Start with the stain
Where the water shows up inside narrows the path before anyone gets on a ladder. Water in the firebox or on the smoke shelf came down the flue, which points at the cap or the crown. A damp patch on the wall beside the chimney breast, or white marks on the brick in the attic, points at the masonry absorbing water. A ring on the ceiling a foot or two out from the chimney, appearing during a storm and drying between them, points at the flashing. None of that is a diagnosis on its own, and all of it saves time.
Flashing, where the chimney meets the roof
The join between a chimney and a roof is two pieces of metal doing one job. Step flashing is worked into each course of shingles as the roof rises against the brick. Counter flashing is let into a mortar joint above it and folded down over the step flashing, so water running down the brick is thrown out onto the roof instead of behind it. One failure to look for is a counter flash that was never cut into the mortar and was sealed with a bead of caulk instead. Caulk weathers and lets go; a counter flash let into the mortar joint does not.
The cricket, and the chimney that needs one
A wide chimney on a sloped roof creates a flat area on its uphill side where water and snow collect against the brick. A cricket, sometimes called a saddle, is a small pitched structure built behind the chimney to split that water and send it around both sides. Without one, a wide stack sits in a pond every time it rains hard, and no amount of flashing repair fixes a detail that is holding water against masonry by design.
Why an uncapped flue is a leak and not just an opening
Rain that falls into an open flue does not evaporate. It runs down the flue wall, soaks into the smoke chamber and the firebox, and works into the mortar from the inside where nothing will ever be seen from the roof. It also lands on creosote, which is what produces the smell people notice long before they notice the water. A cap closes the one opening that leads straight down the inside of the chimney, and fitting one is a small job next to what an open flue does to the masonry.
What happens to the house after the water gets in
A chimney leak is a slow loss, and that makes it worse than a sudden one. Plaster, framing, insulation and the back of a drywall face stay wet between storms and rarely dry out on their own, and wet material has a limited window before the problem changes from drying to mold. The EPA drying window for wet materials is 24 to 48 hours before the mold risk rises, and a chimney leak has usually been running for a season. That is why this repair does not end on the roof.
Efflorescence, the white marks that come with it
White powdery staining on chimney brick, in the attic or on a chimney breast is salt left behind when water that has passed through masonry evaporates on the surface. It is not mold, and it is not damage in itself. It is evidence: it says water has been moving through that masonry and coming out at that point. Wiping it off changes nothing, because the salt is a symptom of a path that is still open.
The season this happens in
The Atlantic hurricane season runs from June 1 through November 30, and the practical peak for wind driven rain in this market is August to October. That is the same window in which the storm and roof leak side of the restoration business is busiest. A chimney that survived four quiet years can open up in one storm.