Is OSB Acceptable as Roof Decking for Metal Roofing?
Oriented Strand Board, commonly referred to as OSB, is a type of engineered lumber similar to particle board, formed by adding adhesives and then compressing layers of wood strands. OSB is widely used as roof decking material.
A homeowner recently wrote and asked me the following regarding OSB roof decking:
Q. We are checking into a metal roof and we have a question that needs to be clarified. We heard that the metal will not stay on as well if you have OSB roof decking. Should it be a different type of wood? Is that true? We have also been told it will be okay. Just wanted some clarification.
Following is my response:
A. OSB in good condition will have every similar fastener pull-out resistance to CDX plywood. The problem, though, is that OSB tends to deteriorate fairly rapidly if exposed to moisture, losing fastener pull-out resistance.
So, what would expose it to moisture? Roof leaks, of course … but also improper attic ventilation can cause condensation in the roof deck that can cause the OSB to weaken.
One option is to try some test fasteners in the OSB. If you get 45 or more pounds of pull out resistance, that’s considered acceptable for most applications. If you have less than that or a lack of ventilation that threatens ongoing performance, I’d replace the decking.
All this said … always adhere to what your roofing manufacturer says. Do not do anything that might later void the wind warranty they offer.
Todd Miller has spent his entire career in the metal building products manufacturing industry. He is president of Isaiah Industries, an organization recognized as one of the world’s leading metal roofing manufacturers. Todd is currently Vice President of the MRA (Metal Roofing Association) and a Past Chair of MCA (Metal Construction Association). Through his website, he strives to raise the bar on standards and practices to provide property owners with the best possible products for successful roofing projects.
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