Why is my chimney crown cracked?

What the crown does

The crown spans the top of the chimney around the flue. It should slope away from the flue and extend past the chimney walls, so water drips clear rather than running down the brick. A crown made of plain mortar, rather than a cement mix made for the purpose, is more likely to crack.

Why crowns crack

• Weather. Freezing and thawing and sun exposure age cement.
• Movement. The flue tile expands and contracts with heat. If the crown is bonded rigidly to the tile, the movement cracks it. A flexible joint between crown and tile is commonly used to allow for it.
• Original build. Thin crowns, or crowns with no overhang or slope, fail sooner.
• Water from inside. With no cap, rain that runs down the flue wets the crown from below.

Reading the crack

Seal or rebuild?

The table above gives the usual route for each condition, and which one applies to your crown is a judgment made up close. Sealing is quicker and less costly, but it only holds if the crown beneath is sound. A coating over a crumbling crown hides the problem for a season.

What happens if it is left

Water enters through crown cracks and reaches the brick beneath. That is a common start to white staining, spalling and loose mortar, and it can work down to the flashing and the smoke chamber. See why is water coming into my chimney? and why chimney bricks flake apart.

A look from the ground

• Binoculars: are there cracks, dark streaks or missing pieces?
• Is there a gap between the crown and the flue tile?
• Is there a cap on the flue?
• Are there pieces of cement in the gutters or on the ground?
• Photograph and date what you see, so you can tell whether it changes.

Common misconceptions

"A crack in the crown is cosmetic." Some hairline cracks are minor, but any crack can let water in, and only a close look shows which kind it is. Wider ones are an open path for water.

"A cap protects the crown." A cap keeps rain out of the flue. The crown protects the masonry. They do different jobs.