You may not know it, but moisture, not heat, is the biggest cause of damage to chimneys. Bricks and mortar are somewhat porous and water from rain and moisture can get in these pores and cause damage to your chimney brick. This damage caused by moisture or water in the brick is called spalling.
What is chimney spalling and how is it caused?
Spalling is when the masonry bricks or stone begin to flake off or become missing due to water penetration. You may first notice this when you find bricks or stones lying on the ground or around your roof. The freezing and thawing of too much precipitation absorbed in the masonry is the main cause of spalling. The damages of spalling can be worsened by the condensation of flue gasses in liners that are not the right size, or gaps and cracks in the mortar joints located on the inside of the chimney. Also, extra water entering the interior of the chimney chase at the top can make spalling more prominent. Softer types of brick are more likely to have moisture penetration affect them than the harder types, but builders will often use the softer types to make up for expenses.
The chimney is one of the first places to show that it has damaged because it is more exposed to the elements than the rest of the home. This will start with the top portion, but when the chimney begins to spall the inside at your fireplace, damage has already started. These bricks and stones cannot be repaired, and should be replaced quickly.
How can you prevent spalling?
There is no way to completely protect your chimney from water, but there are some ways you can help. First, have your chimney waterproofed. Waterproofing can extend the life of the chimney and brick for many years. Your chimney technician can do this on any clear day after your sweep. The sealant itself will need about six hours to dry, so plan accordingly. Also, if you live in an extra wet area you may want to look into having two coats applied. Chimney caps are another great way to help keep water out, but they must be installed properly to be safe.