Wicking Back Without a drip edge and a slight overhang, rainwater runoff and snowmelt can seep (or wick) back underneath the edges of the roof shingles and the roof decking, as well as the sidewall assembly, and behind the gutters. Lack of an overhang can also cause ice dam formation, with similar consequences.
Stop It For new construction, the solution is simple: Design for and provide at least ¾-inch overhang beyond the finished sidewall plane, including trim and fascia, at every roof edge. Use a roofing cement or waterproof mastic to secure a pre-formed, sheet metal drip edge or cap under the roof felt or other protective membrane to further direct water off of the roof and over the edge, ideally into a gutter system.
Fix It If you’ve been called back to fix a moisture-related problem caused by a lack of overhang or drip cap, chances are you’ll have to replace the bulk of the affected roof section (not just the affected shingles or decking) and perhaps the fascia behind the gutters. If you can, extend the new roof deck ¾ of an inch over the sidewall planes and install a drip cap under the new felt and shingles, as described above.