Masonry chimneys are the most exposed area on a home and are the most affected by weather. Rain, freeze-thaw cycles, and high winds cause mortar and bricks to deteriorate over time. With regular maintenance and repair masonry chimneys can last a long time, but without it will eventually need to be torn down and rebuilt.
Many masonry chimneys are built with soft type bricks rather than hard bricks. Soft bricks are more susceptible to moisture penetration than hard bricks and will eventually spall with the brick faces popping off. This is due to water inside the bricks then freezing and thawing. In some cases it only takes a short time for this to occur. There is no way to repair spalled bricks as any repair method will also eventually fall off.
The best choice when building a new chimney or rebuilding an old one is to use hard type bricks. These are virtually impervious to moisture penetration and will last many years. Hard bricks are a little more expensive but well worth the initial cost which will save thousands of dollars in repair work in future years.
Mortar can be expected to deteriorate over time. The mortar used commonly today is not as hard as the lime mortar used years ago. To check the mortar, scrape a screwdriver along some joints. If the mortar scrapes off easily or if you can push the screwdriver into the mortar, it is in poor condition. If there are cracks, gaps, or holes in the joints, this is another sign that pointing is needed. Pointing, often mistakenly called tuckpointing, is usually needed every few years. This involved grinding out of the mortar joint to good, hard mortar, then packing in new mortar and tooling the joint to match the rest of the joints on the chimney. The color should also be matched so it blends in with the other joints.
Cement crowns are another common repair item when it comes to masonry chimneys. The cement crown is a roof for the chimney, protecting it inside and partially outside from the elements. Cracks in the crown, or lifted deteriorated crowns can allow damaging water to enter the inside of the chimney chase, smoke chamber, and fireplace. This can result in washed out mortar joints on the interior where they are not visible. Cement crowns should have a drip edge, to prevent water from running down on top of the top courses of bricks or stones on the exterior chimney. Many masons don't build poured cement crowns with drip edges and this is the cause of many damages to exterior masonry. If the cement crown is not in poor enough condition to need to be replaced it can be sealed with a product that protects the crown from water intrusion. This type of product is available from professional chimney sweeps.
The Midwest Chimney Safety Council says that it is important to have an experienced mason do this type of work in order to make sure the repair is done correctly and blends in with the rest of the house or chimney. Regular maintenance of masonry chimneys will assure many years of use and avoid costly repairs in the future.