The short answer
Most warning signs can be seen from the ground. Look for crumbling or recessed mortar joints, bricks flaking at the face, a cracked or loose mortar cap around the pots, a stack that leans, gaps along the lead at the point it reaches the slope, and brown damp patches on the chimney breast indoors. One sign alone may be minor. Two or three together mean a roofer should look soon.
A ground-level checklist of what a failing chimney stack looks like, outside and in, and which signs to act on first.
What shows from the ground
You do not need a ladder, and you should not use one for this. Stand well back on a clear day, on your own land or the pavement opposite, with a pair of binoculars, or the camera zoomed in. Start at the top and work down the stack, noting anything that looks out of line, uneven in colour or different from the neighbouring stacks on the street.
At the top, look at the cap of mortar around the pots, known as the flaunching. It should be smooth and sloping, to carry rain away from the pots. Cracks, gaps around the pot base or a flaunching that has crumbled mean rain is getting into the flue. Then look at the pots themselves, which should sit straight with no cracks. A pot that tilts or has lost a piece can drop further.
Next, the brickwork. Open or hollowed-out joints show where the mortar has worn away. Faces coming off the bricks and leaving pitted patches are called spalling, and our answer on why chimney bricks spall explains the causes. Finally, check the line of the stack against the sky or the neighbour's stack. A lean, even a slight one, is worth a closer look, and our page on the leaning chimney stack shows what a roofer checks.
Where the stack meets the roof
The join between the stack and the roof slope is where many leaks start. Strips of lead, called flashings, are tucked into the mortar joints and dressed over the tiles to keep water out. Look for lead standing off the brick, split along a fold, or smeared with sealant. A long dark stain under the stack on the roof, or green moss gathering only on one side, suggests water is running where it should not.
You may also see loose or slipped tiles around the base, or a gap where the mortar fillet has dropped out. These cases are different from a general roof fault because the cause sits in the stack, and fixing the tiles alone will not stop the drip.
Aerials and satellite brackets add a strain of their own. Bolted into the brick, they can crack joints as the bracket moves in wind. For the lead itself, our page on chimney flashing replacement explains when a patch is enough and when the whole run goes.
Photos taken from one spot every few months make changes easy to see, and they give a roofer a record to compare against when they visit.
Clues on the chimney breast indoors
Water from a failing stack often shows indoors before it shows outside. Look at the chimney breast in the loft and in the bedrooms below. Brown or yellow stains, peeling paint, wallpaper lifting away and a damp smell that rises and falls with the weather all point upward. Stains that spread after heavy rain, then dry in a dry spell, usually mean the cause is rain getting in, not condensation.
In the loft, check the brickwork for dark streaks, powdery white deposits or damp patches on the timbers next to the stack. Chalky deposits on the brick are another sign that water is moving through the stack.
Fallen debris matters too. Bits of mortar or brick in the fireplace, on the roof or in the gutters are a sign the stack is shedding material. Soot flakes falling into a room can mean a cracked flue lining. If a flue serves a gas appliance and you notice fumes or soot staining, switch the appliance off and get an engineer in before anything else.
What to do once you have spotted a sign
Write down what you saw and where, take photos from the ground, and note the date. Then ask a roofer for a survey. A good survey covers the stack from the roof, with a photo of each part, so you see what the roofer sees.
The urgency depends on the sign. A loose pot, a visibly leaning stack or falling bricks are safety issues: cordon off the ground beneath and arrange a make-safe. Open joints and a worn cap are not an emergency, but water gets in a little more each winter, so do not leave them for years.
As a national guide, Checkatrade's April 2026 chimney repair table gives flashing at £250 to £1,000, repointing at £300 to £1,000 and a cap at £180 to £520. What you would pay is set by the survey findings and the access to the stack, and a table price may exclude VAT, so ask. The chimney repairs hub covers the full list of jobs.
Send us your photos and the address, and we will look at the joints and the lead before anything else.
More on this in Chimney Repairs in Crawley
Two warning-sign questions
Is a hairline crack in the mortar cap a problem?
A fine crack is an early warning rather than an emergency, but rain finds it, freezes in it and widens it. Take a photo and watch it after the next hard frost. If it spreads, or the cap has begun to part from the pots, have the flaunching renewed before water reaches the flue.
Can I check the stack myself from the loft?
You can look at the breast and the timbers beside it from the loft hatch, using a torch, as long as you stay on the joists and never step between them. Do not touch loose brick. Anything that looks damp, dark or crumbling is worth photographing and passing to a roofer.