The cause of most chimney fires is creosote which is a by-product resulting from the incomplete combustion of wood. It accumulates on the sides of your chimney as a liquid and later condenses into a solid. As it builds up it not only blocks the flue, but can ignite into a roaring fire. These fires can be prevented by making sure your flue is lined and in good repair, by having it inspected and cleaned regularly by a professional and by learning how to build fires that reduce the build-up of creosote.