Michael's View: Chimney Crowns

Chimneys are basically very small structures. Just as your house needs a roof to keep water from entering, so does your chimney. Chimney crowns are simply chimney roofs. The crown should slope down from the flue liner at a minimum of 3 inches of fall per foot of run. Flat or improperly sloped crowns can allow water to enter the interior of the chimney. This water can cause efflorescence (white salt deposits on brick surface), brick spalling (the flaking of the brick surface), and the deterioration of the mortar between individual bricks. Not to mention damage to the damper, hearth, fire-screen and wood around the chimney on post-and-beam construction.

All too often, brick masons simply use mortar mix to finish off the top of a chimney. Mortar is easily damaged because it is not designed to handle the elements and freeze-thaw cycles in colder climates. Chimney crowns should be constructed using either pre-cast concrete slabs, cast-in-place steel reinforced concrete, solid stone, or metal. Masonry crown materials should not directly contact the chimney flue liner. This gap should be caulked with a flexible cement stable silicone caulk.

The cracks in crowns sometime occur when the flue liner expands from the heat of the fires below. This expansion can pop a weak mortar cap much like a chick hatching from an egg. Also, excessive shrinkage cracks often develop in cast-in-place chimney crowns that lack adequate reinforcing steel and/or are not cured properly.

Most property owners seldom get up on the roof to monitor the masonry work on their chimney caps, look for tell tale signs of water staining or leaks on the hearth or in the firebox. If water stains are present, contact a chimney sweep or masonry contractor to evaluate the chimney cap. Most likely it has cracked.

If you are handy and like to DYI - There are some good products put out by SaverSystems that can repair damaged crowns.