Apart from the nuisance value of a smoky chimney, chimneys may suffer from a number of structural defects that may or may not be contributing to the nuisance aspect, particularly in older properties.
Flue gases contain a number of ingredients apart from the more obvious ones of smoke and soot. One of the more common harmful substances is sulphur in the form of sulphates, particularly with some domestic coals. These rise up the flue initially as gases, but if the flue is cold due to its exposure on an outside wall or because of its intermittent use, the sulphates will condense on the flue linings. If these are old or in a poor condition, the sulphates will gradually migrate to the outside of the chimney stack where they will be dissolved by penetrating rain and attack the mortar in the brickwork joints causing it to expand. The chimney stack above roof level is most vulnerable, being the coldest section. Often the attack is more severe on one side of the chimney, this is generally on the outside of the stack, ie. on the opposite side of the roof. Consequently. the expansion is greater on this side, pushing the bricks upward and causing the chimney to lean inwards. A closer examination of the stack will normally reveal deteriorating mortar joints, loosening of bricks and severe cracking of a cement render, if used.
Remedial work will depend on the severity of the attack. Any disused flues should be capped to prevent rain penetration but allowing for ventilation, and a vent should be provided in the blocked off fireplace opening. If the chimney is being used, the flue should be relined with a suitable liner.
Where the attack is severe or where the stack is leaning more than 1mm in 100mm from the vertical, it will be necessary to dismantle the chimney stack down to where the brickwork is sound and rebuild it using a mortar made with a sulphate resisting cement. In less severe cases it may only be necessary to rake out the old mortar and repoint with a sulphate resisting cement mortar.
Another common defect more usually observed in the gable walls of the older stone built houses is a black stain following the line of the flue and is normally accompanied by vertical cracking. Sometimes the cracking is apparent without the staining. The cause again is due to the condensation of flue gases and tar deposits on the inner lining. Sulphates from the condensates will again attack the cement in the flue lining causing it to expand and form vertical cracks on the outside. Tar deposits migrate to the outside and cause the black tarry deposits on the outside following the line of the flue. Again, any in-use flues should be lined and the cracks raked out and repointed in a sulphate resisting mortar. Unfortunately, little can be done to remove the black stain apart from wire brushing.
Another source of trouble is damp penetration in the chimney breast usually just below ceiling level on the upper floor often accompanied by black tarry deposits. This is usually the result of water penetration at the point where the stack protrudes through the roof and is normally caused through the cracking and breaking away of a cement fillet flashing or damaged or missing lead flashing. Appropriate remedial work will normally rectify matters