How do you repair roof flashing?
Short answer. Repair roof flashing by first identifying which flashing failed and how: if the metal is sound and only a sealant joint or a few nails have failed, reseal and renail; if the metal is rusted, bent, missing, too short or lapped the wrong way, replace it, which means lifting the shingles and siding around it and weaving new flashing in correctly. Sealant alone over failed metal is a temporary patch.
Repairs by flashing type
- Step flashing at walls: remove shingles along the wall and the bottom of the siding, install new pieces one per course nailed to the deck, and reset the siding with a gap above the shingles.
- Kickout flashing: add one where a roof edge meets a wall, so water from the step flashing goes into the gutter instead of behind the siding.
- Headwall or apron flashing: replace with a piece that laps over the top shingle course, with siding or counterflashing over its vertical leg.
- Counterflashing on brick: grind a new groove into a mortar joint, insert the flashing, and seal.
- Valley metal: replace damaged sections, lapping upper over lower, with membrane underneath.
- Pipe boots and vent flashing: replace the whole boot.
- Drip edge: refasten, or replace sections with 2-inch laps.
When resealing is enough
- The metal is intact, properly lapped and long enough.
- Only the sealant at the top of the counterflashing, or a few exposed nail heads, has failed.
- Use a quality sealant compatible with the metal and roofing, on a clean, dry surface.
When it must be replaced
- Rust holes, splits or bent metal.
- A single long strip where step flashing should be.
- Reverse laps that direct water behind a layer.
- Flashing that only works because of a thick layer of tar.
A gentle hose test confirms which detail leaks before anything is torn open. On older roofs, shingles near the flashing may crack when lifted, so plan for a few replacements. See roof flashing explained and common roof leak causes.