Brick chimneys are basically very small structures that need a roof to keep water from entering just like your house.   Chimney caps (or crowns) are simply chimney roofs.  The cap should slope down from the flue liner.  Flat or low slope caps can allow water to enter the interior of the chimney.  This water can cause efflorescence (white salt deposits on brick surface), brick flaking, and the deterioration of the mortar between individual bricks.

Chimney caps should be constructed using either pre-cast concrete slabs, cast-in-place steel reinforced concrete, solid stone, or metal.  Masonry cap materials should not directly contact the chimney flue liner. This gap should be caulked with a flexible cement stable silicone caulk.   The chimney cap should also extend beyond the outer surface of the chimney on all sides and the bottom of the cap should contain a small drip.   The overhang and drip help to keep water from running down the chimney face.