A chimney is made up of many different parts, and it's easy to assume the important ones are all on the inside. In practice every part plays a role. The exterior exists to support the structure and shield it from the weather — without the masonry, you'd have nothing but a clay or metal liner sticking out of the house, and it wouldn't function for long. Here's what each of the outside components does.
Chimney Flashing
Flashing is the metal material that rises a few inches up the chimney from the roofline. Its job is to stop water leaking into the roof through the gap between chimney and roofing. A thick, continuous bead of caulk should run around all the edges — without it the flashing does nothing, because water simply runs behind it.
Inspect the flashing once a year and check that the caulk is intact, not cracked or split. If the chimney foundation weakens, the chimney shifts slightly, which can pull it away from the flashing and separate the caulk from the brick. Water will find that gap. A chimney that has moved that far needs repair, or at minimum a professional inspection.
Once the seal is compromised it's difficult to restore properly, and replacement is usually the right call. Replacing chimney flashing now costs far less than dealing with moisture and mould damage later. Installation is best left to a professional — this isn't a component you want to get slightly wrong.
Where the Chimney Meets the House
Moving down from the roofline, the chimney butts against the side of the house. That joint needs the same attention as the flashing. If the waterproofing there fails, water damage follows.
The Foundation
The foundation carries an enormous load. Think about the weight of a single brick, then multiply it by every brick in the chimney. Because of that, the foundation must be at least 12 inches thick and extend roughly 15 inches beyond each side of the chimney.
Check it regularly for cracking. Cracks can indicate the soil beneath isn't stable enough to carry the weight. Soil can be strengthened, but not once the foundation is built — by then the only fix involves taking the chimney down.
The Chimney Crown
At the top sits the crown, usually cement, capping the chimney and keeping rainwater and debris out. Each face is angled away from the centre so water runs off rather than pooling. Check it regularly for cracks and hire a professional for any repairs.
The Chimney Cap
The cap does for the liner what the crown does for the chimney as a whole. Without one, rainwater, debris and animals get into the chimney liner and cause problems. Some designs also improve draft, which is what makes a chimney work at all.
A good chimney cap includes wire netting around the outside, forming a barrier against birds and animals looking to nest in the flue.
All of It Matters
Every one of these parts earns its place. Without them you'd have a liner sticking out of a house with no protection and, realistically, no purpose.