Today’s building code requires liners in all new chimneys. These smooth metal or hard clay liners allow for a smooth flow of exhaust, and a more efficient operation of the fireplace. It is also easier to clean a lined chimney.
Before 1954 or so, fireplaces were typically constructed of stone or bricks and mortar, and were generally not lined. If you have an old fireplace, the mortar may have broken down into its original state – sand. That means you might have small holes in the chimney that could lead to fire in the walls.
The absence of a liner also increases the build-up of creosote, a tar-like substance. Creosote can be ignited by heat. The resulting flash fire could be intense enough to crack your masonry, or generate enough sparks to start a roof fire.