I get a few calls about cracks found in the back of peoples fireplaces. And they ask if they need to repair the cracks in the bricks of their fireplace.
Cracks in the walls of a firebox or interior chimney lining are
considered a fire hazard by the NFPA (National Fire Protection
Association). Smoke carries waste particles from a fire and eventually
causes deposits to form on the walls, and inside and behind any cracks.
This oily substance, called creosote, can re-ignite from sparks rising
in the chimney. Therefore, it’s important to keep these walls free of
creosote build-up and to promptly repair any cracks.
Cracked mortar may be repaired by carefully chiseling out cracked
areas and tuck pointing with portland cement or fireclay, but should
never be surface-coated or smeared, as it will not bond with the dirty,
smoky surface. If any brick are broken (typically a back wall will last
15-25 years in North Texas, depending on usage), it is usually advisable
to rebuild the affected wall with new firebrick and fireclay.