A chimney chase cover is the lid that covers the top of a wood-framed chimney. The chase itself is the box that surrounds the metal flue, and without a cover, it would be left open to the elements. It is a critical piece in keeping rain and snow out of your chimney as well as nesting birds and small animals.
Unfortunately, when your home was built, an inferior metal was probably used for the chase cover. Typically, original chase covers are made of galvanized steel, which is prone to rust. Rust, of course, can lead to far more serious problems with your chimney, including structural failure. An inexpensive and virtually permanent solution is a stainless steel chimney chase cover.
A sure sign that it is time to replace your chase cover is the appearance of rust stains on the outside of the home. Not only are these extremely difficult, if not impossible, to remove from the siding but they are a clear warning that the chase cover has begun to corrode. In time, this means water entering the chimney and moisture building between the layers of the flue.
Another problem that can occur with originally installed chase covers is a dipping at the center. This causes water to pool at the center, speeding the rusting that destroys the cover. Chase covers should be raised at their centers, allowing rain and water from melting snow to run down and off it.
Finally, the cover may be improperly sealed, with one or more corners left unprotected from the elements. All corners and nail heads need to be sealed with caulk, which expands and contracts with different air temperatures. Eventually, the caulk will fail and admit water, loosening nails and allowing even more water into the chase. Checking for this should be part of an annual inspection of the chimney.