How to Fix Mortar Gaps in a Fireplace Firebox

Fireplace with fire burning
Image Source / Getty Images
  • Working Time: 3 hrs
  • Total Time: 24 hrs
  • Skill Level: Beginner
  • Estimated Cost: $15 to $45

Your fireplace is a source of beauty and warmth in the cold weather, but the heat generated from a fire will take its toll over time. The interior of the fireplace where the fire occurs is called the firebox. It is made up of refractory firebricks and mortar, which are designed to withstand high heat. Over time, the bricks and mortar can fail due to the expansion and contraction of the materials as they expand from room temperature to 700 degrees Fahrenheit and back again.

Small gaps in mortar can be repaired with a special high-temperature caulk called refractory caulk. It contains silica for heat-resistance and is specifically designed for fireplaces. Refractory caulk, sometimes marketed as fireplace mortar, is suitable for minor repairs only. If your fireplace has broken or crumbling bricks, large gaps in the mortar, or missing mortar, have the fireplace inspected by a professional mason or fireplace contractor. Significant damage in a firebox can be a serious fire hazard and should not be repaired with refractory caulk. ​

What You'll Need

Equipment / Tools

  • Fireplace brush and dustpan
  • Shop vacuum
  • Linoleum knife or putty knife
  • Caulking gun


  • Can of compressed air (optional)
  • Refractory caulk
  • Latex gloves (optional)
  • Paper towels


  1. Remove the Loose Mortar

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

    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 for integrity to make sure they are sound.

    Removing loose mortar
  2. 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.

    gaps in fire bricks being cleaned
  3. 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 helps you visually confirm the degree of joint penetration by the caulk. It may be easiest to caulk all vertical joints first, followed by the horizontal joints.

    fire caulk being inserted in crevice
  4. Tool the Caulked Joints

    Smooth the caulk with your finger to remove excess and to seal the caulk to the edges of the brick. You can wear latex or nitrile gloves to keep your hands clean, if desired. It also helps to wipe off your finger frequently onto a dry paper towel as you work.

    caulked grout joints being finger tooled
  5. Cure the Caulk

    Let the caulk dry as directed by the manufacturer. Its light gray color may appear in stark contrast to the black soot of 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.