Chimney flashing is the metal that seals the gap where a chimney passes through your roof. It is not one part but a layered system: step flashing woven into each course of shingles, base and apron flashing at the front, and counterflashing that tucks into the mortar joints of the brick. When any layer fails, water runs down the chimney and into the deck, often several feet from where you finally see the stain.
Most Columbus homeowners find out the hard way. A brown ring on the ceiling near the fireplace, a musty smell in the closet against the chimney wall, or a drip during a hard rain out of the west. Because the water travels along framing before it drops, the visible spot rarely sits directly under the leak, which is why chasing it with more caulk almost never works.
You likely need this work if the flashing looks rusted, if you can see tar or roofing cement smeared around the base, or if the counterflashing has pulled loose from the brick. On homes built during the Cummins-era boom of the 1950s through 1970s, the original galvanized steel has usually rusted through by now. If the leak only shows up in winter, the cause may be ice, not flashing, and we will tell you that before you spend money.