The old cliché, “better safe than sorry,” has strong applications in the roofing industry. A recent example is the SPRI, RCI Foundation and National Roofing Contractors Association jointly funded evaluation of metal fasteners in contact with preservative treated wood.
SPRI represents sheet membrane and component suppliers to the commercial roofing industry and investigated the role that fastener corrosion could play in roof failures. The big question in the roofing industry’s mind was whether metal fastener corrosion in treated lumber was even an issue. The study went on to assess the relative corrosion levels of various fasteners in different lumber treatments, including CCA, ACQ and CA-B lumber.
The wood industry began using these new preservative chemicals due to environmental and regulatory concerns. There were also concerns that some of these new chemicals may cause corrosion of certain types of metal fasteners, which had been observed in some instances. Fortunately, these concerns tended to be a knee-jerk reaction to the new wood treatments. “The roofing environment - when properly constructed - does not provide the conditions that cause accelerated corrosion,” said Stan Choiniere, co-author of the SPRI study and technical director of OMG Inc., Agawam, Mass. “I have seen no evidence of corrosion since the change in wood treatments, even in less-than-ideal conditions.”
These findings came as a relief for a variety of reasons. For one thing, SPRI knew that post-hurricane investigations by the Roofing Industry Committee on Weather Issues (RICOWI) consistently showed that in many cases damage to a low-slope roof system during high wind events begins when the edge of the assembly becomes disengaged from the building structure. Once this occurs, the components of the roof system (membrane, insulation, etc.) are exposed. Damage then propagates across the entire roof system by peeling of the roof membrane, insulation, or a combination of the two.
Fortunately, this is highly unlikely to happen due to fastener corrosion in treated wood decks. The field studies demonstrated that wood nailers dried from a saturated condition to 45 percent to 65 percent RH within six months of exposure. Because corrosion was only observed on samples exposed to 90 percent RH, corrosion of e-coated or stainless steel fasteners are not an issue. Supporting this conclusion is the fact that there were no reports of excessive fastener corrosion when installed in treated wood nailers.
SPRI’s studies were limited to low-slope roofing applications, and laboratory tests were conducted by SPRI member Duro-Last Roofing of Saginaw, Mich., to identify critical temperature and humidity conditions required to initiate corrosion. TRUFAST of Bryan, Ohio, also conducted various studies that Duro-Last considered as part of its research.
“Duro-Last has stated that in the last four years it has seen no indication that rooftop conditions are sufficient to cause accelerated fastener corrosion in ACQ or CA-B treated lumber,” said SPRI Technical Director Mike Ennis, who co-authored the study. “In fact, for the temperature and moisture conditions observed in the field study, no corrosion issues would be expected.” It should also be noted that in SPRI’s testing, the wood nailers were located in the center of the test roof over a deck. Perimeter edge nailers are usually located at the roof edge over a wall.
However, SPRI recommends that roofing contractors take the following precautions:
- Use either non-treated or SBX treated wood for nailers.
- Use either e-coated steel or stainless steel fasteners if treated wood is used.
- Use a Factory Mutual compliant fastener, or equivalent.
The study is also a great example of the roofing industry working together-and working proactively-to address performance concerns that could adversely affect low-slope roofing contractors.