For prefabricated, or metal chimneys, these chimneys must have a properly installed 'Pan', or sheet metal chase cover or you will have major problems with water in the walls.

Once a chase pan has started to leak, there are very few things you can do to repair them.
Because of the way water flows, it's nearly as expensive to patch a rain pan as it is to replace it.
Repairs on chase covers cannot be guaranteed.

This person really let the problem go for way too long. 
There was water pouring down that large hole for months.
It caused a lot of moisture damage inside his walls. 
If you see rust stains running down the side of your chimney, give us a call. 
You could be having a major water issue and not know it.

This is called a shroud. They go on top of the chase pan on pre fab chimneys.
While you are changing your rain pan, you might want to add this. Bare Pre Fab chimneys look almost like a dryer vent sticking out of your chimney.
A decorative shroud on your chase pan will add style and elegance to your home.