The chimney crown also referred to as the chimney wash, is the top element of a masonry chimney. It covers and seals the top of the chimney from the flue liners to the chimney edge. Most masonry chimneys are built with an inadequate crown constructed from the common mortar mix, the same mixture used to lay the bricks of the chimney. This mortar is not designed for and will not withstand years of weather abuse without cracking, chipping, or deteriorating; situations that allow water to penetrate the chimney. In fact, most sand and mortar crowns crack almost immediately after installation because of shrinkage.
A proper chimney crown should be constructed of a Portland cement-based mixture and cast or formed so it provides an overhang, or drip edge, projecting beyond all sides of the chimney by a minimum of two inches. This drip edge directs the runoff from the crown away from the sides of the chimney, helping prevent erosion of the brick and mortar in the chimney's vertical surfaces.
There are also some modern waterproof, non-shrinking, cement-like coatings for repairing damaged mortar crowns that seem to work fine.