Fully-adhered

A fully-adhered roof is not fastened mechanically but glued with an adhesive. While a fully-adhered roof typically costs more and takes longer to install, certain locations—in particular, windy and coastal regions—call for this method. Fully adhering the membranes prevents sheet flutter. When air gets pulled up under a fluttering membrane, energy is lost and condensation can occur within the roof following cold temperatures. The correct layering and type of insulation can help minimize the effects of wind uplift, but with a fully-adhered roof, the membranes are glued like a sticker onto the insulation below. This method also requires installer expertise on the correct application rate of the adhesive, as well as the right time and temperature windows. Too little can result in an incomplete seal but overcoating can lead to poor adhesion.

There are two types of membranes: smooth-backed and fleece-backed. Generally, after unrolling a membrane, let it relax. Position the membrane, then fold back along its length, and apply fully mixed adhesive with a saturated roller to both the substrate and/or the back of the smooth-backed membrane. One- and two-sided application methods is manufacturer and product specific. (Note to only apply glue to the substrate with fleece-backed membranes.) After the glue reaches the appropriate tack, a weighted roller carefully presses the membrane onto the glue to avoid any wrinkles, and then the seam is welded.

Though still in use, the last 15 years have shown a significant trend away from built-up roofing and similar systems and the industry has moved rapidly towards single-ply materials.

Note that adhesives generally have a temperature minimum for application of at least 40 degrees Fahrenheit, and installers should be well-versed in the listed application rate. Follow the manufacturer’s directions for specific adhesive and membrane type. Be aware that manufacturers often offer multiple adhesive type options: water-based, solvent-based and low VOC solvent-based. Odors differ from one manufacturer to the next and do not always relate to the solvent content. Pay attention to local code requirements and installation conditions when deciding the type of adhesive to use.

Advantages of a smooth-backed membrane on a fully-adhered roof can include a smoother appearance, longer warranty, and reduced likelihood of condensation and thermal bridging through the assembly. Fleece-backed membranes can qualify for warranties up to 30 years, and they allow for additional adhesive options like hot asphalt and urethane foam adhesives. Additionally, there are self-adhering systems with pre-coated membranes covered by a release liner that help reduce labor, flash times and installation consistency.

A fully-adhered roof is leak resistant, but there is a risk of not knowing about a hole until water from rain or snowmelt has pooled and degraded the glue. Ponding or pooling of water on a roof can indicate a drainage problem and compromise a roof system. Care and maintenance—such as annual inspections and ensuring proper drainage—is required by most warranties, and also critical to keeping a roof system durable and effective for years to come.

Wind Uplift and Single-ply Roofing

Roofing problems can occur anywhere geographically, but they are more prevalent and serious in coastal areas that are susceptible to higher wind speeds and torrential rains. According to the Federal Emergency Management Agency (FEMA), damage to roof coverings is the leading cause of building problems during hurricanes and other coastal storms. A roof can be damaged by wind-born debris, or by uplift pressures caused by high winds. Once a roof is compromised, rainwater can then flood into the building envelope, and moisture trapped within a structure can harbor mold growth and degrade the building materials and structure.

Investigation teams from the Roofing Industry Committee on Weather Issues (RICOWI) visited the damaged areas following Hurricanes Charley, Ivan and Katrina. Most damage to single-ply roofing was related to perimeter problems, including inconsistent fastener placement or roofs installed without following manufacturer’s directions. Generally, low-slope roof damage was “a result of the products not being applied according to code or prescribed manufacturer standards for perimeter and membrane attachment,” according to the Hurricane Katrina Investigation report.

Low-slope roofs generally did well where installation and maintenance procedures were followed, leaving little or no damage post-hurricane. In one case, a PVC roof on a hospital wasn’t secured to manufacturer’s standards for the wind zone but still fared well, thanks to little or no air infiltration. Another PVC roof came through with only a few fasteners loose at the corners. At another investigation site, a TPO roof with heat-welded seams over insulation board mechanically attached to the deck virtually had no damage other than a tear, and it likely would been perfect but for sheet metal blowing across it.

Still, a mechanically fastened EPDM system on the roof of a freezer plant was the most extensively damaged low-slope roof observed. The membrane was blown off the 60,000-square-foot facility. Its fasteners weren’t up to the job. “This proprietary system used a ‘non-penetrating fastener’ that was considered to play a major role in the blowoff,” according to the report. “Although fastener screws remained well embedded in the steel deck, either fastener heads failed, allowing the membrane to blow away, or the membrane was torn at the fastener heads, resulting in release of the membrane. In addition, one minor section of steel decking blew out above an area that may have been pressurized from below.”

With so many options, styles, and technologies available, it is more important than ever that architects understand the differences between single-ply roof membrane systems and where best to specify these.

The higher uplift forces in susceptible areas have been integrated into national building codes. It is reassuring that single-ply roofs that are properly installed according to current codes and standards generally perform well in a storm. Still, it’s important to be aware of the following:

• Low-slope roof failures are usually due to the integrity of attachment of the composite system (mainly failed edge details)
• Perimeters and corners are the most vulnerable areas in a roof assembly; peeling on the edge of a low-slope roof can set off a chain reaction of damage
• Inadequate adhesive strength along the bond line and within insulation layers can lead to problems
• Insufficient or poorly installed mechanical fasteners weaken a roof
• Failure to account for higher negative wind pressures at eaves and corners (or pressurization via door or window failures) can weaken a roof system, rendering it susceptible to worse damage
• Keep the edges or perimeter solid and intact; there are low cost options to help prevent peeling on the edges. A simple peel stop in the form of curbs or pipes can be installed about 12 inches from the edge of the roof to prevent peeling.
• Sealing deck and wall prevents air infiltration

A takeaway theme for builders is to plan to prevent possible pressurization build up under a finished roof membrane. Make sure every part of the roof system, including deck, base sheet, layers of insulation, cap sheet and membrane, is adhered or fastened to resist any potential wind uplift. Such measures will mitigate wind uplift damage to a roof assembly. If a roof is designed to current codes and standards, it should hold up well in the event of a hurricane.

According to the report, “older roofs constructed with little concern for the magnitude of the wind pressures at eaves and corners, and roofs installed with poor practices that lead to progressive failure of the roof membrane, were consistently the ones with significant damage. Many roof designers do not seem to understand that winds produce uplift forces that affect not only the roofing system but the roof deck as well.”

Conclusion

TPO, PVC and EPDM are the most popular single-ply roofing materials used in low-slope roof applications today. While each choice has its pros and cons, all three can be used to ensure protection of the roof and structure from the elements. Understanding the physical properties of each of these materials, as well as flashing and installation options, should help you make the best choice for your next commercial project. In addition, you should be armed with information that will help you choose the most durable option for the specific application while still meeting the project’s budget and sustainability goals.

References

¹ “Comparing non-reinforced and fabric reinforced EPDM sheeting for roofing” from Elastomerics.

² “Roofing Systems” from Whole Building Design Guide.

³ “Flat Roof Materials & Installation Costs 2018: PVC vs. TPO, EPDM Rubber, BUR, Modified Bitumen, Spray-on Coating” from Roofing Calculator.

⁴ “What are the best attachment options for PVC and TPO membranes?” from GAF ProBlog.

⁵ “Hot air welding under changing environmental conditions” from Roofing - The Industry’s Voice.

⁶ “Hurricane Katrina: Wind Investigation Report” from Roofing Industry Committee on Weather Issues.

⁷ “Roof Coverings and Best Practices” from FEMA.

⁸ “Design Questions” from EPDM Roofing Association.

⁹ “Low Slope Roofing: Lessons Learned” from The Journal of Light Construction.

¹⁰ “Mechanical Fasteners Get to the Point in Providing Roof System Success” from Interface, RCI, Inc.

Andrew A. Hunt is vice president of Confluence Communications and has been a writer and consultant in the green building and building science industry for over a decade. He has authored more than 100 continuing education and technical publications as part of a nationwide practice. For more information, visit www.confluencec.com