Spalling is caused when the masonry absorbs too much moisture from rain and is subjected to freeze/thaw cycles. Damages can be exacerbated by condensation of flue gasses in improperly sized flue liners and gaps or cracks in mortar joints on the interior of the chimney, and by moisture entering the interior of the chimney chase from the top at the cement crown area where cracks or gaps allow water to enter. If the cement crown is not built with a poured crown with 2” overhand and drip edge per code, damages to the top of the chimney structure will occur more quickly. However, builders seldom build poured crowns due to the added time and expense involved.
Soft type brick is more susceptible to moisture penetration than hard type bricks, but builders often use soft brick because they are less expensive. When building a new home, homeowners should request that hard bricks be used in all construction so they will last many years.
Chimneys usually show damages more extensively because they are more exposed to the elements than the rest of the house. The top portion of the chimney will show the first signs of moisture problems. By the time bricks or stones spall, the damage is done. Spalled bricks or stones cannot be repaired and must be replaced. It may be tempting to save money by replacing spalled bricks and doing partial rebuilding of a chimney, but keep in mind that the entire chimney will need to be rebuilt at some point, so any partial repair is a temporary fix.
Painting chimneys is never a good idea. This servies to temporarily cover up the problem, but will speed up the deterioration process since the bricks can't breathe. Buyer beware if you see a painted chimney.
The application of masonry water repellant sealer can help stop further penetration of moisture, but bricks that already have interior damage will continue to spall. Masonry water repellant should be applied every year to every five years, depending on when it loses its ability to shed water off the bricks. Professional chimney contractors prefer to use Chimney Saver brand water repellant.
To find a qualified chimney restoration contractor in the greater Kansas City area visit the Midwest Chimney Safety Council or the Masonry Heater Association of North America website.