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Underlayment sits between the roof deck and the visible shingle course, doing work that most people never see but that code drafters have paid close attention to, particularly in climates where ice and snow accumulate on roof surfaces for months at a time. The International Residential Code addresses underlayment in two ways: a general requirement covering the entire roof deck, and a more specific requirement addressing ice dam protection in regions subject to freezing temperatures.
The general underlayment requirement calls for a layer, historically number 15 or number 30 asphalt-saturated felt, though synthetic underlayment products meeting an applicable standard are now common and widely accepted alternatives, installed over the full roof deck before shingles go on. This layer serves as a secondary water barrier in the event wind-driven rain or ice gets past the shingle surface, and its installation pattern, including lap dimensions and fastening, is specified in code and often in the shingle manufacturer’s own installation instructions, which the code frequently incorporates by reference.
The ice dam provision is the requirement Minnesota roofers and building officials pay closest attention to, since it addresses a condition endemic to the state’s climate. Where a history of ice forming along the eaves has caused a backup of water, code requires an ice barrier consisting of at least two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen membrane, extending from the eave edge to a point at least twenty-four inches inside the exterior wall line of the building. On roofs with a shallow slope or a wide eave overhang, this can mean the ice barrier extends several feet up the roof plane before standard underlayment takes over.
When a roof is replaced following storm damage, current underlayment requirements, including the extent of ice barrier coverage, apply to the new roof regardless of what underlayment, if any, was present on the original roof. Older homes are frequently found with felt underlayment alone and no ice barrier at all, since that provision has only become a consistent requirement in more recent code cycles across much of the Midwest, and bringing a re-roofed structure into compliance with current underlayment requirements is a routine part of scoping a full roof replacement.
Underlayment choice also interacts with other parts of a roofing assembly in ways that matter for scope and pricing. Valleys, hips, and ridges each have their own underlayment or flashing detail that a roofer builds up before the field shingles go on, and shingle manufacturers often specify a particular underlayment type as a condition of an extended product warranty, meaning the underlayment selected can affect not just code compliance but also whether a wind or workmanship warranty on the finished roof remains valid. Synthetic underlayment products have become the more common choice on new installations in recent years, generally offering better tear resistance and slip resistance during installation than traditional felt, though both remain acceptable under most current code provisions provided they meet the referenced product standard, and the choice between them is typically driven by installer preference, cost, and manufacturer system requirements rather than by code mandating one over the other.
This article is general education about how the appraisal process commonly works. It is not legal advice, and specific procedures can vary by state and policy.
Russ Lis is a working property insurance appraiser and umpire based in Minnesota, serving clients nationwide. Contact Appraisal Resolution.