A modern twin wall insulated flue may need to be installed to replace deteriorating flue accessories in old chimneys, to maintain fire protection.
Older chimneys are commonly lined with lime mortar parging, accorrding to the Society for the Protection of Ancient Buildings, although these may need to be replaced if they show signs of damage.
Obvious indicators of deterioration which may be easy to spot are signs of debris falling down into the hearth, but a keener eye may also be able to spot signs on the external masonry of chimneys and flues.
When it comes to making repairs, the society advises: "Repairs to parging or withes may be possible where safe access can be provided, otherwise the insertion of a modern flue lining might have to be considered.
"Providing some ventilation is maintained, disused flues can be capped after sweeping to keep out birds and help prevent dampness."
If chimneys and fireplaces are still in use, the main reason for maintenance is to reduce the risk of fires, although the upkeep of flues is also key for ensuring the overall longevity of buildings.