Chimney flashing is the metal seal at the point where your chimney passes through the roof. It is the most common source of chimney water intrusion on Chicagoland homes, and it is regularly misdiagnosed as a roofing failure. If you have water stains near your fireplace, if your attic shows moisture at the chimney base, or if a roofer replaced shingles and the leak persisted, the flashing connection is where the investigation needs to start.
A flashing failure delivers water directly to the chimney-roof junction and into the structure beneath it. Unlike a mortar joint leak or a crown crack, failed flashing bypasses the chimney masonry entirely and sends water straight into the attic framing, insulation, and ceiling below. It accounts for a majority of chimney-related water complaints on the 1960s to 1980s homes that make up a substantial portion of the residential stock across Glenview, Northbrook, and the broader Chicagoland service area.
Delta - Masonry and Tuckpointing has worked on chimneys across Chicago’s North Shore and northwest suburbs since 1987. For the full scope of what can go wrong with a chimney, see the chimney maintenance checklist for homeowners and why your chimney leaks when it rains.
What Chimney Flashing Is and How It Works
Flashing is a layered metal assembly that seals the gap where the chimney penetrates the roof. It has two main components that work together.
Step flashing consists of individual metal pieces, typically galvanized steel or aluminum, installed one piece per shingle course along the two sides of the chimney where it meets the sloped roof. Each piece is L-shaped, with one leg under the shingle and the other leg against the chimney face. During a proper roofing installation, these pieces are woven into the shingle courses so that water running down the roof slides off each shingle and onto the next step flashing piece without any gap for entry.
Counter-flashing is the upper seal. It is embedded in the chimney mortar joints, laps over the top of the step flashing, and covers the edge where the step flashing meets the brick. The counter-flashing’s job is to prevent water running down the chimney face from getting behind the step flashing from above. It is anchored by being mortared into horizontal joints cut in the chimney.
The front of the chimney, facing downhill on the roof slope, uses a saddle or apron flashing. The back of the chimney, which faces into the roof slope and receives the most water accumulation, should have a cricket, a small peaked structure that diverts water around the chimney. Chimneys wider than 30 inches without a cricket are chronic leak points. NFPA 211, the national standard for chimneys and fireplaces adopted by most Illinois building codes, addresses chimney construction and maintenance requirements that include proper flashing installation.
When flashing is new and properly installed, this system works well. The problems develop over time as the components age.
How Flashing Fails on Chicagoland Chimneys
The Chicagoland climate creates a thermal expansion problem that is unique in its intensity. A chimney in Glenview or Northbrook goes from below-zero January temperatures to 90-degree August temperatures within six months. The Great Lakes region experiences some of the highest freeze-thaw cycle frequency in the country, and the metal flashing expands and contracts with every temperature swing. The mortar holding the counter-flashing in place does not flex the same way. Over decades, this differential movement works the counter-flashing loose from its mortar seat.
Once the counter-flashing has lifted, even partially, the seal is broken. Water running down the chimney face finds the gap and enters behind the flashing. At this point the step flashing below it is irrelevant because the water is entering from above the step flashing, not from below it.
In Glenview, the community’s documented primary masonry concern is chimney flashing failure. The documented pattern is consistent: a 1974 ranch home on which new step flashing was installed, yet the persistent roof leak traced to the chimney-to-roof connection where deteriorated mortar had allowed the counter-flashing to pull free. The roofer saw intact step flashing and concluded the problem was elsewhere. The masonry contractor found the counter-flashing sitting loose in a mortar joint that had deteriorated to the point it could no longer anchor anything.
This misdiagnosis pattern is not unusual. Roofers are equipped to assess shingles and roof membranes. Counter-flashing embedded in masonry is masonry work. The gap between those two trades is where the diagnosis gets lost.
Caulk over failed flashing is the temporary fix that creates a longer-term problem. Many Chicagoland chimneys have visible beads of roofing sealant or urethane caulk running along the flashing line. This is almost always a previous contractor’s attempt to seal flashing that was already failing. Caulk does not flex with the thermal cycling described above. Within two to three seasons in this climate, caulk over a chimney flashing joint cracks and admits water. The underlying problem is still there.
The Northbrook Pattern: Chimneys Most Exposed
Northbrook split-levels, ranches, and colonials from the 1960s to 1980s building boom have chimneys that are now 40 to 60 years old. The community’s documented masonry concerns identify chimneys as the most exposed masonry element on these homes. On a 1968 Northbrook split-level, the brick walls are partially protected by overhangs, landscaping, and adjacent structures. The chimney stands fully exposed, takes weather on all four sides simultaneously, and has no protection from the prevailing northwest winds that characterize Northern Illinois winters.
On these homes, the original flashing was typically galvanized steel. Galvanized steel has a service life of 30 to 50 years under normal conditions, as confirmed by Chicago’s documented temperature range from the National Weather Service, which spans roughly 130 degrees Fahrenheit annually. That means original flashing installed during the 1960s and 1970s is at or past its service life. If it has not been replaced, it should be inspected. The counter-flashing on these chimneys was mortared into joints that are themselves at the 40 to 60 year mark. Whether the mortar or the metal fails first, the result is the same.
For any Northbrook home built in this era, a chimney inspection that includes a roof-level examination of the flashing condition is not optional maintenance. Flashing failure often has no visible exterior sign until the water stain appears on the ceiling below. For a list of the external warning signs that appear before the interior damage, see 5 signs your chimney needs immediate repair.
Distinguishing a Chimney Flashing Leak from Crown and Mortar Joint Leaks
Three distinct failure points on a chimney produce water intrusion: the flashing, the crown, and the mortar joints above the roofline. They produce different leak signatures, and identifying which is present determines whether you need a roofer, a masonry contractor, or both.
Flashing leaks appear at the chimney base where it meets the roof. Water enters here and travels into the attic at the chimney penetration point. Staining on rafters or sheathing directly adjacent to the chimney, water on the attic floor at the chimney base, and ceiling stains in the room below the chimney are all flashing signatures. If there is a discrete water entry point at the roof line, flashing is the first suspect.
Crown leaks deliver water down through the flue. The chimney crown, the masonry or concrete slab that sheds water off the top of the chimney, sits directly above the open flue. When the crown cracks, water enters from the top, runs down through the flue, and appears as moisture inside the firebox or efflorescence on the firebox interior walls. The spring chimney crown damage post describes how Illinois winters crack crowns and what the damage looks like after freeze-thaw cycling. For a complete comparison of cap and crown functions and failures, see chimney cap vs chimney crown.
Mortar joint leaks occur when the joints between the chimney bricks above the roofline have deteriorated to the point that water is absorbed directly through the wall. This produces efflorescence on the chimney face, staining that runs down the outside of the chimney, and eventually water in the chase or around the firebox from lateral penetration. Mortar joint leaks are addressed through tuckpointing the chimney. For a full breakdown of what happens during a tuckpointing job, see what happens during a tuckpointing job.
In practice, these failures often appear together. A 1965 Northbrook chimney may have all three: a flashing joint that has opened, a crown that cracked two winters ago, and mortar joints above the roofline at 50 percent of their original depth. Addressing one without the others simply moves the water entry point.
Wilmette: Aging Mortar Above the Roofline on 1930s to 1950s Homes
Wilmette’s housing stock is predominantly mid-century brick, with a median home age of 1948. The community’s documented primary masonry concerns specifically identify aging chimney mortar above the roofline on 1930s to 1950s homes as one of the three primary masonry concerns. The mechanism is direct: chimneys are fully exposed to wind, rain, and temperature swings, and on Wilmette homes from this era, the original lime mortar in chimney joints is often the first masonry to fail.
Wilmette’s proximity to Lake Michigan adds a dimension that inland communities do not face. The high humidity from Lake Michigan proximity keeps the chimney masonry wet for longer periods than in drier inland climates. Wet mortar is more vulnerable to freeze-thaw damage. According to the National Weather Service Chicago climate normals, the Chicago metro area sees highly variable temperature swings that accelerate freeze-thaw cycling through the shoulder seasons. A Wilmette chimney from 1940 has experienced roughly 50 to 70 freeze-thaw cycles per winter for more than 80 years. At that cumulative load, the original lime mortar in the exposed chimney joints has long since passed its useful life.
What makes Wilmette chimneys different from a Northbrook chimney of similar age is the moisture content of the brick going into the freeze-thaw cycle. Wilmette’s lake-effect humidity means mortar joints that appear dry are often holding significantly more moisture than comparable joints on an inland home. A Wilmette chimney from 1942 that received no maintenance can be in worse structural condition than a Northbrook chimney from the same year, even accounting for similar physical exposure.
For Wilmette homeowners, chimney inspection encompasses the mortar joints above the roofline, the crown, and the flashing as a comprehensive assessment. Each element is at the age where failure is the rule, not the exception.
What a Proper Flashing Repair Involves
Counter-flashing repair is masonry work, not roofing work. A contractor who is not working in mortar joints is not doing the repair correctly.
The correct sequence starts with removing the deteriorated mortar from the horizontal joint where the counter-flashing is embedded. BIA Technical Note 7B specifies a minimum removal depth of 3/4 inch for repointing preparation. The slot must be clean and fully prepared before the new counter-flashing is installed. NPS Preservation Brief 2 confirms the same minimum depth standard for historic masonry, applicable to any pre-1960 chimney where lime mortar was the original material.
New counter-flashing, typically 26-gauge galvanized steel or aluminum, is bent to the correct profile, inserted into the prepared slot, and mortared in place with appropriate mortar. The counter-flashing should lap over the top of the step flashing by at least 3 inches and should overlap the chimney face by at least 4 inches to prevent water running down the chimney from getting behind it.
The front apron flashing must also be inspected and replaced as needed. On chimneys wider than 30 inches, the absence of a chimney cricket on the uphill side is a structural leak point regardless of flashing condition.
Caulk as the sole repair for failed flashing is not a repair. It is a temporary patch. In this climate it has a service life of one to three seasons. If your chimney was repaired with a caulk application and is leaking again, the counter-flashing needs to be pulled, the mortar joints prepared properly, and the flashing reinstalled and mortared. That is the repair.
The Cost of Not Addressing Flashing
Chimney flashing failure that is unaddressed for more than one or two seasons typically causes damage that extends well beyond the chimney itself. Water that enters through a failed flashing joint on a Glenview ranch does not stay at the chimney. It runs down through the attic insulation, saturates the ceiling drywall below, and may reach the framing before it becomes visible on the ceiling surface.
By the time a homeowner sees a ceiling stain, the water has often been entering for months. Insulation has absorbed significant moisture. If the moisture is persistent, mold follows. The cost of addressing ceiling drywall repair, insulation replacement, and potential mold remediation alongside the chimney flashing repair is substantially higher than the cost of addressing the flashing at the first sign of a problem.
For a detailed look at how chimney maintenance prevents this cost escalation, see the chimney maintenance checklist for homeowners. For cost context across all chimney repair scopes, see chimney tuckpointing cost Illinois 2026. Flashing failures do not self-correct.
Scheduling Chimney Flashing Inspection
For homeowners in Glenview, Northbrook, Wilmette, and Libertyville who are seeing water near the chimney or whose chimney inspection is overdue, Delta - Masonry and Tuckpointing provides roof-level inspections that examine the flashing, crown, mortar joints, and cap as a complete system. We have worked on chimneys across Chicago’s North Shore and northwest suburbs since 1987. Call (847) 713-1648 or contact us online to schedule a free chimney assessment. Our inspection includes photographs of every element at the roof level so you see exactly what we see.
For related reading, see chimney cap vs chimney crown and 5 signs your chimney needs immediate repair. All chimney repair services are available throughout our service area.
The stain on the ceiling near your chimney is almost never a roof problem. The flashing seal at the chimney-roof junction is where most of that water enters.