1. The crown

The crown is the slab on top of the masonry, sealing the brickwork around the flue opening. A proper crown is cast concrete, sloped to shed water, overhanging the brick by an inch or two with a drip edge cut underneath so runoff falls clear instead of running down the face. It should also have an expansion joint around the flue tile.

What most houses actually have is a thin skim of ordinary mortar, flush with the brick, with no overhang and no drip edge. Mortar is not a weather surface. It cracks within a few years, water runs straight into the top of the wall, and freeze-thaw damage starts at the top and works down.

2. The cap, or the absence of one

A chimney with no cap is an open pipe into your house. Rain landing in an open flue runs down onto the damper and the smoke shelf. The damper rusts, first stiffening and then seizing. Mixed with creosote, that water forms an acidic sludge that attacks mortar joints from within the flue.

It is also an open invitation to squirrels, raccoons and nesting birds.

3. The flashing

Where the chimney passes through the roof there is a permanent gap between rigid masonry and a roof deck that moves. Flashing bridges it, and it is two layers, not one: step flashing woven in with the shingle courses, and counter flashing whose top edge is set into a groove cut in the mortar and folded down over the step flashing.

It fails three ways. The mortar groove erodes and the sealant shrinks, so the counter flashing lifts. The metal corrodes after decades of thermal movement. Or — most commonly on re-roofed houses — somebody ran a thick bead of roofing tar around the joint instead. Tar is rigid, the joint moves, and it cracks within about two years. Because it is black and opaque you cannot see that it has failed.

Telling them apart

Water appearing only after wind-driven rain from one direction points at flashing. A stain directly around the chimney breast in an upstairs room or the attic points at flashing or crown. Damp inside the firebox, or a rusted damper, points at a missing cap. White chalky streaks on the outside brick usually trace back to the crown.

In Batavia there is an extra factor. A lot of older property here has limestone in the chimney, and limestone is more porous than fired brick. It absorbs water readily and must be pointed with a soft lime-based mortar — repointing it with a hard modern mix traps moisture and makes the stone itself spall.

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