How to Seal a Crack in a Brick Fireplace

Small Crack

• 1

  Rub the crack in the fireplace with a wire brush to loosen cracked and crumbling mortar and brick. Wipe the surface of the crack with a damp cloth so it is clearly visible and free of residual debris.

• 2

  Insert a fireplace mortar cartridge into a caulking gun. Cut off the tip of the tube diagonally with a knife, so its width matches the size of the crack you want to seal.

• 3

  Position the tip of the gun at a 45-degree angle over a corner of the crack. Maintain steady pressure on the trigger so it releases a straight bead of sealant into the crack, from one end to the other. Release the trigger when you reach the other end of the crack.

• 4

  Run your wet thumb over the exposed bead of caulk in the crack to flatten and smooth it. Wipe off excess mortar on the edges of the crack with a damp rag. Let the caulk set according to the time mentioned on the label directions.

Large Crack

• 5

  Clean the cracked section of the fireplace with a wire brush to remove crumbling mortar. Wipe the surface with a damp cloth.

• 6

  Mix a batch of mortar according to label directions, using a trowel. Use specialized heat-resistant mortar specifically for brick fireplaces.

• 7

  Scoop some mortar in the trowel, and spread it over the crack. Push the mortar into the crack with the tip of the trowel. Continue to fill the crack with mortar until it is smooth with the surrounding brickwork and seals the gaps completely.

• 8

  Wipe the surface of the repaired patch with the edge of the trowel to remove excess mortar and give it a smooth finish. Let the mortar dry for the time specified on the label directions.