How to Fix Mortar Gaps in a Fireplace Firebox

Fireplace with fire burning
Image Source / Getty Images
Project Overview
  • Working Time: 1 - 2 hrs
  • Total Time: 3 hrs
  • Skill Level: Beginner
  • Estimated Cost: $15 to $45

While a fireplace is a source of beauty and warmth in cold weather, the heat generated from a fire will take its toll on the fireplace over time. The fireplace interior where the fire occurs is called the firebox. It is made up of 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 go from room temperature to roughly 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. This fix is a fairly quick, easy, and inexpensive process.


Refractory caulk, also known 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 a professional mason or fireplace contractor inspect it. 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 or nitrile 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 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.

    Removing loose mortar from bricks


  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 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.

    fire caulk being inserted in crevice


  4. 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.

    finger smoothing caulked grout joints


  5. 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.

    brick fireplace