Every other part of a roof has something beside it. A chimney stack rises clear of the ridge, with weather arriving on all four faces and nothing to shelter it. That is why it is often the first part of a house to show the effects of a hard season: a loose pot after a gale, a patch of crumbling brick after a freeze, brown marks up the chimney breast after a week of wind-driven rain.
Winter does not attack a stack in one way. It uses three. Frost works on water held in the masonry. Wind works as a lever on anything standing up from the stack, especially pots, cowls and aerials. Rain driven sideways works on the joints, flashings and flaunching (the sloping mortar collar that seats the pots). This guide takes each in turn, explains how to spot the damage from the ground, and sets out when repairs can be done safely and when they have to wait. The single fault of crumbling brick has its own answer, why chimney bricks crumble; here we look at the season as a whole. For everything else on stacks, see our chimney repairs page.
How frost gets into a stack
Brick, stone and mortar are porous. Even a sound stack takes in water after a spell of rain, and a stack, unlike a wall, sits under no cover of its own and has no heated room behind it to dry it out. When the temperature falls below freezing, the water inside the surface turns to ice and expands. Repeat that often enough and the skin of the brick or stone comes away in flakes or sheets. That is spalling.
The damage is seldom dramatic in any one night. It builds over many freeze and thaw cycles across a winter, and it usually concentrates where the masonry stays wettest: the top courses just below the flaunching, the windward face, and any joint that has already failed.
Soft brick, hard mortar
Older bricks are softer than modern ones. If they were repointed in hard cement mortar, the water cannot leave through the joints, so it leaves through the brick face instead, and the brick takes the strain of every freeze. Cement used on soft stone behaves in the same way. This is one of the commonest reasons we see a stack with sound mortar and ruined bricks. Our answer on the best mortar for chimney repairs sets out why the mix matters.
Spotting it from the lane or garden
Look for brick dust or flakes on the roof below the stack, slate or tile faces pitted with fragments, a rough or powdery look to the upper courses, and mortar that has gone sandy and recessed. Binoculars help. None of this is an emergency by itself, but all of it means water is getting in.
Wind on pots, cowls and the stack itself
Wind does a different job. It does not need water or ice; it simply pushes. A pot stands up from the flaunching like a short post, and a cowl or a TV aerial on the side of the stack makes a longer lever. A gale gives that lever a hard shove, again and again. If the flaunching has cracked or the mortar behind the pot has weakened, the pot rocks. Eventually it can lean, crack or fall.
Why loose pots matter
A pot that falls from a stack lands on the roof or the garden below, or on the lane, and a clay pot is heavy. After a bad blow, it is worth looking up from the ground, from a safe distance, for pots at an angle, missing or sitting off-centre. Do not stand under the stack while you look, and keep children and pets clear. Our guide to chimney pots, cowls and caps explains how these fittings are meant to be fixed.
Aerials and satellite dishes
Brackets bolted to a stack add load and make holes in the masonry. Over a few winters, the movement of the aerial in the wind opens the surrounding joints. If your aerial is on the stack, our answer on whether a TV aerial damages your chimney covers the options for moving it.
Leaning stacks
A stack that has begun to lean is a different and more serious matter than a loose pot, because the whole shaft is moving. If you see a stack out of true after a storm, keep people away from underneath, and call us or another roofer so that it can be made safe first. Our page on leaning chimney stacks covers causes and repairs.
Driving rain: joints, flashings and the back gutter
The third winter problem is rain that arrives sideways. Wind pushes it into every gap on the windward face, and if the pointing is sandy or cracked, the water goes deep into the stack and stays there. A saturated stack is also more vulnerable to the next frost.
Where the water goes in
The weak points are the same each time. Open joints in the brickwork or stonework let water into the masonry. Cracked flaunching lets it down beside the pots and into the flue. Flashings, the strips of lead that seal the stack to the roof covering, can lift in the wind, and a lifted flashing lets water run behind it onto the roof timbers. At the back of the stack, where the roof slope meets the shaft on its high side, a lead channel called the back gutter carries water round to the sides. If it fills with moss or leaves, it overflows.
Signs indoors
The first sign is often damp on the chimney breast or ceiling, brown stains, peeling paper, or a smell of wet soot after rain. Our answers on damp patches on a chimney breast and why chimneys leak help to trace the cause. Not every stain comes from the stack, and a roofer who can see the whole route is better placed to say.
A chimney that is not in use can take in rain through the open pot and send it down the flue. That is one reason an unused flue is closed with a vented cap that shuts out rain and still passes air, a topic covered in should an unused chimney be capped?
After a storm: what to do and what to leave alone
If you think the stack has been hit, the first rule is to stay out from underneath it. Falling brick and clay pots are far more dangerous than they look.
Immediate steps
Photograph the stack from the ground with a phone or a long lens, on the day, and note the date. Keep any fallen pieces: do not throw them away. If anything has dropped onto the roof, check the loft or ceiling below for wet. If water is coming in, put a bucket under it and move belongings. Then call a roofer. Leaks and storm damage get priority with us, and the first job on a leaking roof is making it safe, well before a permanent repair is discussed.
What to leave alone
Stay off the roof, and do not rest a ladder on a damaged stack. Do not try to patch with sealant or a bag of mortar from a ladder. A temporary fix by a non-roofer can hide the problem and complicate an insurance claim. Our page on chimney damage and home insurance explains the evidence to collect.
What a make-safe looks like
A make-safe is the minimum needed to stop the danger or the leak: taking down a loose pot, securing a cowl, covering an exposed flue or sheeting a hole. It is not the repair, and it adds to the cost. On a national basis, MyBuilder (2026) quotes £500 to £1,500 for storm-damage fixes in its roof repair guide. The permanent repair follows once the weather allows and the quote is agreed.
When repairs can safely be done
People are surprised that a chimney repair can sometimes wait for better weather even when the damage looks urgent. There are two reasons: safety for the person working, and what the materials need.
Safety
Working at height on a stack in high wind, on wet or icy slates, or in poor light is dangerous, and a decent roofer will not do it. Scaffolding helps a great deal, but even a scaffold cannot make gale-force gusts safe for lifting heavy pots. In practice, the make-safe may go ahead in poor conditions if there is a real hazard, while the proper repair is booked for a calmer day.
Mortar and frost
Mortar needs time to set. Fresh pointing, flaunching or bedding can be damaged if it freezes before it has hardened, and wet mortar laid on a freezing, saturated stack will not bond well. Lime mortar is particularly slow to set and must be kept from frost and from drying too fast. For that reason, major repointing or flaunching is best done in the milder, drier weeks of the year, usually spring to autumn, and covered with hessian or sheeting if the forecast turns.
Timing the work
If you find damage in the middle of winter, the practical plan is a make-safe now, a full survey, and a repair booked for the first reliable window. Waiting is not always careless; a rushed repair in freezing conditions can fail within the year. Our answer on how long a chimney repair takes gives typical durations and the effect of weather.
Preparing a stack before the cold weather
The least costly chimney repair is the one done before winter arrives. A short check in early autumn can head off most of what this guide describes.
Looking up from below
Stand back in the garden or on the lane and study each stack. Are the pots straight? Is the flaunching a clean, sloping collar, or cracked and crumbling? Do the joints look full, or sandy and recessed? Is there a green or black stain below the stack that suggests water is running down one face? Does any flashing look raised or gapped?
What a roofer adds
Close up, we check the soundness of the brick or stone with a probe, the depth of any recession in the joints, the condition of flaunching and flashings, the back gutter, and whether the pots and cowls are securely fixed. We photograph each face and send you the pictures. That is how we price a repair before anyone goes onto the scaffold. Repointing is the usual cure for open joints; our guide to chimney repointing describes the method.
Costs to weigh
MyBuilder's chimney repair guide (updated 21 August 2026) gives national figures: repointing averages about £750 within a range of £500 to £2,000, and flashing repair runs £250 to £1,200. A few hundred pounds of maintenance in autumn compares well with a rebuild at £2,000 to £10,000 or more. For the wider picture, see our chimney repair cost guide. What we send after surveying your stack is a written quote for the whole job.
On a protected house, ask the council before materials change. The work is guaranteed for 10 years in writing, and we clear the site when we finish.