Chimney caps are the part of the chimney sitting atop the bit of flue that can be seen above the chimney top. A chimney cap does for the flue what a crown does for the chase, but also keeps debris and critters out of the chimney. Chimney caps, too, need to be sloped to direct water away from the flue; they, too, need to be properly designed and installed.
Keep Them Out
They are pretty much the first and last line of defense against birds, animals, nests, and debris. Typically surrounded with mesh of some kind, they are the absolute top of the house and exposed to the worst of everything. Birds grip them with sharp talons, animals try to dislodge them, hail pelts them with balls of ice, and rain steadily wears them down.
Generally lightweight, chimney caps can also be blown away by high winds, so looking for wind-resistant caps may be worthwhile, especially in certain areas. Rust used to be a problem with inferior metal caps, and it still can be, but powder finishes and stainless steels have improved them substantially. With care and a little help from nature, chimney caps can be reasonably durable and some are relatively inexpensive to replace.
Keep Them In
That does not mean you should stick one up there, and forget about it, as any number of things can happen to leave a chimney cap in unsatisfactory condition. What it does for the chimney is too important to overlook, or maybe we should say under-look, since regular attention is needed. A cap improves a chimney’s draft, and not just the caps specifically designed for that purpose.
Chimney caps are also useful blocks against sparks lifted all the way out of the flue. The same mesh that prevents things from entering the chimney can stop wayward sparks from progressing beyond it. Performing a number of important functions, a chimney cap should be the highest thing you see among permanent features of your house.