Galvalume corrugated metal roof assembly

Gregory Nowell
3/9/2013
I am Owner-Builder of a guest house with a simple, straight gable 3:12 corrugated metal roof over an overstack of 2x8's at 24 inches on-center, with unvented closed-cell foam insulation (required because County will not allow attic venting in our VHFH zone), and 5/8" OSB sheathing on top. 7/8" rise straight Galvalume (no Color-Bond) corrugated metal panels are specified for the roof. The 48 inch overhangs are unsheathed - 6x8 rafter tails at 48 inches on-center, with 2x4 stringers supporting the corrugated metal panels at 16" on-center. I am evaluating two roofing contractor proposals offering different roofing assemblies. I would appreciate your help answering a few questions related to the roof assembly: 1) Is a high-temp underlayment required? Mapei Polystick MTS or GAF Tiger Paw are the two options being presented for the underlayment - two widely differing products. Which is the more appropriate? 2) There is differing opinion on the need for foam closures at eaves and ridge - one contractor wants the space under the panels to vent, the other says closures must be used. Which is correct? 3) One contractor cautions the unsheathed overhangs will generate condensation on the underside of the panels. Is this a concern? 4) 24 gauge vs. 26 gauge - is the thicker panel necessary? Thank you, Greg
David Stermer
3/11/2013
Greg, Thanks for the questions. 1) Typically HT products are not required from a code standpoint, but they definitely perform better than, say 30# felt. Of these two, I believe the Polystick product would perform better because it will seal around the fasteners. 2) I recommend the colsures be installed to keep moisture and insects out. 3) Condensation at low eaves, outside the weratherline, is typically not a problem. Condensation inside the building is the usually problem. Condensation at a highside eave could be problematic if not sealed well. 4) I beleive 26 ga should be adequate for the design loads. Regards, David Stermer

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