How to Fix Mortar Gaps in a Fireplace Fire Box

  • 01 of 09

    How to Fix Mortar Gaps in Between Fireplace Bricks

    fireplace fire
    The fire brick and mortar in your fireplace's firebox takes a beating over time. Fotolia

    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 fire box. It is made up of refractory (able to withstand high heat) fire bricks and mortar. Over time, the bricks and the mortar may fail due to the expansion and contraction of the materials as they expand from room temperature to several hundred degrees Fahrenheit and back again.

    When fire bricks...MORE fail and crack or break apart or if you need extensive tuckpointing of fire bricks, you will need to call in a mason to make the repair. However, it's the thin mortar joints between the bricks that more commonly crack and fail over time, leaving a gap in between the fire bricks where the mortar falls out.

    These gaps are dangerous because as a fire rages in the fireplace, the flame can shoot between the gaps of the fire bricks and get behind them and into the wood framing cavity of the home. There a fire can quickly escalate undetected until it is too late.

    It is the repair of these small thin gaps between fire bricks which is the subject of this tutorial. The gaps can be easily repaired by a DIY'er using a special type of caulk called a refractory caulk which contains silica to help gain its high heat capability.

    Needed Tools:

    • Eye Protection
    • Latex or Nitrile Gloves
    • Fireplace Broom / Dust Pan
    • Vacuum Cleaner
    • Linoleum Knife
    • Compressed Air Can
    • Caulk Gun
    • 3M™ Fire Block Sealant FB 136 Refractory Caulk
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  • 02 of 09

    Refractory Fire Block Caulk

    fire caulk
    3M™ Fire Block Sealant FB 136 is a good choice in repairing gaps in fire brick mortar joints. 2016

    This tutorial uses 3M™ Fire Block Sealant FB 136. It is rated to 1,380 degree fahrenheit and properly installed and cured, the caulk will provide the protection between fire bricks needed for safe use of the fireplace.

    Fire Block Sealant FB 136 meets ASTM E-136 and the 2003 International Building Code (IBC), 2003 International Residential Code (IRC) among others, and the NFPA 5000 draft, smoke and fire blocking requirements.

    It is a readily available caulk and features excellent adhesion strength...MORE and can be installed with a standard caulking gun.

    Compare Prices 3M Fire Caulk

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  • 03 of 09

    Gaps in Mortar Between Fireplace Bricks

    gaps in fire bricks
    Gaps between bricks that occur in the fire box are dangerous and need to be filled. 2016

    As the refractory fire bricks and mortar expand and contract from high temperature changes, they can crack and fail. Over time what you will see are gaps in between the fire bricks as shown in this photo.

    These gaps are dangerous because fire can potentially jump between the fire box and behind the fire bricks into the wood framed cavity wall by way of the mortar gaps.

    The gaps can be filled with an actual cement based mortar but it is easier to use a high heat resistant silica based caulk as in...MORE this tutorial.

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  • 04 of 09

    Clean Old Loose Mortar From Fire Brick Joints

    The first step in repairing the thin gaps in the fire brick joints is to clean them out by using a thin, sharp scraping tool. Follow these steps:

    1. Put on eye protection and consider using thin latex or nitrile gloves to keep your hands clean throughout this tutorial.
    2. Remove the fire grate from the fireplace and use your fireplace brush and pan tool to sweep out the fireplace fire box.
    3. Use a vacuum such as a a wet / dry shop vacuum to clean out the fire box of residual ashes and debris.
    4. Use a thin,...MORE sharp metal scraping tool such as a linoleum knife to scrape out any loose mortar from between the fire bricks. Test all the mortar joints for integrity and make sure they are sound.
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  • 05 of 09

    Clean Fire Brick Joints of Debris

    gaps in fire bricks being cleaned
    Use pressurized air or a vaccum cleaner to clean the joints of any remaining debris after being scraped clean. 2016

    Once the mortar joints are scraped of loose mortar, you need to totally clean them of all remaining dust and debris. You can either use a shop vacuum or blow the joints clean using a can of high pressurized air.

    The can of pressurized air is nice in that it allows you to clean deep into the crevices to make sure they are cleaned out. Clean all the joints in preparation for fire stop caulking.

    Once the scraping and blowing-out process is complete, vacuum up the debris that has fallen to the floor...MORE of the fireplace.

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  • 06 of 09

    Fill Caulk Deep into Crevices

    fire caulk being inserted int crevise
    Insert fire caulk deep into the gap betwee the fire bricks. 2016

    Place the caulk into the caulk gun and cut the tip of the caulk tube.

    Slowly force the caulk into the exposed gaps and crevices between the fire bricks. It is easier to work from the bottom up on vertical gaps as you can visually confirm the degree of joint penetration by the caulk.

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  • 07 of 09

    Caulk Joints

    fire caulk beig applied to gaps in fore brick joints
    Fill caulk deep into fire brick gap joints. 2016

    Ensure complete caulk coverage of the joints between the fire bricks. A technique you may want to try is to caulk all vertical joints first and then all horizontal joints.The key however, is to make sure all the gaps are filled with the fire block caulk.

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  • 08 of 09

    Tool Grout Joints in Firebox

    caulked grout joints being finger tooled
    Remove excess caulk with your finger. 2016

    It may be difficult to get a clean lay of caulk into the joints so expect that there will be excess caulk to be cleaned off. You can use your finger or a mortar joint tool to remove the excess material and shape the joint somewhat.

    When applying the caulk you will not be Caulking Like a Pro where great care is taken to hold the caulk to a defined area, but generally try to keep a neat work appearance.

    The caulk will have a stark contrast in its light grey color against the black soot of the fire...MORE brick so where you made a repair will be especially noticeable, but this contrast will fade as the fireplace is used and the caulk darkens.

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  • 09 of 09

    Completed Project

    Once all the gaps are filled with the refractory fire block caulking you may end up with a zig-zag pattern of repaired mortar joints.

    That's fine as the fire block caulk needs exposure to fire for 1 hour in order to properly cure. After that, the caulk will darken and blend in more with the natural soot of a fire box as the fireplace does its job. But your home is now much more safe for fires this winter with this repair than it was with open gaps in the fire box.

    Now check out other tips for g...MOREetting your home ready for winter!