How is roof gutter flashing installed?
Short answer. Gutter flashing, either drip edge or a longer-legged gutter apron, is nailed along the eave so one leg lies on the roof decking and the other hangs down into the gutter, behind the gutter's back. At the eave the underlayment laps over the flashing so water drains onto the metal, and the gutter is hung so the flashing drops inside it.
Drip edge vs gutter apron
- Drip edge has a short roof leg and a flared lower edge that kicks water away from the fascia. It is required at eaves and rakes on asphalt shingle roofs under the residential code.
- Gutter apron has a longer lower leg designed to reach down into a gutter that sits lower on the fascia. It is used where standard drip edge would leave a gap above the gutter.
Installation order at the eave
- Repair or replace any damaged decking at the edge.
- Fasten the drip edge or apron along the eave, roof leg flat on the deck. Code calls for drip edge to extend at least 2 inches back onto the deck and 1/4 inch below the sheathing, with pieces overlapped at least 2 inches and fastened no more than 12 inches apart.
- Lay ice barrier or underlayment over the flashing at the eave, so anything running down the underlayment drains onto the metal.
- Install starter strip and shingles with the proper overhang past the drip edge.
- Hang the gutter so the flashing's lower leg sits inside the gutter's back.
At the rakes, the order flips: underlayment first, then drip edge over it, which keeps wind-driven rain from getting under the metal.
Retrofitting on an existing roof
Adding flashing without a reroof means carefully lifting the first course, slipping the metal under the underlayment, and resealing the shingles. It can be done, but it risks breaking shingle seals, so it is easiest during a replacement. See drip edge explained, ice and water shield in Georgia and roof flashing explained.