How to Fix Mortar Gaps in a Fireplace Firebox

Remove the Loose Mortar

Remove the fire grate from the fireplace and use a fireplace brush and dustpan to sweep out the firebox. Vacuum the firebox floor and walls with a shop vacuum to remove all residual ash and debris.

Next, scrape out loose mortar from between the fire bricks using a linoleum knife or putty knife. As you work, inspect all of the mortar joints to make sure they are sound.

Clean the Mortar Joints

Clean out the mortar joints by sucking out the debris with the shop vacuum or blowing it out with a can of compressed air. Thoroughly vacuum the walls and floor of the firebox again.

Fill the Joints With Caulk

Insert the caulk into the caulking gun and cut the tip of the caulk tube as directed by the manufacturer. Slowly force the caulk into the gaps, filling them as deeply as possible. On vertical joints, working from the bottom up can help you visually confirm the degree of joint penetration by the caulk. It can be easiest to caulk all vertical joints first, followed by the horizontal joints.

Smooth the Caulked Joints

Smooth the caulk with your finger to remove excess caulk and seal the caulk to the edges of the brick. You can wear latex or nitrile gloves to keep your hands clean if you wish. It also is ideal to frequently wipe off the caulk from your finger onto a dry paper towel as you work. That way, you won't accidentally reapply the excess caulk where you don't want it.

Cure the Caulk

Let the caulk dry as directed by the manufacturer. Its light gray color might appear in stark contrast with the black soot covering the firebrick, but this will blend in as the fireplace is used and the caulk darkens. Refractory caulk usually cures fully after exposure to fire for one hour.