Repointing, rebuilding or removing a chimney

A chimney is the most exposed masonry on the house. It is wet on all four sides, nothing shelters it, it dries slower than a wall, and it takes the full wind at the highest point of the roof. Almost everything that goes wrong with one starts at the top and works down.

The flaunching goes first. That is the sloped mortar cap the pots sit in — a thin bed of mortar with nothing supporting it, baking in the sun and shrinking at night until it cracks. Water gets into the crack, freezes, and levers the crack wider. Below it the joints erode: rain washes the fines out of the mortar a fraction at a time and wind-driven grit does the rest, until the joints are hollow and each brick is bearing on its middle instead of its bed.

Then the bricks themselves come under attack from two directions. From outside, freeze-thaw: water sits in the face of a soft London stock brick, freezes overnight, and pops the face off in flakes. From inside, sulphates — decades of flue gases condensing in an unlined flue leave salts in the mortar, the salts take up water and swell, and the joints on the weather side slowly grow. That is why an old stack usually leans away from the direction the rain comes from rather than falling straight.

The test is simple enough to do from a loft window with a car key. Scrape a joint. If the mortar crumbles to sand and you can get 10mm in without effort, the pointing has failed. Then look at the bricks either side: if they are hard, square at the edges and their faces are intact, the brickwork is fine and repointing is the whole job.

Done properly it means raking the joints out to at least twice their width — 20mm or more, by hand — and filling them in two passes so the mortar is compacted rather than smeared on. A joint scratched out to 5mm and skimmed over looks the same on the day and falls out in three winters.

The mix matters more than most people are told. Most Victorian and Edwardian stacks around Camberwell are soft London stock bricks laid in lime mortar. Point them in a hard cement mix and the mortar ends up stronger than the brick, so when the wall moves or salts push outwards, they come out through the brick face instead of the soft joint. The bricks spall, and in ten years you have a rebuild instead of a repoint. A lime or weak hybrid mix is the right answer on old stock brick and it does not cost more.

One thing repointing will not do is stop a leak coming through the lead. If the damp patch is in the ceiling beside the stack rather than in the chimney breast itself, the flashing is the more likely cause and it is a separate job.

Rebuilding is not about age. Plenty of 130-year-old stacks are sound. It is about whether the brickwork can still be relied on to stand, and there is a short list of signs that say it cannot.

The stack is taken down course by course to a level where the brickwork is sound — usually a course or two below the roofline, because the masonry inside the roof space stays warm and dry and is nearly always in better condition than the part in the weather. Then it goes back up in reclaimed or matched brick, with new flaunching and either new pots or the old ones re-set.

Budget for new lead at the same time. The flashings have to come off to take the brickwork down, and old lead that has been dressed once, lifted and re-dressed is not worth putting back on a stack you have just paid to rebuild. Doing the flashing while the stack is open and the scaffold is up is far cheaper than coming back for it.

Scaffold is not optional for this. A single slipped slate is a ladder job; dismantling and rebuilding masonry at ridge height with materials going up and rubble coming down is not.

People say 'take the chimney out' and mean very different amounts of work. The cost and the paperwork change completely depending on how far down you go.

In a South London terrace the stack usually straddles the party wall and serves flues from both houses. Cutting away from a party structure is work covered by the Party Wall etc. Act 1996, which means written notice to the neighbour — normally two months before work starts — and, if they do not consent, a surveyor appointed to settle an award. That is a legal step rather than a courtesy, and it is the single most common reason a chimney removal is still not booked three months after the quote.

There is a practical side to it too. If a flue on the neighbour's side is still in use, you cannot leave it unsupported, unweathered or half a stack tall. Sometimes the answer is that only your half comes down and the exposed face is rebuilt and weathered; sometimes it is that the removal is not worth the trouble and a rebuild is the better buy. We would rather say that at the survey than find it out on the scaffold.

A flue is a shaft of damp masonry running through the middle of your house. While the fire was in use, air moving up it carried that moisture away. Seal it top and bottom and the moisture has nowhere to go, so it moves sideways into the breast.

It gets worse than plain damp. Old soot and flue salts are hygroscopic — they pull water out of the air — and they wick through to the plaster, where they show as brown-yellow tide marks that come back through fresh emulsion again and again. People repaint three times before anyone mentions the capped flue.

The fix is airflow, not a better seal. A capped stack wants a vented cap or capping slab at the top and an airbrick or vent into the flue at the bottom, in the room. A stack that stays up but goes out of use wants a vented cowl on the pot rather than a solid cap. It is a small amount of money at the time and a very large amount of decorating avoided later.

Chimney work is priced per job, not per hour. On smaller jobs the biggest number on the quote is usually not the brickwork.

Access first. A stack at the ridge of a three-storey terrace with rear access only through a neighbour's garden costs more to reach than to repair, and no honest quote can be given for it over the phone.

After that: the size of the stack and how many pots it carries; whether the bricks can be matched, because reclaimed London stocks cost real money and modern equivalents look wrong on a front elevation; whether any flue is still live, which decides how much of the work is masonry and how much is capping, venting and lining; and how much plastering and decorating follows the work indoors.

One more, worth saying plainly: on a stack that is only tired, repointing and re-flaunching is the right answer and it is a fraction of the cost of the rebuild you may have been quoted. We will tell you when that is what we can see.