The outer lay of a chimney is the part you see. Therefore, it is usually designed to be esthetically pleasing and a complimentary part of the house structure. It can be made of brick, stone, concrete blocks, or a facing of brick or stone.
The outer layer is an engineered structure that is designed to be self-supporting and provide structural integrity to the entire chimney system. Usually, it will be the same material from top to bottom. Ideally, a chimney should start on a footer or slab, generally in the basement of a house. Even in a single story house, the chimney could actually be 3 stories tall because it starts in the basement, continues through the main level, then through the attic and finally appears through the roof line of the house–sometimes extending 4 feet or more above the roof line.
If constructed of brick or stone, then there will be mortar holding the material together. The space between the bricks or stone are called joints and are mortar which is struck when the chimney is built and the mortar is still wet. Striking is done with tools of various shapes and sizes, with the outcome being a smooth, even bead of mortar between each brick or stone. This helps prevent water from entering the masonry via the joints.
Joints should be inspected carefully at least annually. Water, ice, and natural shifting of the house structure can cause joints to crack and loosen. When that starts, you have problems. Unattended, these loose joints begin to spread and if not repaired then eventually your bricks or stones will begin to fall out. By then it’s almost impossible to save the structure without major repairs. Sometimes it has to be torn down to ground level and rebuilt.
Another problem is that loose joints cause water to seep into your chimney and then the clay liner can begin to deteriorate from dampness. Also if there is water present and it freezes, the water will expand and cause cracking making the problem even worse.
It’s important to try to view all levels of the chimney as it passes through your home and out through the roof. You want to look for signs of deteriorating mortar, dampness, obvious cracks or signs of leaking gasses. It may not be possible to see some parts of the chimney because of it being covered by interior walls, but you should look for signs of dampness, mold or deterioration on the walls adjacent to the outer layer of the chimney. This will be an indication of trouble in the chimney, whether it’s loose mortar, cracked or loose bricks, or worse yet holes and cracks in your chimney liner. All of these conditions are dangerous and immediate steps need to be taken to repair them or reline the chimney.