Chimney Flashing Leaks: Causes and Repair Options

Flashing failure is a frequent source of chimney-related water damage in homes across Chicagoland. The flashing is the metal assembly that seals the joint between your chimney and the roof, and when it fails, water enters the house at one of the most structurally sensitive locations: where a masonry column penetrates the roof plane. This post covers why chimney flashing fails, how to identify the source of a flashing leak, and what repair involves.

What Chimney Flashing Is and How It Works

Chimney flashing is a multi-layer metal system that waterproofs the joint between the chimney and the roof. A properly installed flashing system includes three main components:

Step flashing: Individual pieces of metal bent at 90 degrees and woven into the shingle courses along the sides of the chimney. Each piece overlaps the next course below, creating a stepped drainage path that carries water away from the chimney face and onto the roof surface.

Counter flashing: Metal embedded in the chimney mortar joints that laps over the top edge of the step flashing. The mortar joint embedding seals the top edge, and the overlap with the step flashing below creates a two-layer barrier. Counter flashing is the component most often seen from the ground; it is the metal band that runs along the chimney face at the roof level.

Rear cricket or saddle: On wider chimneys, a cricket is needed behind the chimney to deflect water around the base. Without a cricket, water ponds against the back of the chimney, accelerating flashing failure and creating a persistent water concentration point at the chimney base.

All three components work together. Failure in any one creates a water path into the structure. The most common failure mode is the sealant between the counter flashing and the mortar joint cracking and separating, which allows water to enter between the flashing and the chimney face.

Why Flashing Fails in Chicagoland’s Climate

Thermal cycling is the primary mechanical cause of flashing sealant failure. The metal flashing expands and contracts with temperature changes throughout the year. Sealant at the counter flashing-to-mortar joint is subjected to these movements for every heating and cooling cycle, and over years of cycling it loses elasticity and cracks. Chicagoland’s temperature range from below zero in January to 90-plus in July creates a large annual thermal swing that accelerates this process.

Freeze-thaw cycles compound the problem. Water that enters a hairline sealant crack freezes and expands, enlarging the crack. In inland Cook County’s repeated freeze-thaw cycles each winter, a minor sealant gap becomes a functional leak path within one to two seasons.

The Roof Replacement Trigger

Roof replacement is the single most common trigger for new chimney flashing leaks. A correct roof replacement should include either replacing the flashing entirely or re-integrating the existing flashing into the new shingle course pattern. When roofers skip this step or do it improperly, the first rain produces a leak.

The error pattern is consistent: step flashing not woven into the new shingle courses (laid flat on top of the shingles instead), counter flashing not re-embedded after the mortar joint was disturbed, or the rear joint between the counter flashing and chimney face left unsealed. All of these produce active leaks that trace directly to the chimney-roof junction.

For homeowners who have had a recent roof replacement and now have interior water staining near the chimney, our chimney flashing repair service covers the diagnosis and repair of post-replacement flashing failures.

Locating the Source of a Chimney Flashing Leak

Water entry at a chimney junction can trace to multiple sources: flashing failure, crown failure, a missing or failed cap, deteriorated mortar joints in the upper chimney, or a combination. A chimney inspection that traces the path from both the roof and the interior is the reliable way to separate these sources.

From the interior, the stain location gives clues:

• Staining on the ceiling immediately adjacent to the chimney, on the attic framing around the chimney penetration: likely flashing or upper masonry
• Staining on the interior chimney wall face below the roofline, not at the ceiling: often a combination of flashing and mortar joint failure
• Staining inside the firebox or on the firebox back wall: more likely crown, cap, or liner issue rather than flashing

From the roof, the inspection checks for lifted flashing edges, failed or absent sealant at the counter flashing-to-mortar joint, step flashing that is not woven into the shingle courses, and rear flashing condition and cricket presence or absence on wider chimneys.

What Flashing Repair Involves

The repair scope depends on what the inspection finds. Three levels of intervention are common:

Sealant refresh: When the flashing metal is in good condition and only the sealant joint between the counter flashing and chimney mortar has failed, cleaning the joint and applying a new polyurethane or EPDM sealant may resolve the leak. This is appropriate when the flashing geometry is correct and the metal is not corroded or lifted.

Counter flashing re-embedding: When the mortar joint embedding has failed or the counter flashing has pulled out of the joint, the repair involves removing the failing mortar in the relevant joint courses, re-embedding the counter flashing, and repointing with Type N mortar (ASTM C270, minimum compressive strength 750 PSI for above-grade residential). On historic brick, the mortar match is critical; using modern Portland-heavy mortar on pre-1920 soft brick accelerates spalling at the joint.

Full flashing replacement: When the step flashing or counter flashing metal has corroded through, has been improperly installed, or needs to be fully replaced to match a new roof system, the repair involves removing the affected flashing components and installing new material. Material choices include aluminum, galvanized steel, and copper. Copper is the longest-lasting option and is appropriate for Wilmette and Highland Park historic homes where the flashing may not be replaced again for decades.

For homes in Des Plaines and Arlington Heights where the housing stock spans multiple eras, the flashing repair approach depends on the chimney and roof configuration. In Des Plaines, the Des Plaines Community and Economic Development Department handles permits. The postwar ranch stock on the southern areas of the city often has side-of-house exterior chimneys with exposed step flashing on multiple roof faces, which creates more total linear flashing length and more potential failure points than a center-of-roof chimney.

How Flashing Failure Connects to Other Chimney Problems

Flashing failure does not occur in isolation on older chimneys. The same thermal cycling that cracks sealant also works on mortar joints, crown material, and cap fittings. A chimney with failing flashing often has concurrent crown cracking or mortar joint deterioration that contributes to the water entry.

Addressing the flashing in isolation without checking the crown, cap, and upper mortar may not fully resolve the leak. A complete inspection documents which components are contributing to water entry so the repair scope covers all of them in one mobilization rather than requiring a return visit.

The chimney crown repair post covers the crown component specifically, and signs your chimney needs repair before winter covers the ground-level indicators that often accompany flashing failure. The chimney inspection guide for Chicagoland homeowners details what NFPA 211 inspection covers at each component level.

Schedule Your Flashing Inspection and Repair Estimate

Delta - Chimney Repair and Services has handled chimney flashing repair across the North Shore and northwest suburbs since 1987. We serve Oak Park, Evanston, Des Plaines, and Skokie, along with the broader Chicagoland area.

We trace the leak source before recommending a repair scope, and we provide a written estimate that separates the flashing work from any concurrent crown, cap, or masonry findings. Call (847) 685-1043 or use our contact form to schedule your inspection.

Failed flashing is the number one cause of chimney-related roof leaks, and the most common repair we do after a new roof goes on a home.