Apron Flashing Repair & Installation
Apron flashing is a one-piece, L-shaped strip of metal that seals the joint where the bottom edge of a vertical surface meets a sloped roof — the front of a chimney, the base of a dormer or wall, or a skylight curb. One leg tucks up under the wall or siding, the other lies over the shingles, so water running down the wall is carried onto the roof surface instead of behind it.
Apron, step, and counter flashing: how they differ
Roofers often use all three at a single chimney or wall. Apron flashing handles the low side, where the wall sits at the bottom of the slope; the others cover the sides and the top of the joint. Knowing which piece failed tells you what a repair should involve.
Apron flashing
Where the wall meets the bottom of a slope
A single long L-shaped piece along the front of a chimney, base of a dormer wall, or headwall. Because it sits at the low side, it collects the most runoff and is usually the first to show wear.
Step flashing
Where a wall runs up the side of a slope
Many small L-shaped pieces, one woven under each shingle course along a sidewall. It follows the slope in steps instead of a single long run.
Counter-flashing
The cover that sits over the top
Set into the masonry or tucked under siding, it overlaps the top edge of the apron or step flashing. On brick chimneys it is cut into the mortar joint, then sealed.
6 signs your apron flashing needs repair
Flashing failures rarely announce themselves with a dramatic leak. They show up as slow stains, damp insulation, and rot that spreads for months before anyone looks up. Check the low side of every chimney, dormer, and wall.
A brown ring that grows after rain, on the ceiling below or beside a chimney or dormer, is the classic flashing leak.
Galvanized steel rusts through at cut edges and nail heads. Streaks running onto the shingles mean the metal is breaking down.
Wind, ice, and settling pull the flashing away from the wall. Any visible gap along the edge lets water run behind it.
Roofing cement and caulk typically shrink and crack within a few years. If sealant is the only thing holding the joint, it has already failed.
Spongy sheathing, darkened trim, or peeling paint just above the roofline points to long-term moisture getting behind the flashing.
Wet insulation, rusted nail tips, or musty odor beside a chimney chase or wall means water has been entering for some time.
How apron flashing is installed
Installation order is what makes flashing work: every upper piece must overlap the piece below it, like shingles, so water always runs over joints and never into them. Exact details vary with the wall material, roof pitch, and local code.
- Remove the old flashing and expose the joint Pull the failed metal and lift the nearest shingles and siding, then inspect the sheathing and framing underneath. Rotted wood is replaced before anything new goes on.
- Install underlayment and a waterproof membrane Self-adhering membrane is lapped up the wall and out onto the roof deck, giving a second layer of protection behind the metal.
- Cut and fit the apron flashing The piece is sized so each leg extends several inches (commonly 4 or more) up the wall and out onto the roof. Corners are folded, not just cut, to keep them watertight.
- Fasten the wall leg, not the roof leg Nails or screws go through the vertical leg under the siding or counter-flashing, where they are covered. Face-nailing the roof leg creates new leak points.
- Lap the shingles over the roof leg Courses are rebuilt so the lowest shingles overlap the roof leg of the flashing, and end laps between flashing pieces are staggered and overlapped.
- Add counter-flashing and seal the top edge On masonry, counter-flashing is set into a cut mortar joint and sealed; on siding, the siding or trim is reset over the metal. Sealant is a backup, never the primary barrier.
Apron flashing repair and installation costs
Price depends on length of the run, wall material, roof access, and how much rot is found once the old flashing comes off. Chimney work costs more than a plain wall because of the masonry cutting and added counter-flashing.
| Job | Typical cost* | What is included |
|---|---|---|
| Minor repair or re-seal | $150 – $450 | Re-securing lifted metal, replacing fasteners, and resealing small gaps. Only a short-term fix if the metal is already corroded. |
| Replace apron flashing, plain wallMost common | $300 – $900 | Removing and replacing a short run along a wall or dormer, including membrane and reworking nearby shingles and siding. |
| Chimney flashing replacement | $500 – $2,000 | New apron, step, and counter-flashing around a chimney, with mortar joint cutting and sealing for masonry. |
| Copper or premium metal upgrade | Adds 2x – 4x the metal cost | Copper and lead-coated copper last far longer than galvanized steel or aluminum and are common on higher-end or historic homes. |
| Decking or framing repair | $300 – $1,500+ | Replacing rotted sheathing or wall framing found behind failed flashing. Scope is confirmed only after the old metal is removed. |
*Estimated 2026 national ranges; local labor, roof pitch, height, and access move totals up or down. Insurance generally covers sudden storm damage but not wear, age, or poor maintenance, so a flashing that simply wore out is typically a homeowner expense. Get at least 3 written quotes, and ask each roofer to say whether they are replacing the flashing or only sealing it.
Apron flashing questions, answered
Flashing is small. Leaks from it are not.
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