“Probably the most common roofing mistake is where there is no overhang at the eave,” says Ken Pfaff of Criterium-Pfaff, a home inspection firm in Spokane, Wash. “Without a drip edge, runoff wicks back up under the roofing and leads to rot in the roof deck and/or the fascia.”

Inspectors and construction defect experts across the country often see the problem as a callback. Some even report situations where roofers neglected to account for the build-out of the siding or fascia of the sidewall, leaving the roof edge behind that intersecting plane. Others bemoan roofers who place the drip edge over the roofing felt (to keep the felt in place, they say), a reverse-flashing boo-boo that’s almost worse than not installing a drip edge at all.

Obviously, providing a slight overhang, ideally with a sheet-metal drip edge (or drip cap) along the entire perimeter of the roof (including rakes and second-floor structures), is most easily done in new construction.

But if you have to come back to replace the shingles, roof deck, and maybe some siding or trim—perhaps extensively so, if only to match the existing finishes not affected—adding a drip edge is a simple and inexpensive fix.