Chimney flashing is the metal seal where the chimney passes through the roof, and on Denver metro homes it is where most chimney leaks start. When a stain shows up on the ceiling near the fireplace, flashing is the first thing I climb up to check.
A real chimney flashing repair is not caulk. It is two layers of metal: base flashing woven into the shingles, and counter flashing cut into a mortar joint on the chimney and folded down over the base layer. If that top edge is glued to the brick with sealant instead of let into the joint, the fix has a shelf life of about two Colorado winters.
Nearly every failure we take apart on the Front Range comes from one of five things: caulk only counter flashing, roofing tar over an old leak, galvanized steel rusted through, a re-roof where the old flashing got reused, or a wide chimney with no cricket behind it.
Quick answers
- Is flashing causing my leak? Usually. Flashing leaks show during wind driven rain and snowmelt.
- Can it be caulked? Sealant is a top edge detail, never the seal itself.
- Roofer or chimney company? The roofer owns the shingles and base flashing, the chimney pro owns the masonry and counter flashing.
- Do I need a cricket? Code calls for one behind chimneys wider than 30 inches.
- How long should it last? In stainless or copper, longer than the shingles. In caulk, two or three years.
What chimney flashing is and why it fails before the rest of the roof
Flashing is the transition between two materials that behave nothing alike. Your roof deck is wood that flexes and swells. Your chimney is masonry on its own footing, moving with heat and with soil that swells and settles under a Colorado foundation.
Nothing rigid survives at a joint between two things that move independently, so flashing is built as two overlapping layers instead of one sealed piece. The base layer moves with the roof, the counter layer moves with the chimney, and they shed water by gravity and overlap, not adhesion. Our guide on why chimneys leak and the four sources to rule out covers the crown, cap, and masonry, while this one goes deep on the roofline.
The four parts of a chimney flashing system
Step flashing on the sloped sides
Step flashing is a stack of small L shaped pieces, one per shingle course, running up both sides of the chimney. Each turns up the masonry a few inches and out onto the deck a few inches, and each laps the one below, which is what walks water back onto the roof. The cheap substitute is one long bent strip up the whole side, which fails fast because nothing catches water under a shingle.
Apron flashing on the downslope face
The apron, or front pan, is one continuous piece across the low side that bends out over the shingles below. It is the easiest detail to get right and the one that leaks least, so a stain at the low side usually means water entering higher and running down the wall.
The back pan or cricket on the upslope side
Behind the chimney, water arrives with all the volume the slope above collected. A folded back pan tucked under the shingles handles that on a narrow chimney, but not on a wide one. Snow sits here too: on a north facing Denver slope a drift can hold for weeks, melt on a sunny afternoon, and refreeze at night.
Counter flashing over everything
Counter flashing is the second layer, the one cut into the chimney itself. It covers the top edge of the step and apron flashing so water running down the masonry lands on metal instead of getting behind it. It is the piece most often done wrong.
Why counter flashing must go into a mortar joint, not onto the brick face
This is the detail that separates a repair that lasts from one that buys you a season. The top edge of the counter flashing has to be let into the masonry, into a groove cut in a horizontal mortar joint called a reglet, usually about an inch deep.
What a proper reglet looks like
We cut into the mortar joint, never through the brick face, because cutting brick weakens the units and opens the soft interior to water. Once the groove is clean, the flange gets bent and driven in, wedged with lead, then sealed or repointed. The metal is locked into the wall mechanically, so even with the sealant gone, water still hits metal and turns outward.
Why surface caulking fails here every time
Sealant on a brick face has almost no mechanical hold, because brick is porous and irregular. It sits fully exposed to sun, and at Denver altitude the ultraviolet load is higher than at sea level, so it hardens faster than the label promises. Then freeze-thaw gets behind the bead and pries it loose a little more each cycle.
The problem people miss is that water in a brick chimney does not only run down the face. It travels through the mortar joints inside the wall, where caulk never touches it. If your mortar is soft, the groove has nothing to hold, so the job starts with tuckpointing the chimney masonry.
The flashing failures we find on Front Range roofs
After enough roofs you stop guessing. Most flashing leaks announce themselves with a specific symptom inside the house that points at a specific failure outside.
Table 1. Leak symptoms, the flashing failure behind each one, and the repair that actually solves it.
| What you see inside | Likely flashing cause | The real fix |
|---|---|---|
| Ceiling stain by the chimney after wind driven rain | Counter flashing caulked to the brick face | Cut a reglet, set the metal into the joint |
| Water down the chase in a summer downpour | Back pan too small, or no cricket | Frame a cricket, lap metal under the shingles above |
| Rust streaks on shingles below the chimney | Galvanized flashing corroded at cut edges | Full replacement in stainless or copper |
| Tar patch that leaks again every spring | Mastic over failed metal, cracked by freeze-thaw | Strip the tar, replace the metal, rebuild the laps |
| Drips in high wind, shiny nail heads on the roof | Nail pops in face nailed flashing | Reflash with fasteners out of the water path |
| Drips during snowmelt but never during rain | Ice damming pushing water under the laps | Ice and water membrane, longer laps, a cricket |
The tar patch is the worst of that list. It hides the real failure, bonds to the shingles so they tear on removal, and turns a short repair into a day of scraping.
How Colorado weather goes after chimney flashing
Hail. The Front Range sits in one of the busiest hail corridors in the country, and thin aluminum dents. Hail also drives full roof replacements, and a re-roof is where good flashing goes to die: crews under deadline reuse the old counter flashing or surface mount a new piece with sealant, because cutting a reglet takes a grinder and a mason.
Altitude and sun. Sealants age faster up here, so any detail depending on a bead of caulk is on a shorter clock in Denver than the label suggests. That is the argument for mechanical details over chemical ones.
Freeze-thaw. Water in a joint freezes overnight and expands, opening the gap so a little more gets in tomorrow. The same mechanism spalls brick and opens mortar joints.
Wind and monsoon rain. Downslope wind pushes rain uphill, under laps that would be fine in still air, and July and August dump more water in twenty minutes than a marginal back pan can move.
When a cricket is required behind a wide chimney
A cricket, or saddle, is a small framed peak built into the roof directly behind the chimney. It ridges at the center of the back wall and slopes away on both sides, so water and snowmelt split and travel around instead of piling against the masonry.
Residential building code generally requires one where a chimney measures more than 30 inches wide across the slope, and inspectors check it. I want one on any chimney near that width on a low pitch roof, because the code number is a minimum, not a design target. For local rules, see our overview of Colorado chimney codes and regulations.
Flashing metals compared for hail and high altitude sun
Material choice matters more here than in most of the country, because our failure modes are impact and sun rather than salt air.
Table 2. Chimney flashing metals, their tradeoffs, and how each holds up in Colorado conditions.
| Material | Advantages | Drawbacks | Colorado notes |
|---|---|---|---|
| Aluminum | Cheap, easy to form on site, will not rust | Dents in hail, fatigues at bends, corrodes against fresh mortar | Common on tract homes, poor in a reglet |
| Galvanized steel | Rigid, inexpensive, takes paint | Zinc wears off, rusts at cut edges and under debris | The metal we pull off Denver roofs most |
| Stainless steel | Strong hail resistance, no rust, holds a crisp bend | Harder to form on site, costs more than steel | Our usual upgrade on roofs with hail history |
| Copper | Longest service life of the common metals, solders into a watertight cricket | Highest material cost, patina can streak a light roof | Right on a chimney you will keep for decades |
Lead sheet and lead coated copper belong on the list too, since they form tightly to the uneven joints on old Denver brick. Whatever you choose, put the better metal on the counter flashing, the piece nobody wants to remove twice.
Roofer or chimney company: who owns which part of the job
This is the question that leaves homeowners stuck between two contractors pointing at each other. Roofers own the roof plane and chimney professionals own the masonry, so the leak happens in the gap between those scopes, which is exactly where the counter flashing lives.
Table 3. How chimney flashing work divides between the trades, and what goes wrong when a piece falls through the crack.
| Part of the system | Whose trade | What happens when it is skipped |
|---|---|---|
| Shingles, underlayment, deck repair | Roofer | Rotted sheathing gets covered, not replaced |
| Ice and water membrane at the base | Roofer | No backup layer during an ice dam |
| Cricket framing and pan | Roofer | Snow and debris pile against the back wall |
| Step and apron flashing | Roofer | Water walks under the shingle courses |
| Reglet cut and counter flashing | Chimney professional | The top edge gets caulked to brick and fails |
| Tuckpointing, crown, cap, flue | Chimney professional | Metal in soft mortar has nothing to grip |
- Newer roof, sound masonry. Start with a chimney inspection. A bad crown looks identical from inside.
- Leak right after a re-roof. Call the roofer, then have the counter flashing checked independently.
- Re-roof coming up. Sequence it: masonry, base flashing and cricket, then counter flashing.
Plenty of roofers do the whole thing well, and plenty of chimney companies, ours included, handle the full system rather than hand a customer back and forth. We take these on as chimney repair calls across the Denver metro, and if it is genuinely a roofing problem we will say so.
What a proper chimney flashing replacement looks like, step by step
A full replacement is a one day job on a typical home, two days with a cricket and masonry repairs. Knowing the sequence tells you whether the crew on your roof is doing it or skipping it.
- Diagnose before demolition: interior check, attic check, then a controlled water test low to high.
- Rule out the crown and cap, since new flashing does nothing if water pours through a cracked crown. See our guide to chimney caps versus chimney crowns.
- Strip shingles back on all four sides and remove every bit of old flashing, tar, and mastic.
- Cut out and replace rotted sheathing rather than covering it up.
- Install ice and water membrane around the base and a few inches up the masonry.
- Frame, sheathe, and membrane a cricket if the chimney is wide enough to need one.
- Set the apron flashing on the downslope face, with fasteners that stay covered.
- Weave step flashing course by course, one piece per shingle, each lapping the one below.
- Install the back pan or cricket pan, lapped under the shingles above and tied into the step flashing.
- Cut the reglet about an inch deep into a horizontal mortar joint and repoint anything soft nearby.
- Set the counter flashing, wedge it, fold it over the base layer, then seal or repoint the groove.
- Water test again and check the crown, cap, and joints while the ladder is up.
Notice how little of that involves sealant. It appears once, at the top of a groove, as a secondary defense. Everything else is gravity, overlap, and metal.
What drives the cost of chimney flashing repair
I will not quote a number for your house here, because the same words cover a one story ranch with a 24 inch chimney and a three story with a wide stack on a steep pitch. National cost guides publish typical industry ranges, and they are wide for that reason.
What moves the price is scope and access: whether the base flashing is still sound, roof pitch and height, metal choice, whether a cricket has to be framed, and the condition of the mortar. For real numbers rather than a national average, our chimney and fireplace pricing page is the honest starting point. If the crown is the real culprit, that lands on chimney crown repair.
How to check your own flashing from the ground
You do not need to get on the roof, and I would rather you did not. Most of what I want to know is visible from the driveway with binoculars.
- Look for black. Roofing tar around a chimney is a confession, and it is almost always still leaking.
- Look for rust streaks on the shingles below, which mean galvanized steel that is already compromised.
- Check the top edge of the metal. A clean line disappearing into a mortar joint is right. A ragged bead of caulk on brick faces is not.
- Look for lifted metal, gaps at the corners, and fastener heads visible where water runs.
- Look behind the chimney for a flat spot holding needles, leaves, or old snow, which is where a cricket should be.
- Go into the attic the day after a soaking rain and look for dark staining on the framing near the chimney.
- Check the firebox for damp brick, efflorescence, or a rusting damper. For the whole-chimney walkthrough beyond flashing, follow our DIY chimney checkup.
Anything on that list is worth a professional look, and a proper evaluation covers the flashing, crown, cap, and flue together. That is what our guide to the three chimney inspection levels walks through.
Frequently Asked Questions
How do I know the leak is the flashing and not the crown or cap?
Flashing leaks usually show during wind driven rain from one direction and during snowmelt, and the stain lands near the ceiling beside the chimney. Crown and cap leaks show as damp brick in the firebox, a rusting damper, and staining inside the stack. The two overlap enough that only a water test from the roof, low to high, settles it.
Can chimney flashing just be caulked or tarred?
Sealant has one legitimate job, sealing the top edge of counter flashing already set into a mortar joint. Caulk on a brick face has no mechanical hold, sits exposed to high altitude sun, and gets pried loose by freeze-thaw. Roofing tar is worse, because it bonds to the shingles and hides the failure until the deck rots.
Does my chimney need a cricket?
Measure the chimney across the slope, since residential code generally requires a cricket or saddle above 30 inches wide. On a low pitch roof or a north facing slope where snow sits, I want one at anything close to that, because the code figure is a minimum.
How long should chimney flashing last in Colorado?
Stainless or copper counter flashing set into a cut reglet should outlast the shingles around it. Galvanized steel runs on a much shorter clock here, especially behind the chimney where debris holds moisture against the metal. Caulk only counter flashing often starts leaking within a few winters.
Why did my chimney start leaking right after a new roof?
Flashing is the one part of a re-roof that requires masonry work, so it is the part most likely to get shortcut in a busy hail season. We find old flashing reused under new shingles, and fresh counter flashing screwed to the brick face and caulked instead of cut into the joint. Neither is visible from the ground.
Can flashing be replaced without redoing the whole roof?
Yes. A flashing replacement pulls shingles back only around the chimney, a few courses in each direction, and they are reinstalled or replaced locally. On a roof near the end of its life, or with brittle sun baked shingles that crack on removal, combine the flashing work with the re-roof.
Get the flashing checked before the next storm cycle
Flashing is a small part of a house that quietly protects a large one. It takes the worst of our hail, sun, wind, and freeze-thaw, and when it fails it does its damage inside the walls.
If you have a stain near the chimney, a tar patch you have been meaning to ask about, or a re-roof you never had verified, get eyes on it before winter. We look at the flashing, crown, cap, and masonry together, tell you which one is passing water, and put the fix in writing. Book a chimney inspection in the Denver metro, call (720) 207-9232, or reach us through the contact page.
