Chimney Anatomy: Crown, Flashing, Caps, and Mortar

The crown is the sloped masonry slab on top of the chimney that sheds water off the structure; when it cracks — which freeze-thaw makes inevitable over time — water pours straight into the chimney body. It is repaired with crack sealant for minor cases or rebuilt with concrete and a proper overhang and drip edge for serious ones. The cap is the metal cover over the flue opening, usually stainless or galvanized, that keeps rain, snow, and animals out. A cracked crown and a missing cap are two of the most common reasons a chimney leaks.

Flashing is the metal — typically aluminum, galvanized steel, or copper — that seals the joint where the chimney passes through the roof, in two layers: step flashing woven into the shingles and counter-flashing let into the mortar joints. It is a leading leak point because the chimney and roof move differently and old flashing pulls loose or rusts. The brick itself can spall (the face flaking off) when water soaks in and freezes; damaged units are cut out and replaced with brick matched as closely as possible to the original.

Mortar is the binder in the joints, and matching it matters: the new mortar should be compatible in strength and color with the original, since mortar that is too hard for old soft brick can actually damage it. Tuckpointing repacks failed joints with the right mix. Finally, a breathable masonry water repellent can be applied to help the brick shed water while still allowing it to dry — the key word is breathable, because a non-permeable coating traps moisture and makes spalling worse. A good mason chooses each material to suit the age and condition of your chimney.