The chimney crown does exactly as its name suggests: it tops off the upper end of a chimney. Another name for a chimney crown is a chimney wash. It’s one of many essential chimney parts, but few people realize that its main purpose is to act as a protective roof or chimney cap that shields the chimney’s structural elements from weather-related deterioration. This is important because, in many cases, mortar is naturally weaker in the chimney’s upper section and will slowly fall apart if exposed to repeated rain and snow.
Since a good crown is vital to the longevity of the chimney itself, homeowners should know how to spot both bad and good crowns when buying a home with a chimney attached. The most basic thing to watch for is how far out the crown extends. The crown should reach well beyond the edges of the chimney walls, preferably 2 to 2.5 inches (5 to 6.35 cm), with a lip that is known, oddly enough, as a "kerf." Kerfs act as miniature rain gutters to prevent water from rolling to the underside of the crown and finding its way onto the chimney.
Having all the right elements on a properly built chimney crown can give a chimney enough structural integrity to last for a hundred years. One of these elements is called a "flashing" and although it isn’t technically part of the crown itself, all good crowns are conventionally installed alongside a flashing to provide an added measure of security. Flashings can be made of copper or steel. They look like a simple metal band placed directly below the chimney crown, and they deflect any excess water that may have reached the upper chimney bricks despite the kerf’s protection.
When a chimney crown is added to a home, the homeowner should make sure it is made of the right materials. Masons will sometimes build a crown that isn’t up to par, and if the homeowner doesn't know the difference, he’ll end up paying only to watch the chimney rapidly deteriorate over the next few years. The number one rule is to never use bricks. The whole reason for a chimney crown in the first place is because mortar isn’t weatherproof, so if a mason is using mortar and bricks to construct the crown, that should be a warning sign. Instead, opt for metal, stone, or concrete slabs.