O'Fallon homes built from the 1970s onward often feature masonry chimneys with step flashing that meets asphalt shingles. Freeze-thaw cycles through winter push metal away from brick mortar joints, allowing meltwater to run under the roof deck along the chimney sides. We remove deteriorated flashing without disturbing the surrounding shingles, then fabricate new pieces from matching gauge aluminum or copper to maintain the original counter-flashing detail required by local building practices.
Edge cases appear when homeowners have added gas fireplace inserts that raise flue temperatures. The extra heat accelerates sealant breakdown at the top of the cricket. In these situations we install a wider cricket with soldered seams and extend the step flashing an extra course up the chimney to keep water moving past the penetration even during heavy spring rains common in the O'Fallon area.
Proper integration with existing ridge vents and valley metal is critical. Misaligned flashing on one side of the chimney can direct runoff into the attic insulation along the opposite wall. We always pull a few courses of shingles on the high side to verify the underlayment laps correctly before reinstalling.
Homeowners sometimes confuse flashing failure with a cracked flue crown. We inspect both during every visit because a failed crown can send water straight down the flue chase and appear as a leak several feet away from the chimney on the ceiling.
Technician insight
In O'Fallon the thing that catches most people out with Chimney Flashing Repair is how quickly the freeze-thaw cycles in St. Charles County push aluminum step flashing out of the mortar joints on 1980s brick chimneys. Homeowners often think a bead of caulk will hold it, but once the metal has walked even a quarter inch the only fix is pulling shingles and resetting the flashing with proper counter-flashing depth.
Hidden leak behind chimney on a split-level in O'Fallon
Water stains appeared on the second-floor ceiling after a heavy March rain. We found the step flashing on the north side of the chimney had lifted from repeated freeze-thaw movement. The old aluminum was replaced with new pieces tucked deeper into the mortar, a small cricket was added, and the ceiling stains have not returned.
Chimney Flashing Repair Services We Offer
✓Step Flashing Replacement
Remove and replace corroded or lifted step flashing along chimney sides to restore a watertight seal where shingles meet brick.
✓Chimney Cricket Installation
Fabricate and install a new cricket behind the chimney to divert water around the penetration and reduce ice dam pressure in O'Fallon winters.
✓Counter Flashing Repair
Re-seat or replace reglet-mounted counter flashing that has pulled away from mortar joints due to thermal movement.
✓Chimney Flashing Sealant Refresh
Clean and apply high-temperature, UV-stable sealant at all flashing laps and brick interfaces without full replacement.
✓Leak Tracing at Chimney Base
Use moisture meters and targeted shingle removal to locate hidden water paths entering at the chimney-to-roof transition.
Chimney Flashing Repair Cost in O'Fallon, MO
| Service | Typical Range | What Affects It |
|---|---|---|
| Step flashing replacement only | $350–$650 | Price rises with roof pitch and number of chimney sides affected. |
| Cricket fabrication and install | $450–$850 | Higher when custom soldering is required for copper. |
| Full chimney flashing refresh | $700–$1,200 | Includes sealant refresh and minor shingle replacement around the chimney. |
| Leak tracing and targeted repair | $200–$400 | Diagnostic visit applied to repair if work proceeds same day. |
Ranges are typical for O'Fallon, MO and confirmed with an exact written quote before any work starts.
How it works
- 1
On-Site Assessment
We climb the roof from O'Fallon neighborhoods and document flashing condition, shingle age, and any visible water stains inside the attic.
- 2
Material Match and Fabrication
Pieces are cut on-site from the same profile and gauge as the original to avoid telegraphing under the shingles.
- 3
Removal and Prep
Old flashing and sealant are stripped, mortar joints are cleaned, and underlayment is inspected for rot.
- 4
Installation and Integration
New step flashing, counter flashing, and cricket are set with proper overlaps, then shingles are relaid and sealed at the top edge.
- 5
Final Water Test
We run water across the repair and check the attic for any remaining moisture before leaving the site.
Pricing
Most chimney flashing repairs in O'Fallon fall between a few hundred and just over a thousand dollars depending on access and whether a cricket must be rebuilt.