Why is water coming down my chimney breast?
Possible causes include failed chimney flashing, cracked mortar or render, defective chimney pots or caps, penetrating rain and, in some cases, condensation within an unused flue. A proper inspection should distinguish the chimney defect from nearby roof-covering problems.
Why chimneys leak
A chimney is a masonry structure passing through the weatherproof surface of the roof, so it depends on several details working together: the flashing where it meets the covering, the pointing and brickwork, the flaunching at the top, and the cap or terminal.
Any one of those failing can let water into the stack, where it travels down and shows on the chimney breast, sometimes in a room well below the roof.
Staining that is not a leak
Not all chimney breast staining is rainwater. Disused flues can produce salt staining from historic soot deposits, which draws moisture and marks plaster in dry weather too.
A roofer will look at the whole stack rather than one element, because patching the flashing on a chimney with failed pointing generally moves the problem rather than solving it.