Business Caveats
Before specifying a TPO for the rooftop, facility executives need to understand what is being purchased. For example, does the manufacturer produce its own products? How many square feet or jobs has the contractor installed? Are there any roofs in the area that can be examined? Does the membrane meet FM and UL requirements and local building requirements?
“The contractor should have experience installing that particular manufacturer’s TPO,” suggests Gallivan of Stevens Roofing Systems.
“Make sure the company you choose has experience in the single-ply market, a history with TPO roofing systems, and invests time and money into research and development,” recommends Paul Oliveira, thermoplastic product manager for Firestone. “You want to make sure you’re getting a reasonable cost, but also a quality roofing system.”
Facility executives concerned with energy consumption and reflectivity can check to see which TPO membranes carry ENERGY STAR listings. “To be recognized by the Environmental Protection Agency’s ENERGY STAR program, you must not only start with a white membrane, but you must maintain a certain degree of reflectivity over time,” explains Head of Carlisle SynTec Incorporated.
As in any capital investment, repair and preventive maintenance are crucial to the roof’s success.
“When you buy a new car and receive its warranty, you know that it requires you to change the oil and do other routine maintenance,” explains Harrison of GAF. “TPO roofs are the same. You have to check the seams regularly and do certain things once or twice a year to get optimum performance from your roofing system.”
Rita Tatum has covered facility management and technology issues for more than 25 years.
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Copyright Trade Press Publishing Company Jul 2001
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