There are three prominent membrane attachment methods used in the U.S. low-slope commercial roofing market. They are fully adhered, mechanically attached and self-adhered. There are advantages and disadvantages to each of these attachment methods which should be considered prior to selection for a specific project.
Fully AdheredFully adhered membrane attachment dates back over 100 years, as this was the primary method of attachment for built-up roof systems. In these applications, the roofing felt is set in a full and even application of bitumen. It was not until the advent of single-ply membranes in the early 70s that other attachment methods were available. Attachment methods for modified bitumen sheets include application in modified asphalt, torch application and attachment in cold adhesive. Single-ply membranes (thermosets and thermoplastics) are adhered in bonding adhesive.
A primary advantage of these applications is that they provide excellent waterproofing (full coverage) and wind-uplift capacity. They are higher in cost than the other attachment methods due to labor and material costs, however they provide the longest average service life of all the systems.
A disadvantage of fully adhered membranes has been the safety concerns associated with hot bitumen and torch applications and environmental issues surrounding solvent-based cold adhesive applications. Due to the increased Volatile Organic Compound (VOC) regulations there have been changes in material formulations of solvent-based adhesives. The current VOC level of adhesives is approximately 20 percent of the total material content. Solvents are primarily added to adhesives to improve their adhesion capabilities. In the older adhesive technologies, low solvent content produced a thick adhesive that was hard to apply. As more solvent was added the adhesive became lighter and was easier to apply.
Since the late 90s the adhesive manufacturers have been working on producing quality adhesives that meet the current VOC content regulations. These regulations have led to the increase of water-based adhesives. There has been a significant amount of improvement in the development of low-solvent-based adhesives and some U.S. manufacturers can now produce solvent-based adhesives with 0 percent VOC content.
Some of the single-ply manufacturers are offering latex/neoprene bonding adhesives that are produced with no solvents, and one manufacturer offers a polymer formulation that is 100 percent solids. Water-based adhesives are available for use with cold process built-up systems.
Another disadvantage is the temperature constraints associated with adhesives, which require an ambient application temperature of above 40 degrees Fahrenheit. On the other hand, hot-applied bitumens can be applied in any temperature, which makes them an advantageous choice in cold-weather applications.
Mechanically AttachedMechanically attached systems are primarily used with single-ply membranes (thermosets and thermoplastics). An advantage of these systems is reduced labor and material costs. However, one of the primary concerns regarding mechanically attached membranes has been their susceptibility to wind flutter, which can create substantial interior noise and contributes to membrane-fastener separation. This limits their service life and adds to maintenance costs associated with membrane punctures from fastener movement.
A primary reason for the membrane’s roof flutter is due to the width of the sheets. When mechanically attached systems were first introduced, the width of the sheets was 5 feet. This allowed for sufficient fastening attachment and limited the chance of wind flutter. Over the years, the effort to reduce labor costs led to wider sheets that were 10 feet to 12 feet in width. These expanded widths have contributed to the susceptibility of wind flutter.
In an effort to eliminate roof flutter in these sheets, OMG Roofing Products has developed a securement system that uses an induction welder to adhere the membrane to the plates without membrane penetration. The system provides increased attachment points, which ultimately eliminates roof flutter. OMG states that the system includes three primary components - the induction welder, a set of magnetic cooling weights and specialty plates that are designed for use with both PVC and TPO membranes. Most of the membrane manufacturers have approved the use of this system with their membranes. Consult the specific manufacturer for proper application methods.
Self-AdheredSelf adhered technology (or “peel and stick” as it was referred to early on) evolved from steep-slope underlayments. Self-adhered membranes are now available with modified bitumen and TPO membranes.
In modified bitumen systems, a self-adhered polymer - such as styrene isoprene styrene (SIS) - along with other agents and resins are blended into the SBS engineered sheet. This formulation permits the lowering of the blends viscosity making it soft enough to achieve good adhesive properties, high long-term peel strength and the proper tackiness to perform at temperatures as low as 50 degrees. This advance in technology has permitted two layers of modified sheets to be successfully self-adhered without torch fusing as was required by early incarnations of the sheets.
When they are properly applied, self-adhered sheets do not require the cure time typically associated with cold adhesive applications. These membranes can be applied directly to approved substrates and achieve immediate initial peel strengths to provide watertight laps at ambient temperatures. Most manufacturers require the self-adhered membranes be applied when ambient temperatures are above 50 degrees. Peel strength at the laps is usually achieved in a few days from heating methods such as air, sunlight or any other external source.
Self-adhered membranes offer several initial advantages for contractors. First and foremost, they appear to address the current safety and environmental issues. The application of these sheets is completed with no heat and no fumes, which makes them relatively safe to apply. They are also environmentally friendly because there are no volatile organic compounds in the material.
Another advantage for contractors is the ease of application. No special equipment is required and they are less labor intensive than many other roof system applications. Application is completed in a quick three-step process. The membrane is rolled out over the properly prepared substrate. Once the membrane has been rolled out and adequately relaxed, the membrane sheets are installed by removing a separator sheet from the roll as the membrane is adhered to the substrate. Subsequent rolls are applied in similar fashion utilizing manufacturer required overlaps at end laps and seams.
Recently, some thermoplastic manufacturers have begun producing self-adhered TPO membranes. Factory-applied (specially formulated) adhesive is set on the bottom side of the membrane during the manufacturing process and covered with a release film. The application concept is similar to self-adhered modified sheets. The substrate is cleaned and prepared for installation; the membrane is then unrolled and positioned for proper placement. The release film is removed from the roll as the membrane is adhered to the substrate and all edges, seams and the field of the roof are rolled for additional adhesion.
The primary advantages to these types of applications are that they are solvent free and there are no volatile organic compounds contained in them. Environmental issues concerning the use of solvents in certain parts of the country, particularly with the use of single-ply bonding adhesives, have forced single-ply manufacturers to investigate the use of alternative attachment methods for fully adhered applications. The other advantage to this attachment method is that it provides for more uniform application rates, eliminating insufficient adhesion and less chance of adhesive related wind uplift problems.
This application method is also less labor intensive and take less time to install because there is no wait time associated with adhesive curing. One manufacturer states that these types of applications are up to 50 times faster than taped seam applications, up to five times faster than water-based adhesives and up to three times faster than solvent-based adhesives. Another advantage is that this method of attachment eliminates the chance of airborne particles contaminating the membrane application, a key concern with fully adhered single-ply applications.