The most exposed
masonry on
your house.
A chimney gets weather on four sides and above. It fails before anything else on the roof, and when it does the water comes in somewhere that looks nothing like a chimney problem.
Four sides,
no shelter.
Every other wall on the house has something protecting it. A chimney stack stands clear of the roof with weather hitting it from all directions and driving down into it from above. It also goes through the biggest temperature swings on the building, because a flue that is in use gets hot and then cools.
That combination erodes mortar joints on a stack far faster than on the elevations below. It is normal for a chimney to need repointing when the walls beneath it are still perfectly sound, and it is why a stack is often the only masonry on a house that has been repointed twice.
The other reason to take it seriously is what is underneath. A stack sits directly over the roof covering. Water that gets into the masonry does not stay in the masonry, it runs down into the roof structure and into the ceiling below.
The cement collar
at the very top.
The flaunching is the sloped mortar bed at the top of the stack that holds the pots in and sheds water off the top course. It is the single most exposed piece of mortar on the house and it is the first thing to crack.
Once it has cracked, water goes straight down inside the stack rather than off it. That is how you get damp staining on a chimney breast in a bedroom with no obvious external cause, and it is why people spend money on the inside of that wall repeatedly without success.
Pots themselves crack, and disused pots are a specific issue. A flue that is no longer used should be capped so rain cannot get in, but capped with a vented cowl rather than sealed solid. A completely sealed flue has no airflow, so any moisture in the masonry has nowhere to go and the chimney breast goes damp from the inside instead.
That distinction, capped and ventilated rather than simply capped, is the detail most commonly got wrong.
Where the stack
meets the roof.
The joint between a stack and the roof covering is closed with lead: a front apron, side soakers and stepped flashing, and a back gutter behind. It is a system, and it fails at the weakest part.
Common failures are lead that has been repaired with mortar fillets, which crack within a couple of winters, and cement rendering over the joint instead of proper lead. Both are cheap fixes that guarantee a return visit. Failed flashing produces damp at ceiling level near the chimney breast, usually only after driving rain from one direction.
The back gutter is the part most often missed because you cannot see it from the ground. On a stack sitting mid slope it is the piece doing the hardest work.
All of this needs a scaffold, which is why chimney work should be planned alongside the roof, the gutters and any upper level pointing rather than as a separate visit. Access is usually the largest single line on a chimney job.
What your report
probably said.
Typical wording includes the flaunching is cracked and requires renewal, the stack requires repointing, the lead flashings are showing signs of deterioration, and disused flues should be capped and ventilated.
Reports also raise something structural about chimneys. Where a chimney breast has been removed in a room below but the stack above has been left, surveyors ask whether the remaining masonry is properly supported, because a chimney breast taken out without a gallows bracket or a steel beam leaves several tonnes of brickwork resting on nothing. That is a different job and a more serious one.
If your survey mentions both the stack and support to removed masonry, treat them as two separate items. One is weathering, the other is structural.
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