Four places:
The stain indoors only tells you where the water ended up; the trace runs top-down through each entry point to find where it came in.
The repair happens at the entry point:
In this county's wet months a small chimney leak becomes interior damage fast.
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Soot and stage-one creosote brushed out with drop cloths and a vacuum at the hearth.
Level I through III, camera down the flue, photos of what the liner, crown, and flashing actually look like.
Tar and glazed stage-two and stage-three creosote taken out with rotary tools and treatment.
Cracked stacks, failed mortar joints, damaged crowns and caps, rebuilt or repointed back to weather-tight.
Stainless steel liners sized to the appliance, for cracked clay liners or conversions to gas.
Factory-built pipe systems, chase covers, and rusted sections on newer homes and manufactured housing.
Stainless caps and screens that keep rain, birds, and fir needles out of the flue year-round.
Breathable vapor-permeable repellent on exterior brick, stops absorption without trapping moisture inside.
The metal seal where chimney meets roof, lifted or rusted sections replaced and tucked in.
Repointing, spalling brick replacement, tuckpointing, and stack rebuilds on older chimneys.
Burner, pilot, glass, and vent checks on gas fireplaces and inserts before the season.
Where the water is getting in, crown to flashing, traced and sealed at the source.
Birds, squirrels, and raccoons removed from flues, plus the cap that keeps them out.