Why chimneys are the most common roof leak point
Chimneys leak more often than almost any other part of a roof because they are a large opening where two very different materials meet and move independently. The roof is wood framing that swells, shrinks and deflects under snow load. The chimney is heavy masonry on its own foundation. The flashing is the only thing bridging that moving joint.
Calgary makes that joint work hard. Freeze-thaw cycles run from fall to spring, and a chinook can lift temperatures well above freezing in an afternoon before the cold snaps back. Metal expands and contracts, mortar cracks, sealant pulls away from brick, and meltwater gets pushed into every gap. A wide chimney also catches a lot of water and snow on its uphill side.
How chimney flashing works
Chimney flashing is a system of overlapping metal pieces that lets water run over the roof-to-chimney joint instead of into it. Each layer laps over the one below, so water always flows outward and downhill, just like shingles.
Base or apron flashing
The apron is a bent piece of metal across the downhill face of the chimney. It sits on the shingles below and turns up against the brick, carrying water back onto the roof.
Step flashing woven with shingles
Along the sides, L-shaped pieces of step flashing are installed one per shingle course, each tucked under the shingle above and over the shingle below. Metal and shingles are woven together, so the overlap keeps shedding water as the roof moves.
Counter flashing set into the masonry
Counter flashing covers the upturned edge of the step flashing. It is set into a mortar joint or a slot, called a reglet, cut into the brick, then bent down over the step flashing. The two layers are not fastened together, so roof and chimney can move without tearing the seal.
Back pan, cricket or saddle
On the uphill side, a narrow chimney gets a back pan that turns water around the corners. Wider chimneys need a cricket or saddle: a small peaked roof framed behind the chimney that splits water and snow around both sides.
Ice-and-water shield underneath
Beneath the metal, a self-adhering ice-and-water membrane covers the deck and laps up the chimney sides. It seals around nails and catches water that backs up during an ice dam or heavy melt.
Signs your chimney flashing is failing
The most common sign of failing chimney flashing is a water stain on the ceiling or wall beside the chimney that appears after heavy rain or snow melt. Watch for:
- Stains or peeling paint on ceilings and walls next to the chimney.
- Water in the attic around the chimney chase: damp framing, dark stains on the deck or wet insulation.
- Rust streaks on the brick or on the shingles below the chimney.
- Lifted, loose or bent metal, especially counter flashing pulled out of the mortar.
- Gaps filled only with caulk or roof tar, often layered from earlier quick fixes.
- Damaged shingles at the base: curled, cracked or missing granules.
- Efflorescence, a white chalky residue, on interior brick in the attic or around the fireplace.
Flashing leak or masonry leak?
A chimney leak is a flashing leak when water gets in at the roof line, and a masonry leak when it comes through the chimney itself. They can look alike from inside, so match the symptom to the likely source.
| Symptom | Likely source | Who fixes it |
|---|---|---|
| Stain low beside the chimney during heavy rain or melt | Step flashing, apron or back pan | Roofer |
| Leak on the uphill side after snow builds up | Missing cricket or ice dam behind the chimney | Roofer |
| Counter flashing pulled out of the joint | Failed reglet or worn mortar at the flashing line | Roofer, sometimes with a mason |
| Water in the firebox, damp brick high above the roof | Cracked crown or missing cap | Mason or chimney specialist |
| Brick stays dark and wet long after rain | Porous or spalling brick | Mason |
| Crumbling or recessed mortar between bricks | Mortar joints needing repointing | Mason |
| Attic frost around the chase in cold snaps, no rain | Condensation from warm indoor air | Roofer for attic airflow, plus air sealing |
We work on the roof-to-chimney connection: flashing, crickets, and the shingles and underlayment around the chimney. Crowns, repointing, caps, liners and fireplace work belong to masonry and chimney trades. If the masonry is part of the problem, we will tell you, because new flashing cannot stop water coming through the brick above it.
Why caulk and tar aren’t a repair
Caulk and roof tar are not a chimney flashing repair because they depend on a sealant bond, not overlapping metal, to keep water out. That bond does not last on a Calgary roof: prairie UV bakes the sealant, and freeze-thaw cycles and chinook swings flex the joint until it cracks and peels off the brick.
Cracked tar can make things worse by holding water against the masonry, hiding rusted metal and gumming up shingles. Properly lapped flashing does not need a perfect seal. Water simply runs over each layer and off the roof.
How we repair chimney flashing
A proper chimney flashing repair rebuilds the whole connection in layers, from the deck up:
- Remove the shingles around the chimney to expose the old flashing and deck.
- Remove the old flashing and failed sealant, including built-up tar on the brick.
- Check the deck and underlayment for rot and replace damaged sheathing.
- Apply ice-and-water shield to the deck and up the chimney sides.
- Install new apron flashing on the downhill face.
- Weave new step flashing into each shingle course along both sides.
- Frame and flash a cricket where the chimney is wide, or install a back pan on a narrow one.
- Cut reglets and set new counter flashing over the step flashing.
- Reinstall or replace the shingles to match the roof.
- Clean up old metal, nails and debris.
Chimney crickets (saddles) and Calgary snow
A chimney cricket keeps snow and meltwater from piling up against the uphill side of a chimney, where many Calgary chimney leaks begin. Without one, a wide chimney acts like a dam: snow drifts in behind it, melts on sunny days and refreezes overnight into ice that holds water against the flashing until it finds the top edge of the metal.
A cricket splits that flow around both sides so nothing sits still long enough to find a gap. On roofs that hold heavy snow, roof snow removal after a big storm also takes pressure off the chimney.
Flashing repair during a roof replacement
The best time to replace chimney flashing is during a roof replacement, when the shingles are off and the whole connection is exposed. New step flashing only works when it is woven into the new shingle courses.
Reusing old flashing is a common shortcut. That metal may be rusted, bent during removal or sized for a different shingle, and it has already flexed through years of Calgary winters. With asphalt roofs lasting about 15 to 20 years here, the flashing should last as long as the shingles over it.
Metal chimney chases and other penetrations
Framed chimney chases and other roof penetrations follow the same flashing principles as a brick chimney. A chase clad in siding still needs apron, step and counter flashing, plus a cricket if it is wide.
B-vents and plumbing stacks use collars or boots lapped under the shingles above and over the shingles below. Skylights use their own layered flashing; see skylight installation. Frost dripping around a chase in cold snaps is usually a roof ventilation and air sealing issue, not a flashing one.
What affects the cost of chimney flashing repair
The cost of chimney flashing repair depends mainly on chimney size, how hard the roof is to work on, and how much damage the leak has done:
- Chimney size and cricket need. Wider chimneys take more metal and may need a cricket.
- Roof pitch and height. Steep or two-storey roofs need more safety setup.
- Masonry condition. Crumbling mortar affects how counter flashing is set, and some chimneys need a mason first.
- Deck damage. A long-running leak can rot sheathing that must be replaced.
- Shingle matching. Older or discontinued shingles can be hard to match.
- Season. Winter work may mean temporary protection now and permanent work later.
Ready to stop the leak for good? Book a free inspection or contact us with a few photos of the chimney.