Chimneys easily suffer damage from exposure. The pointing often gets weathered and washed out at a greater rate than on the rest of your buildings walls. This, combined with the effects of flue gases, may lead to chimneys being rebuilt several times during the life of a building.

Bird nests can block flues. Use vented caps or metal cages to prevent this from happening.

What we see from the ground is the chimney stalk rising above roof level. In stone chimneys, if the stones are not well bedded, they can be at risk of collapse or being dislodged in high winds. Repointing or rendering may be required. Problems may not be easily visible from ground level and need professional inspection from scaffolding or a cherry picker.

Chimneys that are still used to draw smoke from a fire should be swept once a year to remove soot deposits from the inside of the flue. If this is not done, the deposits can build up and catch fire.

Some insurance policies require an annual chimney sweep – ask for a certificate to prove this. Chimneys that are in use should be lined – flexible steel liners may be best.

Even if chimneys are not used, they still require attention as falling pots or stones are dangerous. Unused flues should be vented top and bottom to prevent damp.

Types of chimneys

Wallhead chimneys

Wallhead chimneys are built on top of the front and back walls of a tenement. They normally carry flues from bedrooms. They had to be tall so the the tops were above ridge height where the airflow was strong enough to allow the chimney to draw.

Rebuilt wallhead chimney. Note the wide coping designed to throw rain clear of the stonework below.

Technical information on repairing wallhead chimneys

Mutual chimneys
Mutual chimneys are built on top of the gable wall and will often carry flues from the neighbouring tenement as well as from your own. Repair costs will be shared between both tenements.

Mutual chimney. Note render problems.

Technical information on mutual chimneys

What to look out for

Leaning or cracked chimneys and leaning chimney pots are dangerous and require immediate attention.

TV aerial and satellite dish fixings

Satellite dishes on chimneyhead. Damage can be caused not just by the fixing straps but also workers accessing the roof.

Even if corner plates are used on the cables fixing aerial masts or satellite dishes, they can work loose and the cables damage the render or stone. Try to organise a communal TV aerial system as this can help reduce unnecessary access to the roof.

In situ concrete copings installed on rebuilt chimneys may have no drip or weathering to throw water off the face of the chimney stalk.

Plant growth indicates problems. Plants grow in damp conditions and in open joints. Roots then force stones apart.

Plant growth can be a problem and the build up of plant growth and moss will accelerate the decay of the chimneyhead.


Copings are important because they protect the chimney underneath by ensuring water is thrown away from the face of the building, preventing damage to the stone or render underneath. A throating (a channel on the underside of the coping) stops water running along the underside and down the face of the chimney.

Rusting iron cramp (staple). The stone has been forced off, damaging the coping and letting water in.

Coping stones are sometimes held together with iron cramps. These tend to rust and can then damage the stone. Loose copings can be dangerous. Poorly weathered copings lead to saturation of the render or the stone of the chimneyhead, causing eventual damage.

If the coping is in reasonable condition, then it can be retained. If there is no throating, then the decision has to be made as to either renew the coping or to install a stainless steel channel under the coping to throw water away from the face of any new render.

New concrete coping being formed. Copings are required, even on lowered chimneys.

Replacement copings may be made by forming a timber shutter around the chimney and pouring in concrete. As they are formed in one piece they often crack with time. A stainless steel channel can also be used to provide an effective throating to the coping. Some contractors use old cable or rope fixed to the shuttering to make a throating, but this rarely forms a good drip.

Replacing the coping with stone is a more durable repair solution than using concrete, and may be required in listed buildings and conservation areas.

Stonework problems

Decayed stonework in chimney. Caused by weather, flue gases, poor copings and inappropriate cement repairs.

The stonework on gables is exposed to the weather, and salts inside the flues can damage the stonework as well. This leads to a breakdown of the stones that bond the flues together, and you sometimes get bulges in gable end chimneys and weathering of the stonework adjacent to the chimney flues.

If you have a stone chimney and the stone is largely intact, then try to keep it that way by replacing damaged stone with new stone that closely matches the existing stone.

Sometimes all that is needed is some repointing of the stonework and coping. Use one of the more durable lime based mortars.

If the cracks are wide and the chimney shows signs of leaning or missing pots, then it could be dangerous and may need rebuilding.

Replacing (indenting) new stones may cost more than rendering the chimney with a strong lime based render, but will last a lot longer. The decision of how best to repair will depend on the extent of the defect and the rest of the fabric.


Chimney previously rebuilt in brick. Render repairs now required. Note the ventilation cowls. The concrete coping is also faulty being built off-level.

Although the chimneyheads would originally have been formed in stone, many have been repaired over their lifetime and often get rebuilt in common brick, then rendered over in cement.

This lasts for a while but often starts to crack and come loose from the brick or stone base. This render may crack due to frost and chemical action, and eventually come loose. If the render needs replaced, it may be worth applying it to a stainless steel mesh fixed to the chimney. A stainless steel channel may also be used to provide a throating. Your architect or surveyor should advise you on this.

Flues and flue liners

Each flue serves a single fireplace. The flues were often built of brick. The ‘bridges’ or ‘feathers’, the bricks separating the flues, were often poorly tied in. As combustion products (salts from coal and gas fires) can damage the flue bridges, the bricks may have fallen into the flue. This can either block the flue or allow smoke and fumes to drift from one room to another.

A chimney sweep can usually tell you how good the flues are by carrying out a smoke test, where a smoke bomb is let off in one fireplace and then the flues are checked to make sure the smoke is coming out of a singe flue.

Flues and flue liner. There are different grades of flue liner for gas and coal/wood fires.

New flue liners can be installed to deal with the problem of broken flues. The cowl should be fitted either directly into the coping or as an insert to a chimney pot connected to the flue liner. These flexible liners are held in place with a collar at the head of the flue inside the coping.

Where the chimney is used for a gas appliance then it will need to comply with the Gas Regulations with a regulation liner and gas terminal. Gas inspections may reveal a need for common or mutual repairs. You may also need to have gas safety checks on all appliances connected to the chimney.

Stainless steel flue liners should be checked periodically as they can corrode and may need replacing.

When repair work is underway, gas appliances and flues may need to be disconnected and the flues checked for any blockages before reconnection.


Even if the fireplaces have been removed, it is wise to retain the ventilation that the flue provided. Ventilation through the flue helps to keep the hidden areas behind wall linings dry. Ventilation can be maintained by installing a grilled vent at the sealed fireplace and having the chimney pot capped with a vent cowl. Clearly there is a balance to be made between controlling ventilation and reducing draughts, but flues should never be completely sealed. An alternative is to partially seal the flue with an inflated ‘chimney balloon’, although newspapers are also often used.

Pots and cowls

New coping. Note the flaunching holding the pots in place

The chimney pot should be set into a recessed seating in the top of the cope and then ‘flaunched’ with mortar in the gap.

Cracked pots can work loose in storms, so can be dangerous. Most pot types can be replaced with matching pots.

If a chimneyhead has been reformed with an in situ cope, then the chances are that the pots have been cast into place. Whilst this holds them securely, if a pot breaks, it can make replacement much more difficult.

Chimney pots (cans) should have appropriate terminals or cowls depending on how they are used and may need bird guards.

When to rebuild a chimneyhead

This requires judgement and close inspection. In most cases, the condition of the chimney will be worse than when seen from ground level. Close inspection will reveal how badly stones are weathered or if the render applied to a chimney is loose and needing replaced.

Stone chimneys that are leaning, out of alignment or clearly have large cracks in them, will almost certainly need to be rebuilt. If your building is listed or in a conservation area, you may need consent to get the chimney rendered, lowered or capped.

Rebuilding a chimney in stone is the preferred solution, if the repair is to last. Rebuilding in a good quality facing brick may last for some time, although this rarely matches in with the look of the stone.

Who pays?

You need to check your title deeds to see if they say chimneys are common property, in which case every flat owner pays a share. If they say nothing about chimneys, then costs are shared between the flats whose flues share the stack. The cost of repairing a mutual chimney is shared between flat owners on both sides of the gable wall.

Professional help recommended?

Although the work may appear straightforward, ensure your builder or tradespeople have the skills for the job. If in any doubt, get professional help to specify and organise the repair.

Further information

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