How chimneys are supposed to be flashed
A properly flashed chimney has step flashing running up both sides, individual pieces woven in with each course of shingle, a cricket or saddle built on the uphill face for any chimney wider than about thirty inches, and counter flashing let into a cut mortar joint on the brick itself, overlapping the step flashing from above.
When a chimney was built without a cricket, snow and leaf debris pile up against the uphill face all winter, and that is almost always where the first leak shows up, whether the chimney is on a cape in Fall River or a colonial out in Dartmouth.
Where the shortcuts show up
The most common shortcut on an older repair is caulk in place of proper flashing, either sealing the joint between counter flashing and brick or, worse, replacing the counter flashing altogether. Caulk moves with temperature and dries out within a few seasons, and by the time it cracks visibly the leak has usually been running for a while already.
Reusing old step flashing while installing new shingle is another shortcut that produces a callback within a winter or two. Every flashing repair here replaces all of the metal rather than working around what is already in place.
Mortar and repointing
Where the mortar joint that holds the counter flashing has gone soft or crumbled, repointing that section comes before the new flashing goes in. Setting counter flashing into a joint with no grip to hold it just relocates the same failure a season later, so this step is not skipped even though it adds a day to an otherwise short repair.
Older chimneys on the harbor-area homes in Dartmouth and the mill-era buildings closer to downtown Fall River tend to need this more often, since decades of freeze and thaw work the mortar loose long before the brick itself shows any wear.
Signs it is time to look
A stain on the ceiling or upper wall near a chimney, daylight visible around the chimney chase from inside the attic, or visibly rusted and lifted flashing metal from the ground are all reasons to have it looked at before the next storm rather than after.