Wm Service
10/19/2017
I have a two year old 2/12 pitch shed roof house with 40 foot long galvanized metal roof panels with exposed fasteners. The roof deck is OSB sheathing and the roof panels are attached directly to the OSB (no purlins). There is an ice and water guard membrane between the OSB and the roof panels on the entire deck. When I went on the roof I recently I discovered that the #12 roof screws are backing out and that maybe 5% of them are stripped. The builder is suggesting a fix with larger screws (special order for larger than #12's) and a spot of roofing adhesive in each penetration to seal the screws. The roofing material fabricator suggested that expansion/contraction of the panels is causing the screws to loosen, a problem made worse by the use of OSB as opposed to plywood. Initially he agreed that screw replacement and adhesive was advisable, but when we got into details and he understood that we have exceptionally long roof panels (40 feet), and that the builder had used the largest screws they stock (#12's) for installation he changed his recommendation. He is now suggesting that 1x4 pine purlins be installed over the OSB and membrane. This seems like a reasonable recommendation, the builder is reluctant. His objection is that condensation will form on the underside of the roof and cause the roof deck to rot. Recall that there is an ice/water guard membrane over the OSB. There is batted r14 insulation in the attic on the interior side of the OSB. First, is the fastener replacement worth a try? I'm concerned it will last a few years and then I'll be on the hook for the more thorough repair. Second, is condensation an issue if we put the roof deck on purlins? Will the ice/water guard membrane be adequate protect the OSB? Thanks for the help. Will
Todd Miller
Classic Products, Inc.
10/19/2017
I do not think condensation with purlins is more of a concern than it's been in the past. I assume this roof is unventilated but has decent insulation. Are there any signs of past condensation in the roof assembly? So, his reasoning is that the purlins will hold fasteners better than the OSB did? Those are, as you know, very long panels for an exposed fastened system. I feel that, even with purlins, you will still have the fasteners loosening ... or the panels themselves having the fastener holes wallow out, which can also lead to problems including leaks.
Guest User
10/19/2017
Thanks for the quick reply, Todd. The ceiling is raised; that is the attic is the just the space created by the roof rafters and joists. The attic is ventilated with small soffit vents and is well insulated. Yes, the factory guy (really a small fabricating shop nearby) suggested the fasteners would hold better in solid purlins than in the OSB. He did raise the possibility of the roof material getting wallowed out as you mentioned. Fortunately the roof is easy to access and I can do regular inspections of the fasteners and panels pretty easily. I haven't seen signs of condensation. We did have a leak in one location but it appears that was a result of a failed plumbing vent boot. Any suggestions for how I might look for condensation without removing roof panels? Is the answer to put new roof panels up in shorter lengths, and overlap them? If so, any suggestion for a maximum length?

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