How much should roof flashing overlap?
Step flashing
- Size. The International Residential Code requires base flashing against a sidewall to be at least 4 inches high and 4 inches wide. GAF's preferred piece is at least 5 inches up the wall and 5 inches onto the roof, and at least 2 inches longer than the shingle exposure.
- Overlap. With a common 5 5/8 inch exposure, that means pieces about 7 5/8 inches long or more, so each overlaps the one below by roughly 2 inches.
- Placement. Each piece sits on the shingle below and under the shingle above, nailed to the deck only, never to the wall.
Drip edge
- Adjacent pieces overlap at least 2 inches.
- The roof flange extends at least 2 inches back onto the deck, and the drip leg extends at least 1/4 inch below the sheathing.
- Fasteners go no more than 12 inches apart. At eaves the underlayment goes over the drip edge; at rakes it goes under.
Valleys
For open valleys lined with metal, the code requires the lining to be at least 24 inches wide. Where valley metal pieces join, the upper piece laps over the lower one by at least the manufacturer's minimum, and many installers add sealant in the lap. Closed valleys can be lined with a 36-inch roll or a self-adhering membrane.
Counterflashing and walls
Counterflashing on a chimney should lap down over the vertical leg of the step flashing far enough to cover it fully without being nailed through it, with its top edge set into a mortar joint. On sided walls, the housewrap and siding lap over the step flashing's wall leg, and the siding should stop slightly above the roof surface so it does not wick water.
Always check the shingle wrapper and local requirements, since Georgia uses the 2024 IRC with state amendments as of January 1, 2026. See roof flashing explained and drip edge explained.