Your fireplace comprises of three parts: the firebox assembly or combustion chamber, the fireplace opening and the hearth. If you spot a cracked brick or mortar along the firebox walls or interior chimney lining, it must be repaired immediately. The NFPA ranks cracks or other signs of deterioration within the firebox assembly as a fire hazard. Cracks are sometimes caused by mortar joints that may be too wide, allowing oily deposits called creosote to wedge inside and behind these narrow openings. Creosote buildup may also form along the firebox walls. As a result, flying sparks can re-ignite creosote and cause deadly fire outbreaks. The NFPA lists creosote buildup as the No. 1 cause of chimney fires.
If you have a firebrick fireplace, use mortar high in calcium and lime in motor joints no larger than 0.25 inch. Apply fireclay mortar, which has a high fire resistance and can result in joints 0.125 inch in width or less. Portland cement can also be used to patch up cracks. Carefully chisel out cracked mortar in the firebox, and shape thin strips of fresh mortar into the joints. Avoid smearing or applying cement or fireclay over old mortar. Surface-coated or smeared mortar will likely not stick to the fireplace's sooty surface. Back walls with broken bricks should be rebuilt with new fireclay or firebrick material.
Build a fireplace that adheres to local building codes and uses quality materials to prevent cracks or other serious problems from forming. For example, coat chimney flues with parging made from fireclay or similar mortar material to prevent hot gases from escaping through cracks and mortar joints. Also, consult a building inspector or experienced mason on appropriate dimensions and materials and possible design constraints. Dealing with problems prior and during construction will give you peace of mind and extend the lifetime of your fireplace.
Cracks found in fireplaces built from firebrick pose immediate safety issues, according to Ask the Builder author Tim Carter. He states, “Cracks in granite countertops, dents in new drywall, blemishes in ceramic tile do not present life safety issues. But a gap or crack in firebrick that is poorly installed can cause a house to burn to the ground.” Hot gases can sneak into these cracks and ignite the surrounding wood framing. Cracks in such fireplaces can cause irreparable fire damage to your house or serious injuries and death.