When the entire roof membrane consists of a highly flexible roof membrane, such as EPDM rubber, the membrane itself is turned down the outside edge of the wood blocking. But then, often the failure is due to an adhesive failure between the metal and the EPDM sheet. Water can then enter through the nail holes from the attachment of the gravel stop metal to the wood blocking.
The Architectural Sheet Metal Manual by the Sheet Metal and Air Conditioning Contractors National Association (SMACNA) tells us that a 10-foot-long piece of galvanized steel edge metal can be expected to move 5/64ths of an inch with a 100°F temperature change (hot day and cold night). That cyclical movement is enough to fatigue and eventually rupture the stripping plies.
In addition to the thermal problems, studies of roofing failures during high wind events reveals that most blow-offs begin with roof edge failure. As demonstrated by Metal-Era, one of the leading manufacturers of improved fascia design:
The roof is widely recognized as one of the most vulnerable parts of a building. Of the various components of a roof, the roof edge is the most critical because of the way in which wind acts on a building. Commonly the roof edge receives little attention. It is considered simply an add-on accessory; however, careful selection of an appropriately tested edge is necessary to guard against the effects of potential wind damage. In addition to longevity issues, a tested edge is required by building code in many states and municipalities.
A document that addresses wind effects on roof edges — ANSI/SPRI/FM 4435-ES-1 2010 Wind Design Standard for Edge Systems Used with Low Slope Roofing Systems — is available for download free of charge from Single Ply Roofing Industry (SPRI) at www.spri.org. Inside, you’ll find test methods to determine wind resistance of fascia and coping systems and resistance to membrane shrinkage. Building codes are incorporating ES-1 2010 into their requirements.
Still another issue with roof edges pertains to the wood nailers. Our industry has generally used treated wood for nailers. The treatment, copper chromium arsenate (CCA), is no longer available, and in its place, wood treated with higher levels of copper but no arsenic is supplied. This, in turn, has raised questions about galvanic corrosion between steel fasteners and the high copper content wood. This has lead to NRCA now recommending that untreated wood be used as blocking, rather than treated wood.
The good news is that we now have edge details available which meet both the wind resistance requirements of ES-1 2010 and the thermal problems of embedded metal.
Richard (Dick) L. Fricklas was technical director emeritus of the Roofing Industry Educational Institute prior to his retirement. He is co-author of The Manual of Low Slope Roofing Systems and continues to participate in seminars for the University of Wisconsin and RCI Inc. - The Institute of Roofing, Waterproofing, and Building Envelope Professionals. His honors include the William C. Cullen Award and Walter C. Voss Award from ASTM, the J. A. Piper Award from NRCA, and the James Q. McCawley Award from the MRCA. Dick holds honorary memberships in both ASTM and RCI Inc.
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