In the past few years in London and surrounding counties, we have experienced an increase in calls concerning dampness to internal chimney breasts.
Chimney stacks are open to the elements and should the chimney stack not be correctly maintained, defects like damp can occur. The main causes of damp in chimneys are rain, condensation or salt contamination.
Salt contamination especially in older chimney breasts is a common problem and is usually a result of fossil fuels like coal being burnt in the fire place. Because coal comes from the ground it can contain salt minerals like nitrates, chlorides, sulphates etc. When the coal is burnt these salts manifest themselves in the brickwork of the fireplace. Over a period of time and especially if the fireplace is no longer used as such the salts can migrate from the brickwork and settle in the wall plaster. These salts then become what is technically known as hygroscopic, which means that they can attract and hold atmospheric moisture from the surrounding environment.
This is why damp patches associated with salt contamination are often worse when the weather is wet.
However, dampness may also be occurring due to defective flashings to the base of the chimney stack, mortar pointing and or the mortar flaunching which have become cracked and defective or chimney pots which may be open to the elements. Repairs can be carried out to rectify these issues.
If the chimney is left open to the elements, rain can enter down the flue chambers and cause water ingress and dampness. Therefore, chimney pots should be capped off with the appropriate terminals whether the chimney stack is in use or not which will prevent water entering down the flue chambers and allow for the adequate ventilation. If the chimney stack is no longer in use, then the stack should be sealed with mortar or coping stones, removed or taken down to below the roof level.