Chimney cap (crown)
Masonry and stone chimneys usually have a concrete top, called a cap or crown, to help shed water off of the top of the chimney area. Chimney caps are designed to keep water from entering into the chimney’s walls or cavities, which will cause deterioration of the masonry or mortar. They also help to keep rainwater from entering the space between the flue tile liner and the masonry face brick.
Cracks in the chimney cap
It is not uncommon for cracks to develop in this concrete cap. When these cracks occur, water may penetrate and pass into the chimney itself. These cracks get worse over time, especially in areas that experience freezing and thawing. When water gets into these cracks and freezes, it expands causing more damage or widening to the cracks.
A good chimney cap will have steel rebar in it. When the moisture reaches the rebar, it tends to rust, and as it rusts, it expands. This expansion then causes more cracking and spalling to the chimney cap.
Patching and repair
The repairing or sealing of these chimney caps is usually a very easy task and seldom costly. Generally, chimney cap patching or repair should be considered part of the home maintenance program.
While check the chimney top for cracks, also check the following (links below):
Home owners will often seal or repair their own concrete chimney cracks . Fireplace contractors or qualified handymen can easily and inexpensively do this type of work.