Like a good novelist, I am going to torture you by forcing you to read this story prior to revealing a super- secret answer.
One of our clients will be constructing a Hansen Pole Building in Colorado soon. This particular building is very typical post frame construction as it has steel roofing over open purlins. There is not a “roof decking” of OSB (Oriented Strand Board) or plywood.
When applying for his permit to build his new building, he was told an “Ice Barrier” would be a requirement.
2015 International Building Code deals with a myriad of roofing products in Chapter 15 (check it out yourself here: https://codes.iccsafe.org/content/IBC2015/chapter-15-roof-assemblies-and-rooftop-structures). These include Section 1507.2 Asphalt shingles, 1507.3 Clay and concrete tile, 1507.4 Metal roof panels, 1507.5 Metal roof shingles, 1507.6 Mineral-surfaced roll roofing, 1507.7 Slate Shingles, etc.
Most of these roofing choices list a requirement such as:
“1507.2.8.2 Ice barrier.
In areas where there has been a history of ice forming along the eaves causing a backup of water, an ice barrier that consists of at least two layers of underlayment cemented together or of a self-adhering polymer modified bitumen sheet shall be used in lieu of normal underlayment and extend from the lowest edges of all roof surfaces to a point at least 24 inches inside the exterior wall line of the building.”
IBC 1507.4 Metal Roof Panels does NOT include a subsection for Ice barrier.
Now it is possible for any local permit issuing authority to make amendments to their adopted version of codes. However if my Building Department had such an amendment I would be asking to see it first, then ask how they propose to install it over widely spaced purlins?