Have you ever wondered what that dome of mortar or concrete is on top of your masonry chimney and what purpose it serves? Here at Moore Masonry we get asked this question a lot from homeowners when they find out that their chimney cap may be in need of repair or replacement. Since most people don't get on top of their roof and inspect their chimney, people usually find out from a roofer or someone cleaning the gutters who is nice enough to alert them to a potential problem. As you can see from the above picture, it doesn't take an expert to know when there might be a problem.
A chimney cap or chimney crown or chimney wash (it has many names) is a dome of mortar or concrete on top of a chimney that sheds water off of the chimney top while also locking the masonry together at its point of termination. Although the dome shape helps to shed water, it also absorbs water due to the porous nature of concrete and masonry. Through multiple winters and summers the chimney cap and moisture inside it is subjected to freeze/thaw cycles. This causes the water molecules to expand and contract which over time causes cracking and dislodging of the chimney cap. If caught early in the cracking stage sometimes the cap can simply be coated in a breathable cement-based waterproof coating. If caught too late then the chimney cap needs to be removed and replaced. These before and after pictures show examples of concrete chimney caps that Moore Masonry replaced.
Another problem concerning failed concrete chimney caps is the ability of water to enter the rest of the chimney and even the home. If water gets past the concrete chimney cap then it can get absorbed into other parts of the chimney creating further damage. The most obvious is when water gets past the cap, is absorbed in the brick veneer, undergoes freeze/thaw cycles and causes spalling. "Spalling" is where the expansion and contraction of the water molecules in the bricks causes the faces of the bricks to pop off. People generally notice this by finding pieces of brick in their gutters and on their roof. These pictures show examples of brick "spalling" due to failed concrete chimney caps. The mortar joints themselves are also susceptible to this process.
At Moore Masonry we have 35 years of experience in building, evaluating and repairing chimneys. In fact, we pride ourselves on being one of Central Virginia's leading chimney experts. Whether it's a new chimney project, repairing an existing one or preserving the one you have, please contact us if we can be of help in any way.