Why do I need a chimney cap?
Imagine how much water would come into your home during a rainstorm if you had a hole in your roof twelve inches square. Chimney flues can be even larger than that, so where does the water go?
It runs down the chimney and seeps into the bricks and mortar, settling behind the firebricks. There is little air circulation in the chimney (unless you're using it) and no sunlight, so the inside of the chimney stays wet long after the rain stops. This moisture erodes the mortar joints and weakens the brick.
Add to this a freeze/thaw cycle during the cold months, and you have all the needed ingredients for a deteriorating chimney. Even though you can't see the damage happening, it's there, slowly deteriorating your fireplace and chimney. This is a particularly important consideration if you have a metal firebox incorporating a metal damper and smoke shelf.
All fireplace dampers made of metal are subject to this deterioration as the water mixes with creosote and mild acids are formed. Even chimneys serving woodstoves are in need of a cover.
Installing a chimney cover or chimney cap on top of your flue can save you a lot of money in the long run. Caps and covers keep out damaging moisture, which wears away masonry and steel chimneys. They also keep out the birds and other critters.