Chimney Crowns perform an important function; They keep water from soaking into the masonry system.
Movement: Proper crowns should allow for expansion and contraction. This is accomplished by using expansion materials that keep the concrete from bonding with the flue.
Slope: The surface of the crown should slope away from the flue so water will shed easily off the top.
Overhang: The outer edges of the crown should extend beyond the surface of the brick so water can drip down to the shingles.