Generally, chimneys are constructed of masonry (brick, concrete, stone) or are prefabricated from metal or a cement-asbestos material. Masonry chimneys are typically supported by their own footings, which in northern communities extend below the frost line. Such chimneys do not depend on the main structure for support.
If a masonry chimney that extends up to the side of a building has open joints between the chimney and the sidewall, some settlement has occurred but often that is not a problem. The open areas, however, should be sealed to prevent water intrusion. On the other hand, if the chimney is not vertical, open joints indicate excessive settlement due to the lack of a footing, or one that is improperly designed, and it should be checked by a professional. In this case, schedule a chimney inspection & repair with a qualified contractor. Depending on the degree of settlement, the flashing at the joint between the chimney and the roof might need repair, as the movement of the chimney could result in open and loose sections of flashing.
With a brick chimney inspection, the area above the roof line should be checked to make sure it is vertical. In time, the mortar joints can weaken on one side and cause the chimney to lean. A leaning chimney represents a potential safety hazard, so have it inspected by a professional mason to determine whether corrective measures are needed. Moreover, a leaning chimney is an indication of weakening mortar joints that question the structural integrity.
In some cases, brick chimneys on relatively new houses are coated with white mineral deposits called efflorescence, a condition that often occurs when water is absorbed by the bricks. The minerals in the bricks dissolve in the water, then resurface when the water evaporates. Though efflorescence is common in new brickwork, it can easily be scrubbed or washed off with a dilute solution of muriatic acid. Usually, recurrence can be controlled by covering the bricks with a vapor permeable water repellent.
Note that efflorescence on a brick chimney that has been up for many years generally indicates that water is getting inside the chimney, through cracks in the joints on top of the chimney or cracks in the bricks or mortar joints. If you find heavy efflorescence on an older chimney, make a note for future inspection & repair. The top of a masonry chimney should have a cement finish that slopes from the flue to the edge of the chimney. This finish deflects rain and protects the joints between the flue and the chimney, but it is vulnerable to cracking so it should be resealed periodically.
The top of the chimney must extend above the roof line to prevent downdrafts caused by the wind as it sweeps past nearby obstructions or over sloping roofs. Also, the top of the chimney must extend at least three feet past the highest point where it passes through the roof, and two feet higher than any roof part within ten feet measured horizontally. When a chimney is ten horizontal feet beyond a roof ridge, builders sometimes terminate the top of the chimney below the ridge. This, however, cannot be considered quality construction, although it does satisfy the code. If the top of the chimney terminates below the ridge downdrafts can still occur as a result of air currents formed when the wind hits the side of the building. The downdrafts can not only affect the efficiency of the heating system, but they can result in backsmoking of the fireplace. Installing a concrete or stone cap about eight inches above the top of the flue can usually control this problem.
When it comes to older homes, more often than not they have unlined chimney flues. These chimneys might operate satisfactory, however they are a potential hazard. In time, the corrosive gases can have a deteriorating effect on the mortar joints, so make sure your home inspection includes checking for chimney flue hazards.
All masonry chimneys should be inspected for cracked, loose, chipped, deteriorating and missing sections of brick and mortar joints. If the chimney has a stucco finish, check for cracked, chipped and loose sections of stucco. Sometimes, brick chimneys on older homes are covered with and asphalt-like coating, especially above the roof line. This technique is frequently used to prolong the life of the chimney when there are many cracked mortar joints and deteriorating bricks. Such chimney repair methods are considered makeshift.
Prefabricated chimneys generally need very little maintenance. Metal chimneys tend to rust and should be checked regularly for corrosion holes. If the metal chimney has no rain cover, consider installing one. Rain covers should be mandatory components for metal chimneys.