Chimney Spalling and What Can Be Done
Spalling is the crumbling of bricks that occurs when moisture has penetrated them, which in turn has frozen. When the water absorbed by the chimney freezes, it expands, pushing the brick face outward and off. Not only unsightly but ultimately the downfall, quite literally, of the chimney itself, spalling cannot be ignored.
First, the damaged brick will need to be removed and replaced by a professional trained to do the job correctly. Crumbling mortar will also have to be addressed and removed before fresh mortar is applied. Finally, the cause of the problem will have to be identified and corrected so that it does not recur.
In all likelihood, the culprit will be either the chimney chase cover or the chimney cap. When these are cracked or rusted, they allow water to penetrate the chimney and chase, and a variety of problems result. The one signaled most by chimney spalling is the expansion of the brick when the moisture in it freezes.
Generally used by builders because they are less expensive, soft bricks are much more susceptible to penetration by moisture. Another added expense that builders tend to avoid is a chimney crown, with an overhang designed to carry water away from the chimney. Without it, water can enter the chimney chase, and even with it, routine inspections need to confirm that no cracks have appeared in the crown.
Once moisture is allowed to enter the chimney chase and flue, serious damage to the chimney begins to unfold. It does not help that flues can also be compromised by the steady flow of corrosive gases to which they are exposed. When cracks occur, more moisture is admitted, and when that moisture freezes and thaws, the bricks expand and spalling begins. Chimney spalling is a sign of serious water damage and needs to be corrected by a trained professional as soon as possible.