Installing a chimney cap can prevent roof fires, as its spark arrestor will trap the hot embers. A cap inhibits downdrafting. Backpuffing of smoke can result from several factors. One of these is downdrafting, blowing smoke back down into the room. A cap keeps out leaves.
Leaves can choke a flue and set off a chimney fire in a dirty flue.
Dust-like carbon deposits called creosote collect on the inside of a chimney flue, impairing the draft. Creosote is a natural by-product of burning wood. The build-up will vary depending on the type of wood most used in the fireplace. Soft woods, including pine and artificial logs, produce the highest level of creosote. These burn fast and leave a high deposit of creosote.