When it comes to appreciating Britain’s chimney covered, rooftop panoramas, watching Coronation Street or Mary Poppins is about the closest many of us get. But many of the nation’s chimneys are in a dire state of repair, concealing potentially life-threatening defects.
Badly neglected antique masonry could become dislodged by strong gusts of wind, sending loose bricks or even whole chimney pots crashing down and they can even become leaky and damp compromising their structural integrity. It therefore pays to occasionally cast a skyward glance in the direction of your chimney stacks and periodically check their condition.
- Chimney construction
How to repair a leaking chimney
- How to prevent damn in a chimney
- Repairing cracked or damaged brickwork
- Leaning chimney stacks
- Damaged chimney pots and flaunching
Even in parts of the country where stone was the traditional building material, the Victorians often used brick for chimneys, as it is better able to withstand intense heat.
Chimney pots perform a very practical purpose in improving efficiency by raising the height of the chimney without the need to build excessively tall and bulky stacks. This helps to prevent down-draughts and smoky fires, which can still be a problem today where properties are overshadowed by tall nearby trees, roofs, hills or high-rise buildings.
Replacing a chimney pot
Where original chimney pots have been removed, it is normally possible to source replacements online, from salvage yards, or at garden centres, where they are frequently sold as outdoor ornaments.
Pots were secured in place into the upper chimney brickwork and bedded in thick layers of mortar, or ‘flaunching’. Victorian chimneys were commonly built into the party walls of terraces and semi-detached houses as very wide structures containing multiple flues. As well as being economical to construct, this helped insulate them from the cold.
Flues, which comprise the internal space enclosed by the chimney brickwork, were lined internally with a layer of lime render, or ‘parging’, to protect the masonry and prevent smoke escaping through any gaps in the mortar joints.
Dealing with chimney defects
Exposed for well over a century to the worst the British weather can throw at them, and under attack internally from intense heat and chemical erosion, many chimney stacks are now in need of a little TLC. There are a number of defects that can afflict old chimneys, but as scaffolding is a major part of the cost of repairs, it can make sense to have any roof works carried out at the same time.
Damp patches and brown water stains on upstairs ceilings, chimneybreasts and internal walls are commonly the result of leaks around the edges of chimneystacks where they meet the roof. Victorian houses sometimes had flashings made from cheaper zinc or just thick strips of mortar, which are prone to cracking.
How to repair a leaking roof and chimney stack
- Durable, long-lasting lead is the best form of weatherproofing around these joints.
- Defective junctions between stacks and roofs, or walls, should be stripped and covered with new lead flashings cut into a stepped pattern, tucked into the mortar joints, and pointed up.
- Avoid cheaper modern GRP (glass-reinforced plastic), short-life tapes or cement strips.
- Where flashings have come loose they can be refixed into existing joints with fresh mortar, or a new stepped line cut into the side of the chimney and the flashing fixed into the groove and sealed.
A small amount of rain entering an open chimney pot is to be expected and is normally harmless, soaking into the internal chimney brickwork. Originally the flow of hot air from fires below would facilitate evaporation, but in disused flues rain can reach further down, where it mixes with old soot, potentially seeping through to the plasterwork.
Where flues have been protected with modern flue liners, rainwater that was previously soaked up may run straight down the new liner forming puddles in the fireplace. Damp in fireplaces can also be caused by condensation from burning unseasoned logs.
How to prevent damp in your chimney
Protect chimney pots from rain ingress by ‘capping’ with a specially designed cowl or ‘rain hat’. The choice of fitting must be suitable for the type of fire or appliance. Cap off redundant flues and ventilate to prevent condensation.
Exposed chimney masonry can erode, sometimes resulting in small patches of the brickwork outer face dropping off. Rendered stacks are especially at risk as small cracks tend to develop as the masonry repeatedly heats and cools, allowing water to penetrate and freeze, loosening the render.
Chemical erosion from acidic gases inside the flue is another cause of damage. Any resulting gaps in joints can allow fumes to seep into the house. This is especially dangerous where gas fires have been installed in old fireplaces without first fitting a flue liner, as they can be a source of deadly carbon monoxide.
How to repair damaged chimney brickwork
- Cut out and replace any severely damaged brick or stonework, although the odd ‘spalled’ brick isn’t necessarily an issue.
- Any badly eroded old mortar joints will need to be repointed.
- In extreme cases where cracking or erosion is advanced enough to threaten structural problems, the stack will need to be taken down and rebuilt using a suitable sulphate-resistant mortar.
- Defective render can be hacked off and made good with a fresh coating.
- Installing a flue liner should prevent erosion internally.
It’s not unusual for old chimneys to have a slight lean but in most cases they will still be stable. A structural engineer can confirm whether a lean is beyond acceptable tolerances. The cause is usually due to a combination of external wind pressure and persistent dampness and frost action on one side of the stack, exacerbated by chemical attack inside the flue on the cold side causing uneven movement.
Thinner chimneys built on outer walls are generally most at risk. On rare occasions movement may be a result of incompetent structural alterations, such as removal of chimneybreasts or poorly undertaken loft conversions.
How to deal with a leaning chimney
- Stacks can sometimes be stabilised with stainless-steel straps or traditional ‘stay bars’.
- Where leaning is severe, at least partial rebuilding may be necessary, or stacks may need to be taken down to below roof level or entirely rebuilt.
- To prevent existing movement getting worse, strengthen the stack by repointing eroded mortar joints and installing a flue liner.
Over time the mortar flaunching at the base of the pots can eventually start to crack and disintegrate. Despite the weight of the pots, there is a risk that storm-force weather could then displace them. Using binoculars, look for signs of frost erosion and flaking, or cracks and damage on the surface of the pots.
Fixing a damaged chimney pot and flaunching
- Take down and replace broken pots, if possible with matching reclaimed ones.
- Secure unstable pots by repairing any damaged brickwork and flaunching, and also check the condition of any TV aerial fixings and, if possible, relocate them to the loft.
- Cut out and replace any very loose flaunching.