Whether made of precast or cast-in-place concrete, a properly constructed chimney crown (also called a cap or top plate) sheds water and seals the top of the chimney. But much of the time, chimney crowns are poorly built. Poor crown construction is a leading cause of chimney deterioration and failure. Chimneys frequently suffer water damage, freeze-thaw erosion, spalling, and efflorescence as a result of faulty crown construction.
The Brick Institute of America (BIA), which receives numerous complaints about "disintegrating" brick near the top of chimneys, has issued guidelines for chimney crown construction.
PRECAST VERSUS CAST-IN-PLACE
Precast concrete crowns are easy to install and provide consistent quality, but they generally are used only in new construction or on smaller jobs, such as 16x16-inch chimneys. The heavy weight of concrete makes hoisting large precast crowns impractical. Adjustable steel forms make it easier to construct cast-in-place crowns today. Steel forms require only half the setup time that wood forms do. Steel forms also last forever if properly maintained.
BUY THE RIGHT FORM
Follow this rule of thumb when buying a form: No more than two- thirds of the length of the form should extend beyond the chimney. Another consideration when buying a form is how easily it can be attached to the chimney and stripped out after the concrete has set. Also, look for a steel form with a welded-in drip edge to save setup time.
SUPPORT SYSTEM AND BOND BREAK
Install the crown support system, which covers the open area between the flue or flues and the masonry and serves as a bridge from masonry to masonry. The support system also establishes a bond break between the concrete and masonry. It's important to have a bond break because masonry and concrete expand and contract at different rates.