WHAT IS THE CAUSE OF BRIDGING THAT HAS BEEN SEEN IN THE PAST AT ANGLE CHANGES IN SOME EPDM LOOSE-LAID APPLICATIONS?

June 3, 2018

Some membrane bridging has been observed in the past in some loose-laid ballasted membrane assemblies. The bridging was not seen in adhered or mechanically-fastened applications where the membrane is adhered or secured in place. The cause of this bridging relates to membrane movement experienced in ballasted roofing systems and insufficient securement in the angle change at the base of parapets. This phenomenon was not seen in every ballasted loose-laid assembly but only some (mostly in colder regions) where improper securement is present. Examples include:

Roofing nails used to anchor the roofing membrane to a horizontal wood nailer
Improperly anchored beveled wood nailers fastened to the vertical wall (warping and buckling of the wood occurred)
Rubber nailing strips fastened horizontally or vertically
In 1987, the method of securement was changed to incorporate 2″ diameter plates and threaded fasteners spaced at 12″ on center or a reinforced strip (glued to the underside of the roofing membrane at the angle change) to anchor the roofing membrane. This change in securement method eliminated the bridging. Both NRCA and SPRI have endorsed this securement.