How to Caulk the Cracks in the Walls of a Fireplace

Cracks in the walls of fireplaces are a hazard.
Fill cracks in fireplace walls.Fill cracks in fireplace walls.
The residue of burning wood fires, called creosote, can work its way through the cracks and build up behind the masonry walls. This unseen creosote is just as dangerous and as flammable as creosote that is built-up on the interior walls of the fireplace's chimney. Lower the risk of this happening by mending the cracks in the walls of your fireplace using special fireplace mortar. It is available premixed in the same kind of tubes in which regular builder's caulk is available. Apply this special fireplace mortar using the same type of applicator used to apply regular caulk.

Step 1

Remove loose mortar or cracked brick residue from the cracks in the walls of your fireplace using a small metal rod or mason's tuckpointing tool. Ensure that no chunks of residue remain in the cracks so the caulk used to repair adheres to the bricks when filling the cracks.

Step 2

Go over the cleaned-out cracks with a dry paintbrush to remove any remaining small particles of loose mortar or crumbling brick.

Step 3

Apply special fireplace mortar, which is actually a silicate-based refractory cement caulk, packaged in the same type of container as regular caulk. Insert the tube of fireplace mortar into a caulk gun to apply it to the cracks. This special mortar withstands temperatures up to 2,000 degrees F once it is properly cured. If the cracks are large, apply multiple layers of this caulk, with each layer approximately 1/8 inch deep and 1/8 inch wide. Overlap the layers of caulk slightly to ensure that you cover the entire crack. Allow the fireplace mortar/caulk to dry to the touch before continuing.

Step 4

Start a small, low fire in the fireplace after the caulk is dry to the touch. The ideal temperature of this fire should be below 212 degrees F. Burn the fire at this temperature for one to four hours to completely dry the special fireplace mortar/caulk. After four hours, increase the intensity of the fire to about 500 degrees and keep it at that temperature for another one to four hours. After this curing by fire, the fireplace is ready for normal use.

Things You Will Need

  • Metal rod or mason's tuckpointing tool
  • Dry 3- to 4-inch paintbrush
  • Plastic spray bottle
  • Caulk gun
  • Special fireplace mortar/caulk
  • Firewood

Warning

  • Do not use regular builder's caulk or regular mortar to mend cracks in the interior of your fireplace -- they will not withstand the high temperatures in the firebox.

About the Author

Sharon Sweeny has a college degree in general studies and worked as an administrative and legal assistant for 20 years before becoming a professional writer in 2008. She specializes in writing about home improvement, self-sufficient lifestyles and gardening.