A chimney stack takes weather on every side, sits above the roof line where the wind is strongest, and on most houses has not been touched since it was built. It is also the one element whose failure can be dangerous rather than merely expensive, which earns it a closer look than its size suggests.
All of the useful inspection can be done from the garden with binoculars and from the loft with a torch.
The four things to look at
The pointing between the bricks, which weathers faster up there than anywhere else on the house. The flaunching, which is the sloping mortar bed holding the pots. The flashing where the stack meets the roof. And the line of the stack itself against a vertical reference such as a door frame or a downpipe.
Missing pointing, cracked or crumbling flaunching, lead lifting away from the brickwork and a stack that is visibly out of plumb are the four findings that matter, in ascending order of urgency.
A leaning stack is urgent
Stacks lean for a reason, and the common one is sulphate attack: combustion products in an old flue have reacted with the mortar, which expands on one side and bows the stack. It will not correct itself and the mortar that is holding it is the mortar that is failing.
That one warrants getting a roofer out to look at it, not a line added to a list of jobs for the spring. The same applies to a stack with open cracks, with visibly displaced brickwork, or where a pot is loose in its bedding.
Damp on the chimney breast
Staining on the breast inside, often in the upstairs room beside the fireplace, is usually one of four things: failed flaunching letting water down the flue, failed flashing at the roof junction, eroded pointing on the stack, or an unventilated capped flue holding condensation.
The pattern helps. Damp that worsens days after rain suggests water coming down the flue. Damp present in cold dry weather, sometimes with a brown tar staining, suggests a sealed flue or old soot drawing moisture.
Capping a flue, properly
An unused flue should be capped against rain but left ventilated, which means a cowl or a vented cap at the top and an airbrick or vent at the bottom. Air then moves gently through the flue and keeps it dry.
Sealing both ends is the classic error and it reliably produces damp: the flue becomes a cold closed shaft, moisture condenses in it, and old soot deposits are hygroscopic enough to hold that water against the brickwork.
Flashings and the roof junction
A proper junction has stepped lead dressed into the mortar courses, with an apron at the front and often a back gutter. Mastic smeared along the joint is a temporary measure, and finding it means that junction has already leaked and been patched.
Look for lead that has lifted, slipped or torn at a fold. Lead work has a long life but it moves with temperature, and the fixings at the top edge are what usually fail first.
If the fireplace is still used
Have the flue swept at the proper interval for the fuel and keep the certificate. That is a safety matter rather than a maintenance one, and it also means somebody competent is looking up the flue annually.