Since their introduction in the late 1990s, wide thermoplastic single-ply roofing membranes have been touted as the best thing since the staple gun. But is wider really better?
Wider roofing membranes first came out as un-reinforced EPDM for fully adhered or ballasted applications, and became popular among roofing contractors for installation efficiencies. As EPDM manufacturers entered the thermoplastic arena, they brought the “wider” mentality with them in order to appeal to EPDM contractors.
Although building owners might benefit from lower installation costs when wide membranes are used as fully adhered or ballasted systems, the benefits are less clear for mechanically attached installations.
But one thing IS very clear: Wider membranes have placed the integrity of the roofing system squarely on the shoulders of the roof deck and its attachment method. Consider that the wind load on each fastener in a mechanically attached 6.5-foot-wide membrane fastened every 12 inches in the seam is approximately 540 pounds of force in a Factory Mutual FM 1-90 design. The wind load goes to 855 pounds of pressure per fastener for the same system on a 10-foot-wide sheet and to 1,055 pounds of pressure on a 12-foot-wide membrane.
As such, the performance of mechanically attached 10- and 12-foot-wide membranes remains questionable, as they have yet to be tested by a significant wind event such as a hurricane. As Rene Dupuis, president of Structural Research Inc., of Middleton, WI, points out, “Wider sheets make wind uplift resistance more difficult, as a single row of fasteners has to take up more load and transfer it to the deck.” And while wider membranes have received FM approvals, FM wind testing is a static pressure test, not a dynamic load test, and therefore may not mimic real rooftop performance.
As a result, caution should be used when mechanically attaching wide membranes on existing decks – or on new construction projects where the deck fastening has not been enhanced – because many steel decks may not be able to accommodate the increased lateral load of a wide membrane simply because the deck has not been designed and attached to the structural members sufficiently to withstand the added pressure.
Put all that aside, and sheet flutter is a concern. Since fasteners on wide membranes are spaced further apart, simple geometry suggests that there is a greater likelihood that the membrane will flutter or billow on the roof. Not only can this dynamic flutter create an annoying slapping sound, but it may also cause foam insulation to pulverize and compress over time. As such, the damaged insulation can cause the fastener and stress plate to lose proper loading with the membrane interface and leave tented fasteners and potential puncture points on the roof. Additionally, this reduces the wind uplift resistance of the system, leaving it susceptible to wind-related damage or blow off.
With all of these potential problems, it’s no wonder why Wal-Mart has revised its roofing specifications to limit membrane width to seven feet, and other retailers limit sheet width to eight feet. If these building owners are cautious about using wider membranes, perhaps you should be, too.
Steve Moskowitz is vice president, Technical and Warranty Services, at Stevens Roofing Systems (www.stevensroofing.com), Holyoke, MA.