The top of the chimney must extend above the roofline to prevent downdrafts caused by the turbulence of the wind as it sweeps past nearby obstructions or over sloping roofs. The top of the chimney should extend at least 3 feet above a flat roof and 2 feet above the ridge of a pitched roof. When a chimney is 10 horizontal feet beyond a roof ridge, builders often terminate the top of the chimney below the ridge. This is not quality construction, but it satisfies the code. In this case, downdrafts can still occur as a result of air currents formed when the wind hits the side of the building. The downdrafts can affect the efficiency of the heating system or result in backsmoking of the fireplace. This type of problem can usually be controlled by installing a concrete or stone cap about 8 inches above the top of the flue.
Older homes very often have unlined chimney flues. Although these chimneys might operate satisfactorily, they are a potential hazard. Over the years, the corrosive gases can have a deteriorating effect on the mortar joints. If an unlined chimney in your house is connected to a fireplace, and if you intend to use the fireplace, you should have a flue liner installed down the existing flue. This is an important fire safety measure and should be recorded on your worksheet.
In many homes when the flue damper in the firebox has deteriorated, it is replaced with a chimney-top damper. (See FIG. 3-4.) This spring-loaded damper is mounted over the flue opening at the top of the chimney. The device has a stainless-steel wire that runs from the damper down the flue into the opening of the fireplace. If you see a chimney-top damper during your inspection, don’t forget to check its operation when inspecting the fireplace.
All masonry chimneys should be inspected for cracked, loose, chipped, deteriorating, and missing sections of brick and mortar joints. Some masonry chimneys have a stucco finish. In this case, look for cracked, chipped, and loose sections of stucco. Brick chimneys on older homes are sometimes covered with an asphalt-type coating, especially above the roofline. This technique is often used to prolong the life of the chimney when there are many cracked mortar joints and deteriorating bricks. It is considered makeshift. If you see any of the above items, they should be recorded on your worksheet.
Is there a cricket (also known as a saddle) behind the chimney? (See FIG. 3-5.) There should be one if the chimney is located along the slope of a roof and is more than 2 feet wide. The cricket prevents debris or snow and ice from piling up behind the chimney. This can cause rain or melting snow to back up under the shingles and leak into the house. The cricket also deflects water running down the roof around the chimney.
For the most part, very little maintenance is needed for prefabricated chimneys. Metal chimneys have a tendency to rust and should be checked for corrosion holes. If a metal chimney has no rain cover, note it on the worksheet; a cover is recommended.