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
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.