By Chris Mader
From the January/February 2016 Issue
Today’s facility managers have essentially three options when it comes to installing a new single-ply roofing system. The roofing system can be fully adhered, fully ballasted or mechanically attached. Each has pros and cons. White thermoplastic materials represent the highest percentage of new commercial roofs being installed in North America today, and the vast majority of thermoplastic materials are installed using mechanical attachment methods. These single-ply systems include polyvinyl chloride (PVC), thermoplastic polyolefin (TPO), as well as black rubber (EPDM). The material is typically provided in rolls 10′ wide by 100′ long. In a mechanically attached thermoplastic system, the membrane is rolled out over the insulation then fastened with screws and plates along the edge. The next roll overlaps the fasteners and the seam is fused using hot air (900°F+) to create a bond that can be stronger than the membrane itself.
Fully adhered. In a fully adhered system, the membrane is bonded or glued to the substrate. Typically in these assemblies a layer of insulation is mechanically fastened to the structural deck, however, it can also be glued, depending on the deck type. Frequently there are several layers of insulation in the assembly all glued together before the waterproofing cover is adhered to the top level of insulation or cover board. Ironically, fully adhered systems often require more fasteners per 4’x8′ insulation board than mechanically attached systems, and are also dependent on the installation temperature and may be impacted by the humidity levels as well.
Ballasted. By contrast, ballasted systems use stone or paver materials to hold the roofing membrane in place. The roof cover is loose laid over the insulation, which is either mechanically fastened or loose laid in place, and then ballast is installed on top. Historically, ballasted roofs represent the least expensive option. The biggest challenges with these roofs are preventing ballast from scouring during periods of high wind, ensuring that the building can handle the dead load weight of the ballast, and difficulty finding leaks after the fact.
Mechanically attached. The majority of the single-ply materials are mechanically fastened to the structural roof deck using screws and “plates” or oversized washers designed for the application. While the majority of new roof decks in North America are steel, there are many other deck types, including gypsum, concrete and wood, which can all accept mechanically fastened roof systems.
Induction Welding. Rather than relying exclusively on in-seam fastening techniques for mechanically attached thermoplastic membranes, many facilities have had their contractors employ an alternative technology, induction welding. The system, called RhinoBond, is a non-penetrating option for installing TPO and PVC membranes, which spreads the wind load more evenly across the roof deck. Based on microprocessor-controlled induction welding technology, the system significantly limits roof flutter, regardless of membrane width, and reduces the load per installed fastener.
Oversimplified, this technology—accepted by all major roofing system manufacturers—is used to heat a specific metal target through a non-metallic material. In this case, the “target” is a metal plate, coated with a heat-activated adhesive, installed under the TPO or PVC roofing membrane. This process creates a strong bond between the bottom side of the roofing membrane and the coated plate, without penetrating the membrane. In static testing, these bonds have demonstrated the ability to resist more than 500 pounds of force.
Most mechanically attached roofing assemblies are designed around in-seam fastening patterns and are based on a specific wind loads (i.e., Factory Mutual [FM] 1-90). In traditional mechanically attached systems, insulation is secured to roof deck using a specified number of fasteners and insulation plates per square. A second set of fasteners and seam plates or a batten bar is installed through the membrane in the lap or seam, and protected by an overlapping heat-welded seam that eliminates potential entry points for moisture.
With the induction system (see photo above), only one fastener and plate is used to secure both the insulation and the membrane to the deck. As such, the number of required fasteners is reduced by up to 50% depending on the specific wind requirements. Since the grid pattern distributes the wind load more evenly, the system can achieve higher wind performance ratings with fewer fasteners, while complying with the steel deck stress limitations of FM 4470.
For facilities and light manufacturing plants with standing seam metal roofs, the induction system eliminates many issues that roofing contractors find difficult with these types of projects. “I think we came in a couple of hundred thousand dollars less than if we had gone with a metal roof,” says Brent Howell, facility manager for Salmon Bay Center about the Stimson Marina project in Seattle. “If we had done a complete tear-off we were probably looking at another million dollars.”
Mader is a codes/approvals support engineer for OMG Roofing Products. He is responsible for helping to evaluate new products as well as for developing and maintaining technical product specifications, maintaining code approvals and keeping abreast of technical changes and advancements in the commercial roofing industry. Mader is a member of the NRCA, SPRI and of RCI.
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