Question: I recently discovered a horizontal crack across the bottom of my brick fireplace. Is this dangerous? Who do I contact to fix this? How will they fix it and approximately how much will it cost? Thanks Answer: crack is between the hearth extension and the actual fire box and occurs due to settling of the chimney structure. In most cases this can be repaired by simply filling the crack with refractory (fireplace) cement. Many fireplace and hardware stores sell this in tubs or in caulk tubes. Dampen the crack before inserting the cement - this will make it stick better.