Excessive Moisture in the Attic
Bricks are typically made of clay and shale, with filler materials that act as binders to hold the bricks together. Once baked into brick form, this material is sturdy, but it is susceptible to moisture, especially over time. An improperly vented attic will have higher humidity levels. The moisture in the air will degrade the bricks over time, as well as the mortar that holds the bricks together. This is the most common reason for the degradation of bricks in an attic.
Age of the Brick
If your home is more than 50 years old, you are dealing with old building materials that have weathered numerous climate changes. Extreme temperature changes can create nearly invisible hairline cracks in the brick, and over time, these cracks will worsen. The bricks will begin to break down and crumble to dust. In addition, if the bricks were not made properly, they also will be more prone to breaking and cracking.
Age of the Mortar
If the mortar holding the bricks in place is old and has not been touched since the chimney was installed, there's a good chance it is also going to crumble over time. Mortar typically is made up of a combination of cement, lime and sand. These organic materials are sensitive to climate changes and movements in the home. They will eventually break down. Removing bad mortar and replacing it can help maintain the structure of the chimney, but eventually, it will need to be replaced.
House Settling Issues
When a house is built, it goes through dramatic settling changes as the building materials acclimate. These changes settle down after the first two years, but your building will continue to shift and move back and forth, sometimes as much as 1/4 inch per year or more, depending on the type of foundation and the area in which you live. This settling places stress on interior structures such as a chimney, and the bricks may simply break due to this stress.