In the February 2012 issue of RCI’s Interface magazine, the author, Helene Hardy Pierce, warned us to stay alert in order to protect our single-ply roofs. One example was a long run of cast iron pipe sitting on wood blocking. By consulting the SMACNA Architectural Sheet Metal Manual, we find that a 10-foot length of steel pipe on a roof may be expected to move five 64ths of an inch with a 100-degree F. change in temperature (e.g. summer to winter). As the blocking is dragged back and forth, it will scrub the membrane, especially significant on thin single ply systems (45 to 60 mils) compared to traditional bituminous roof systems at a quarter-inch thick or so.
When long runs of ballasted solar racking are installed over a single-ply membrane, despite the overburden, the same thing can happen, Hardy Pierce points out. Not only may these cumulative forces put stress on the membrane, especially at laps in the roof system, but long runs of conduit may also be pulled apart, causing electrical short circuits. (nfpa.typepad.com/files/target-fire-report-09apr29.pdf) In fact, Zurich insurance affiliates are concerned enough about this phenomenon to advise clients not to mount their PV panels on the roof.
Still another concern involves evaluating fire exposure when solar panels are raised off of the roof membrane surface. ASTM E108 involves “burning brand” tests, where wood brands are ignited, placed directly upon the roof membrane and observed to make sure the brands do not penetrate the roof system and ignite the combustible deck below. When the solar panels are elevated above the surface, there are questions about a chimney effect with resulting changes in both burning brand and surface burning.
Moisture movement under winter conditions traditionally moves from the interior of a building toward the colder outside. Vapor retarders were typically placed near the warmer inside so that the retarder’s surface temperature would be above the dew point of the inside air. However, it is now thought that air leakage is more important than moisture migration and that membrane systems that are loose-laid (ballasted or mechanically fastened) permit migration to the cold underside of the membrane, where it can saturate the roof insulation or cause disbonding of the insulation facers. A 1-perm vapor retarder might pass 5 pounds of moisture per month, whereas a 1-square-inch hole in that barrier could pass 80.
Insulated roofs can be observed to self-dry – during the warm season, rooftop temperatures will drive moisture towards the cooler building interior. However, with light-colored single ply membrane, this reversing phenomenon may not be effective, and that moisture content will build up in this system year by year. For more information on this subject, click here
These subjects will require more attention, even though light-colored, reflective membranes and highly insulated roof systems have already captured a significant share of the market. From an economic point of view, there may be little return beyond R-values of 10 or so. Energy codes may prove to be overzealous leading to condensation problems. Perhaps properly designed air barriers will prove to control the potential for moisture migration and billowing of flexible membranes.
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