If this were the only reason for installing a cap, it would be enough. I have seldom seen an uncapped chimney over five years old that was not suffering from some kind of water damage. Go over to your fireplace right now and look at the back firewall near the base. Take a screwdriver or coin and run it across the mortar in the bricks. Seem a little crumbly? Or maybe it's obvious just looking at it. Rain puddles up on the smoke shelf, mixes with creosote in the chimney, and turns into a highly corrosive acid. It then seeps down and attacks the mortar joints on the back wall of the fireplace. It becomes weak and, presto, the bricks on the wall become loose.
Rain can set off a bad smell in the chimney. This will happen in warm weather, especially if the chimney is dirty or has bird droppings on the shelf.
Rain speeds the rusting of the metal parts of your fireplace. Damper hard to move or frozen?
A Cap Keeps out Birds, Squirrels, and Other Varmints
Birds are fun to look at, but they do have a few bad qualities when perched on your chimney.
They chirp and flutter constantly, driving sane folks nuts and crazy folks bananas.
Their droppings accumulate in a huge, vulgar mass on the smoke shelf, causing a bad smell the following summer, a breeding ground for mites, and a breeding ground for a rare, but serious, lung disease.
A squirrel can wreck a house faster than a dozen two-year-olds. Once they get inside the chimney, the only way out for them is through the damper. Wait till you see a soot-covered squirrel panic in your living room or den. You will quickly lose interest in them as furry, cuddly creatures.
A Cap Inhibits Down Drafting
Backpuffing of smoke can result from several factors. One of them is downdrafts blowing smoke back into the room.
A Cap Keeps out Leaves
Leaves can choke a flue and set off a chimney fire in a dirty flue. As you can see, there is probably no single investment that you can make for your chimney that will return so much to you. You have invested at least $2,500 in your fireplace; why not protect that investment?
When Was the Last Time You Had Your Chimney Cleaned and Safety Inspected?
While installing the safety cap, it might be a good idea to have us check the chimney, too!