Published in Buildings (February 2005)
Why Single-Ply?
A closer look at non-asphalt-based single-ply roof systems reveals the benefits, costs, and application methods.

By Curtis L. Liscum

 

If you’ve sipped the alphabet soup of single-ply roofing systems, then you probably have a taste for their characteristics and advantages. However, if you have yet to experience membranes such as TPO, PVC, EPDM, or Hypalon, get ready – Buildings is dishing out everything from the basics and beyond to help you better understand these non-asphalt-based systems.

Single-ply roofing membranes have grown in popularity over the past 30 years, and due to their flexibility, relative ease of installation, and competitive price, it’s no wonder that building owners and facilities professionals are interested. According to the Waltham, MA-based Single Ply Roofing Industry (SPRI), a commercial roofing industry association, one of the benefits to single-ply membranes is their production; they are “manufactured in a factory to strict quality control requirements that minimize the risk inherent in built-up systems.” This consistency in the quality of the products is undeniably one of its selling points.

Careful consideration should be taken so that the proper number of fasteners is used in mechanically attached single-ply roofing applications. An insufficient number of fasteners could result in blow-off.

But owners aren’t the only ones hailing the praises of these membranes. “I think most roofing contractors [and] roofers would generally find single-ply roofing easier to install, neater, and cleaner to work with. For example, with a single-ply roof, there’s not a need for one of these big kettles to heat up bitumen, so it’s cleaner,” says Vincent McPartland, vice president, sales and marketing, Stevens Roofing Systems, Holyoke, MA. Installation of a single-ply membrane is often faster as well.

At the same time that single-ply membranes were gaining market share in the United States, concerns for the environment and an emphasis on energy conservation became paramount. Single-ply membranes offered a solution – a very cool one. Light-colored, reflective roof surfaces could offer building occupants a significant reduction in energy use, points toward a U.S. Green Building Council LEED™ building certification, as well as the potential to downsize HVAC equipment. Additionally, single-ply membranes offer a lightweight solution that can accommodate the sometimes unconventional roof designs of modern architecture.

After spelling out so many wonderful attributes, you may be wondering what you’re giving up with a single-ply membrane. The answer is in the question. Because these systems are only one ply (as their name suggests), you’re sacrificing the security of a redundant system. As with any installed roof, caution should be taken to avoid punctures and damage. “That’s probably the biggest drawback in some people’s eyes,” says Tim McFarland, technical director, Mule-Hide Products Co. Inc., Beloit, WI.

Thermosets

Single-ply membranes can be generally categorized into two groups – thermosets and thermoplastics. “Thermosets, or those materials that you can’t hot-air weld because it will change their physical characteristics, use a tape or a contact cement to make up the lap,” explains Curt Liscum, senior consultant, Benchmark Inc., Cedar Rapids, IA. According to SPRI, “Thermoset membranes are compounded from rubber polymers. The most commonly used polymer is EPDM [ethylene propylene diene monomer].”

EPDM membranes are known for their long-term weathering ability, and perform well even when subjected to fluctuations in temperature, ultraviolet rays, and ozone. The membranes are available in both unreinforced and reinforced, the latter of which increases resistance to tearing. EPDM is available in thicknesses ranging from 30 mils to 90 mils, and is offered in wider sheets that may be desirable. “EPDM may be a good choice because you can get it in sheets up to 50 [feet] by 100 [feet]; for very large roof areas, it’s an ideal product to minimize the seams,” says McFarland. While it’s true that wider sheets can reduce the number of seams on a roof, there has been some debate about whether long-term wind resistance could be compromised in mechanically fastened applications where wider sheets are used.

Although most people think of thermoplastic membranes when a cool roof is mentioned, EPDM membranes can be energy-savers, too. By applying white, acrylic coatings, the transformation of an EPDM roof is like night and day. According to the Alexandria, VA-based EPDM Roofing Association (ERA), “In addition to the oils and polymers used to make an EPDM membrane, another ingredient is added to the mix to enhance UV resistance. In the case of a black membrane, carbon black is added, which converts UV rays into heat. In the case of a white membrane, in lieu of carbon black, titanium dioxide is typically used to reflect UV rays and prevent it from attacking the polymer.”

Hypalon is another thermoset, although it is quite different from EPDM. “Hypalon is a unique material because it is manufactured as a thermoplastic; but, because it cures over time, it becomes a thermoset. Hypalon materials are heat sealed at the seams,” explains SPRI. It’s important to note that, unlike thermoplastics, Hypalon becomes unweldable with age.

Thermoplastics

As opposed to thermosets, which must use adhesives, the seams on thermoplastic single-ply membranes can be hot-air welded together to form cohesive laps. When thermoplastic membranes cool, they return to their original material type. PVC and TPO are two of the most commonly installed thermoplastic single-ply membranes. According to SPRI, “Most thermoplastic membranes are manufactured to include a reinforcement layer, usually polyester or fiber glass, which provides increased strength and dimensional stability.” And because these membranes are characteristically light in color, they provide excellent reflectivity, resulting in potential energy savings.

PVC membranes are manufactured from a combination of PVC resin, stabilizers, pigments, fillers, plasticizers, biocides, and various processing aids. Unlike TPO membranes, which require additives to increase fire resistance, PVC membranes are naturally fire retardant. While problems back in the 1960s with PVC membranes may have colored some facilities professionals’ views on these thermoplastic roofs, the use of chemical plasticizers that evaporated (causing embrittlement and shrinkage) have long since been discontinued.

The other popular thermoplastic membrane on the market today is TPO. “TPO polymers are blends or alloys of polypropylene plastic or polyprolylene and ethylene propylene rubber (EPR) or ethylene propylene diene terpolymer rubber (EPDM),” SPRI reports. TPO can be installed in a multitude of ways and with the introduction of peel-and-stick products, installation time can be reduced significantly. Like PVC, TPO remains hot-air weldable throughout its service life, simplifying repairs and maintenance.

Fastening Methods

Mechanically Fastened

Using fasteners to secure the roof (see figure above) is one single-ply roofing installation method. When considering the fastening method most appropriate for your application, it’s important to make a selection based upon many factors, including the type of roof deck (wood, metal, concrete, etc.) on your building. “Mechanically fastened single-plies will work on just about any type of roof deck. Where you have to be careful is when you get into some of the very lightweight decks, such as the cementitious wood fiber and gypsum decks. Lightweight insulating concrete you have to be careful with, because you’re relying on that material to hold the roof down,” explains McFarland. In many cases, manufacturers will recommend fastening through the insulating concrete into the structural deck. Some of these applications may require narrower sheets and more fasteners to ensure that there isn’t a greater likelihood of fastener pull-out (referred to as pull-out value).

Careful consideration should be taken so that the proper number of fasteners is used. Installing a roof with an insufficient number of fasteners can increase the chances of blow-off. When one fastener pulls out, its load is transferred to surrounding fasteners, increasing the chances of their pull-out as well.

If the facility is located in an area prone to high winds, the means for attaching your single-ply roof is especially important. “If you’re in a wind zone or a place where you’re exposed to high wind uplift conditions (hurricanes, etc.), the fully adhered applications (with mechanical attachment at perimeters) tend to resist wind uplift better than mechanically attached,” explains Ray Dengler, vice president, sales manager (Midwest Service Area), D.C. Taylor Co., Cedar Rapids, IA.

Fully Adhered

“[With] fully adhered [systems], you’re totally reliant on the substrate that you’re adhering the membrane to [in order to] hold the roof on. You’re not using fasteners and plates or bars, you’re just gluing the membrane to the substrate – sometimes, that’s right to the deck material, insulation, and in some cases, an existing membrane,” says McFarland. While mechanical fastening involves the expense of increased labor time and materials, fully adhered systems can be quite economical – especially when considering self-adhered products like peel-and-stick TPO.

Again, it’s important to think about the roof deck and its role in this decision. “We don’t recommend mechanically attaching a roof – any kind of roof – to a concrete deck. We just don’t think it is the right thing to do (hammer-drilling holes in a concrete deck to attach the roof). Now, we do recommend adhering single-plies to a concrete deck with an adhesive. We think that’s an appropriate solution,” says Liscum. While it’s not uncommon to mechanically fasten a roof to a concrete deck, fully adhered systems have been found to provide excellent wind uplift resistance in these applications. Additionally, various peel-and-stick TPO membranes must have a suitable substrate in order to bond properly.

Ballasted

While adding ballast to a loosely laid roof is most commonly associated with EPDM, other membranes can also take advantage of this fastening method. However, PVC is one membrane where it’s best to seek out an alternative means of installation. “In the ’90s, the PVC industry abandoned the idea of a ballasted PVC, as it was thought that the silt associated with the ballast was to blame for extracting plasticizers from the membrane,” explains Richard Fricklas in the May 2004 issue of Roofing News, an e-mail newsletter published by Buildings.com. Ballasted systems typically use aggregate that is 0.75 to 1.5 inches in diameter and is applied over a loosely laid membrane at 10 to 12 pounds per square foot, Fricklas goes on to explain. As an alternate to aggregate, 2-inch concrete pavers that can withstand freeze-and-thaw cycles can be applied over a non-woven fleece pad to anchor the membrane.

According to ERA, “… generally, ballasted [EPDM] roofing systems have the lowest installed cost per square foot, followed by mechanically fastened and fully adhered systems.” But don’t let cost alone be the deciding factor. As Dengler points out, every method has its pros and cons; in the case of a ballasted system, a facility professional should be aware that locating the source of eventual leaks will be more difficult. It’s also important to determine if the weight of a ballasted system can be sustained. “I would advocate that you shouldn’t put a million pounds of rock up on a metal deck [or on] a building that has a lightweight deck. There [are] better ways to do it,” Dengler explains.

As noted earlier, areas prone to high winds must take special precautions when designing and installing a ballasted system – or consider avoiding it altogether. “Once upon a time, you used to be able to put a ballasted roof on buildings in Houston. [With] the hurricanes, they completely outlawed it. It wasn’t because the roofs failed,” Dengler explains. “[It was] because the rocks blew off, broke all the glass in the surrounding buildings, and all the glass shards fell on people.”

Proper Installation and Maintenance are Critical

Regardless of the roofing system you choose, proper installation and regular maintenance (including inspections) is a must. When searching for a roofing contractor, make sure the company you hire is trained to install the single-ply membrane you are specifying. Contact the National Roofing Contractors Association (NRCA at [www.nrca.net]) or the membrane’s manufacturer for a list of approved applicators. “The other thing I recommend to owners is, if it’s possible, go out and visit a job – and I prefer that they visit a job in progress, not just a completed job. When you visit one in progress, you get to learn about the way the contractor sets up, cleans up, and manages the project,” advises Liscum.

After storms and when tradespeople are doing rooftop work, make sure to survey the roof for potential damage. The best way to make sure your single-ply membrane lives up to its 15- to 20-year life expectancy is to provide it with the TLC it needs and requires.

Jana J. Madsen (jana.madsen@buildings.com) is managing editor at Buildings magazine.

Making Dollars and Sense Out of Single-Ply

While cost shouldn’t be the single determinant of which roof system you select, it is undeniably a consideration among facility professionals. “I’ve seen [single-ply membranes cost] as low as $1.50 per square foot, up to (in New York City where things get very expensive) $10 a square foot or higher. On a whole, [they average] maybe $1.50 to $3 or $4,” says Tim McFarland, technical director, Mule-Hide Products Co. Inc., Beloit, WI. Some of the factors that will affect the selection and cost of your next single-ply roofing project include: