Why chimneys leak so much more than roofs
A roof slope is one continuous surface doing one job. A chimney is a column of brickwork punched straight through that surface, sticking up into the worst of the weather, made of a material that soaks up water and held together with mortar that does not last as long as the bricks do. It is the most exposed and most complicated thing on the building.
It also leaks in a way that disguises itself. Water enters at the top or the sides, runs down inside the stack or along a rafter, and appears somewhere else entirely - often on a bedroom wall a metre away from the breast, which sends people looking at their windows or their pointing. Damp on a chimney breast in a loft is worth a proper look even if nothing downstairs has stained yet.
The places it actually gets in
Cracked flaunching at the top. Open joints where the pointing has washed out. The back gutter behind the stack, which fills with debris and then overflows under the tiles. Perished lead soakers and step flashings down the sides. And, on a disused stack, straight down an open pot that nobody ever capped.
Repointing, and doing it with the right mortar
Repointing a stack is straightforward work that is easy to do badly. The old mortar gets raked out to a decent depth - not scratched at the surface - and replaced with something appropriate to the brick. On the older soft reds you see around Whitstable and Faversham that means a lime-based mix, not a hard cement one.
This is not fussiness. A cement mortar is stronger than a soft Victorian brick, so when the wall moves or when trapped water freezes, the brick face blows off instead of the mortar. You end up with a stack that sheds its bricks rather than its pointing, which is a much more expensive problem. It is one of the few things on a roof where the cheap option actively causes damage rather than just wearing out sooner.
Salt air and lead
Close to the front at Herne Bay, Tankerton or Reculver, the thing that shortens a chimney's life is not rain, it is salt. It draws moisture into the brickwork and keeps it there, it accelerates the corrosion of any ferrous fixing near it, and it crystallises inside the pores of soft brick and pushes the face off from the inside.
It is also a reason to be generous with lead rather than clever with it. Lead does not care about salt, which is why it is still the material of choice on the exposed coast a couple of thousand years after somebody first worked out you could beat it into shape.
Capping a stack you no longer use
Most houses around here have at least one flue doing nothing. Left open, it collects rain, and every litre that goes down it has to evaporate back out through the brickwork and the plaster of the room below. Sealed up completely, it goes damp and musty for the opposite reason - no air movement at all.
The answer is a capping pot or a vented cowl that keeps the rain and the birds out while letting the flue breathe, plus an airbrick at the bottom if the fireplace has been closed off. It is a cheap job that solves a surprising number of "rising damp" diagnoses in upstairs bedrooms.