The short answer
Not bad for every job, but frost is hard on fresh mortar, so repointing, rebuilding and flaunching are best done in milder spells. A dangerous or leaking stack should not wait: it is secured at once, and the lasting repair is booked for suitable weather. Replacing a cowl, a pot or a flashing can often go ahead in winter if the access is safe.
Why mortar and frost do not mix
Mortar sets by a chemical reaction that needs time and moisture. Fresh mortar holds a lot of water, and if it freezes before it hardens, the ice expands inside it. The result is a weak, crumbly joint that has to be raked out and redone.
A chimney is a tough place for the job. It is at the top of the house, wind passes round all four sides, and a stack dries quickly in some weather and stays wet in others. Cold air removes warmth from a small thick mass of masonry faster than it does from a wall.
That is why repointing, rebuilding courses of brick, re-forming flaunching and rendering are weather-sensitive jobs. The mortar manufacturer's instructions give the minimum temperatures, and a careful crew reads the forecast for the days after the work as well as the day itself.
Some workarounds exist, such as covering fresh work with sheeting or hessian, but they limit the risk and do not remove it. Where frost is forecast, the sensible choice is to wait.
Lime mortars on older brickwork are especially slow to set and especially vulnerable to frost. Our guide to chimney repointing with lime or cement explains why the choice of mortar matters.
What can go ahead in cold weather
Not every stack job depends on curing. Replacing a loose pot that can be bedded in a suitable mix on a mild day, fitting a cowl to a sound pot, putting a mesh bird guard on or taking off an unsafe section are all possible when the surface is safe to work on.
Lead work is often possible in cold weather, as the metal is dressed by hand and held with clips and wedges, but the surface behind it must be dry. A flashing fitted to a wet wall will trap moisture, so a dry day matters more than a warm one.
Taking a stack down is a mechanical job. If a stack is dangerous, dismantling it and capping the roof opening can proceed in the cold, because it does not rely on mortar setting.
The limiting factor is again safety. Wet slate, ice on a ladder and wind round the top of a roof make work at the chimney hazardous, and a good crew will stand down when it is not safe.
Our answer on roofing work in winter covers the wider roof, and the one on chimney cowls describes the options for the pot.
Making safe first, repairing properly later
Winter is when stacks fail, so many winter chimney jobs are in two parts. The first part is the make-safe: loose bricks or pots are taken down, a leaning top is dismantled, the opening is capped and the roof is covered so water cannot get in.
This takes away the hazard and stops the damage getting worse. It is not a repair, and you should not be told it is. The aim is to give you time.
The second part is the lasting repair, planned for a mild, dry spell. It might be a rebuild of the top courses, a full repoint, new flaunching and flashing, and a cowl. The survey and photographs taken at the first visit define the scope, and a single written figure covers it.
In between, watch the loft and the chimney breast for damp and tell us if anything changes. A temporary cap needs to stay in place, and a tarpaulin that flaps in a gale has to be checked.
If you have an insurance claim, the insurer will want photographs and receipts for the make-safe. Our answer on storm-damaged chimneys and insurance explains how.
Planning the work around the weather
Weather decides the programme, so ask for a plan that allows for it. A fixed finish date in winter is a promise nobody can keep. A better arrangement is a stated sequence, a note of which tasks depend on conditions and a statement of what happens if a spell of frost arrives.
We do not give response times, because the speed of a repair depends on access, weather and what the survey finds. What you can expect is a plain account of what is waiting on conditions and what the next step is.
If you know a stack is tired, a survey in autumn gives time to plan the work for a settled spell. That is often cheaper than an emergency in January.
The best conditions for mortar are mild, dry and calm, with no frost forecast for several days. Late spring and early autumn often offer them, but the weather has the last word.
Our chimney repairs page shows the jobs that usually go with a stack repair, and the yearly maintenance checklist helps you spot problems early.
