If a stain on your ceiling only shows up after a hard rain and disappears once things dry out, most homeowners assume the chimney itself is cracked. It usually isn't. The chimney and the roof meet at a joint that has to shed water two different ways at once, and that joint is flashing, thin strips of metal woven into the shingles and tucked against the brick. Flashing fails a lot more often than the masonry around it, and it's the first thing worth ruling out before anyone talks about rebuilding a chimney.
Check your attic tonight
You don't need a ladder for this. After the next rain, or right now with a flashlight, go up into the attic and look at the underside of the roof sheathing right where the chimney chase passes through. A dark ring, a water stain, or damp insulation on one side of the chimney and not the other points straight at flashing, since water tracks along the wood until it finds a low spot to drip from.
From outside, stand back from the house and look at the line where the roof meets the brick. You're looking for a strip of metal that's lifted, rusted through, or missing entirely under a bead of caulking or roofing cement. That caulk is the tell. Nobody smears roofing cement along a chimney that's never leaked. If you see it, someone already tried to stop a leak there once, and caulk alone rarely holds for more than a season or two.
What's actually supposed to be there
Proper flashing at a chimney is two separate pieces working together, not one strip of metal. Step flashing gets woven into each course of shingles as the roofer goes, so every piece overlaps the one below it and sheds water downhill. Counter-flashing then covers the top edge of the step flashing and tucks into a cut groove in the mortar joint, called a reglet, so water running down the brick can't get behind the metal.
We see two failures on London roofs constantly. The first is a roofer who re-shingled the house but left the old flashing in place instead of pulling it and starting fresh, figuring it still looked fine from the ground. The second is older homes where the reglet was cut into original lime mortar decades ago, and that mortar has since crumbled from freeze-thaw cycling, so the counter-flashing that was tucked into it now just sits loose against the brick with nothing holding the top edge tight. Either way, water gets behind the metal instead of running over it, and it takes almost no water at all once it's inside the wall or roof deck.
Aluminum flashing set directly against certain masonry or in contact with a different metal will also corrode faster than you'd expect, which is one more reason a quick patch with whatever's on the truck isn't the same as flashing installed for the actual materials on your roof.
Where a caulk gun stops being the answer
Sealing a small gap with a good exterior sealant is a reasonable stopgap if you've got an active drip and no one can get on the roof for a week. It's not a fix. Caulk sits on top of a problem that's happening underneath the metal, and it breaks down with the same freeze-thaw cycling that likely caused the leak in the first place.
An actual repair means pulling back the first few courses of shingles, replacing the step flashing so it's woven properly again, cutting a new reglet if the old mortar joint is gone, and setting new counter-flashing into it. That's roofing work and masonry work in the same repair, which is why it often gets missed. A roofer who doesn't want to touch the brick leaves the reglet as is, and a mason who doesn't do roofing won't reweave the shingles. If your chimney is leaking and the flashing looks original or the mortar around it is soft and crumbling, that's worth a proper look rather than another tube of caulk. Our chimney repair work includes flashing and reglet repair alongside the masonry, and every job gets priced after we've actually seen what's there, since the fix depends on whether it's a flashing swap or the brick around it needs work too. If the leak has already gotten into the chimney chase or the masonry itself, our piece on what chimney cracks actually mean covers how to tell a surface issue from something structural.
Frequently asked questions
Is caulking around my chimney flashing a real fix?
No, it's a stopgap. It can buy you time before a repair, but caulk breaks down under freeze-thaw cycling and doesn't address whatever let water behind the metal in the first place.
How do I know it's flashing and not the chimney crown or brick?
Location is the biggest clue. A stain that appears right where the roofline meets the chimney, and tracks along the sheathing rather than straight down the inside of the flue, is almost always flashing. Crown and brick leaks tend to show up as damage on the chimney itself, not on the roof deck around it.
Does home insurance cover a chimney flashing leak?
Sometimes, if the damage is sudden and the flashing failure caused it, but ongoing leaks from flashing that's been failing for years often get treated as a maintenance issue rather than sudden damage. Check with your provider before assuming it's covered.
How often does chimney flashing need to be replaced?
There's no fixed schedule. Good flashing installed correctly can last decades. What usually forces a replacement is a re-roof where the flashing gets reused instead of redone, or mortar around the reglet failing well before the metal itself does.