DIY Firebox Mortar Repair

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The firebox within a fireplace, oven or similar structure is the internal chamber where the fire burns. This structure, made with firebricks and a special heat-resistant mortar known as refractory mortar, insulates the surrounding structure from damaging temperatures and reflects heat into the room. In many situations, especially with older fireplaces where regular mortar was used instead of refractory mortar, the joints between firebricks may have started to deteriorate and crumble, requiring repair. Firebox mortar repairs are fairly simple but must include a thorough cleaning of the joints and proper application of the correct type of mortar.

Things You'll Need

  • Safety glasses
  • Shop light or head lamp
  • Fireplace cleaner, if needed
  • Mortar raking tool
  • Masonry chisel
  • Hammer
  • Stiff-bristled brush
  • Refractory mortar
  • Trowel
  • Joint filler
  • Jointing tool
  • Sponge
  • Clean the firebox so that all of the joints can be visibly inspected. Use a fireplace cleaner according to the manufacturer's instructions to remove soot.

  • Remove faulty mortar to a depth at which sound mortar is reached. Attempt mortar removal first with a mortar raking tool. If the mortar is stubborn, use a masonry chisel and hammer to chip mortar out.

  • Brush the joints with a stiff-bristled brush to remove any remaining mortar still attached to firebricks, and clean debris out of the empty spaces.

  • Dampen the surfaces of the firebricks where mortar will be placed. The brick surfaces should be moist but not have standing water.

  • Prepare a refractory mortar. Use a hydraulic-setting refractory mortar, preparing it according to manufacturer's directions if it was not purchased in a ready-to-use form. Consider purchasing a mortar with admixtures that are mixed into the mortar to reduce shrinkage and improve bonding.

  • Apply refractory mortar to the joints in layers using joint filler. Each layer should be no thicker than 1/4-inch to limit air pockets. Let each layer of refractory mortar sit for 30 minutes before the next layer is applied.

  • Smooth the joints with a suitable jointing tool once the final layer of mortar is applied. Use a jointing tool that matches the existing joints for aesthetic purposes.

  • Brush any excess mortar off the surfaces of the bricks using a stiff-bristled brush. Allow the mortar to cure for several weeks if possible before utilizing the firebox unless the mortar manufacturer provides a more specific curing time requirement.

Tips & Warnings

  • Use a camp sponge to clean off any mortar that drips onto the face of the bricks.
  • Wear safety glasses when chipping mortar or using powerful cleaners.

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References

  • "Masonry and Stonework"; Tom Lemmer
  • Photo Credit Comstock/Comstock/Getty Images
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