Why water runs behind gutters: This Old House shows the drip-edge gap many roofers skip

The segment, posted Nov. 1 on the show's YouTube channel, runs water over a correct and an incorrect roof edge built side by side. Silva has made the same case in earlier This Old House lessons. When the metal lies flat on the board, surface tension keeps water clinging to it, so the water wraps around the edge, runs down the front of the fascia and ends up behind the gutter, where it rots the wood. Teaching apprentices in a 2020 lesson, he called a drip edge nailed snug to the fascia, with shingles cut flush on top of it, the first big mistake he sees roofers make.

His numbers come from the show's written guides. Keep the drip edge away from the fascia with a 3/8 to 1/2 inch gap, and set it so the roof edge stands 1 1/2 to 1 3/4 inches out from the fascia board. Let the shingles run past the metal by about a finger's width, which the guides put at 1/4 to 1/2 inch, so water falls into the gutter instead of being pulled back under them. Hang the gutter so its front lip touches a straightedge laid on the roof and run out past the eave, and pitch it 1/4 inch for every 4 feet toward the downspout, or 1 inch over a 16-foot run.

The IRC sets minimums but no gap. Section R905.2.8.5 requires drip edge at the eaves and rakes of shingle roofs, lapped at least 2 inches, reaching at least 1/4 inch below the sheathing and 2 inches back onto the deck, and nailed no more than 12 inches apart, with underlayment over the metal at the eaves and under it at the rakes. High-wind areas need a second look. Builder Mike Guertin, writing in JLC, notes that the IBHS Fortified Home guides want starter shingles flush with the drip edge or overhanging it by 1/4 inch at most, and nearly all of the seven shingle makers he checked allow flush to 1/4 inch. On those roofs, the manufacturer's limit sets the overhang.