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Underlayment, the layer of felt or synthetic material installed on the roof deck before shingles go down, does most of its work unseen, but the way it is lapped at ridges and hips has a direct bearing on how well a roof resists wind-driven rain and ice damming over its service life. At a ridge, where two roof planes meet at the top, underlayment installed on each side needs to extend up and over the ridge line with enough overlap to create a continuous water shedding surface, rather than stopping short and leaving a gap that shingles alone would need to bridge.
Standard practice, reflected in ASTM standards for underlayment application and in most shingle manufacturer installation instructions, calls for underlayment courses to be installed starting at the eave and working up the slope, with each higher course overlapping the one below it, so that water always drains onto the top surface of underlayment rather than finding a seam that runs against the flow. At the ridge, this means the last course of underlayment on one roof plane should lap over the top of the underlayment coming up the opposite plane, rather than the two courses simply butting together at the ridge line. The same principle applies at hips, the sloped external corners of a roof, where underlayment needs to be lapped and, in some installations, supplemented with an additional strip running along the hip itself.
Fastening pattern also plays a role near ridges and hips, since underlayment held only by the minimum number of cap nails or staples along its edge can lift slightly in high wind before the shingles are installed over it, potentially shifting the lap out of alignment. Many installers add extra fasteners near these transitions for exactly this reason, and some synthetic underlayment products include printed overlap lines on their surface specifically to help installers maintain a consistent, correct lap as they work across a ridge or down a hip in changing wind conditions.
This detail matters most in exactly the conditions common across Minnesota and similar cold climate regions, where wind-driven snow can work its way under cap shingles at a ridge before an ice dam even forms, and where ice damming along eaves and in valleys can back water up the roof slope far enough to reach the ridge in extreme cases. A ridge or hip with correctly lapped underlayment provides a continuous secondary barrier if wind or ice moves water past the shingles themselves, while a gap or reverse lap at that location removes that backup protection entirely.
Because underlayment is installed before shingles and generally not visible once the roof is complete, verifying its condition and lap direction at a ridge or hip typically requires removing shingles at that location, which is not something done as a matter of course during a claim inspection unless there is a specific reason to investigate, such as documented interior water staining near a ridge with no other apparent cause. When underlayment issues are suspected, a limited test area is a common way to examine the condition and lap pattern without committing to removing the entire roof, giving the parties to a claim physical information about this normally hidden detail to evaluate alongside everything else found during inspection.
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.