How to Repair Your Brick Chimney

  • 01 of 07

    Introduction

    Do-it-yourselfer fixing a brick chimney
    Lee Wallender

    Masonry fireplaces, whether wood-burning or gas, are cozy features that represent the epitome of home for many owners. Yet cracks in a brick or other type of masonry chimney are the first warning sign that your chimney is on the road to ruin. Patching cracks in your brick chimney can save you thousands of dollars worth of repairs later on, as well as keep you and your family safe by reducing the possibility of chimney fires.

    Small Cracks in Brick Lead to Larger Cracks

    Small cracks in the summer can become surprisingly large cracks by next spring. Letting those larger cracks remain deepens the penetrating effect of rain, snow, and ice, further coring into the brick. Then the water begins to work its way down, sometimes between the outer brick and the flashing, sometimes farther inside, between the outer brick and the flue.

    Left unchecked,these tiny events accumulate and result in disaster for roof systems and interior ceilings, insulation, wall studs, and even floors.

    Repair Cracks to Prevent Chimney Fires

    But even worse, cracks in a brick chimney that continue from the flue to the exterior are one cause behind a terrifying, devastating phenomenon: a chimney fire.

    Chimney fires can ignite instantly—those who have experienced them describe it as an explosion followed by a sound similar to a freight train. Once a chimney fire has started, only the fire department can stop it since it must be extinguished from the top-down.

    The good news is that cracks in chimney brick, mortar, crown, and cap are very easy for a do-it-yourselfer to fix with only a few simple tools and materials. Pick a warm, dry day for this project, as some of the materials need a few hours of curing time.

    Codes and Regulations

    In most communities, minor brick chimney repair that does not involve replacing any elements of the chimney should not require permitting. If you are removing and replacing bricks in your chimney, check with your local permitting office.

    When to Repair Your Brick Chimney

    Brick chimney repair is dependent on seasons and weather. Do not apply mortar when temperatures are below 40 degrees F. If you are applying the mortar in warm weather but expect temperatures to drop below freezing within 24 hours, do not apply the mortar. Instead, wait until the temperatures are favorable.

    Project Metrics

    • Working Time: 60 minutes
    • Total Time: 75 minutes
    • Skill Level: Beginner
    • Material Cost: $20 to $40

    What You'll Need

    Equipment/Tools

    • Joint raker, used for removing brick mortar
    • Tuckpointing tool for adding and smoothing down brick mortar
    • Ladder
    • Chisel
    • Hammer
    • Pointing trowel
    • Wire brush
    • Putty knife
    • Garden hose and water source
    • High heat mortar and caulking gun
    • Clean paintbrush
    • Safety glasses
    • Latex gloves
    • Clean rags

    Materials

    • Mortar
    • Water-based silane/siloxane water repellent such as Prosoco Sure Klean
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  • 02 of 07

    Repoint Your Chimney Brick Mortar

    Person using a tool to repoint chimney bricks.
    Ian Lishman/Getty Images

    Mortar holds bricks together. New mortar is smooth and solid. But as the seasons go by, the elements batter the mortar, causing it to crumble. Since mortar is softer than brick, the mortar will always deteriorate first.

    Repointing, or tuckpointing, is a common method of scraping out the crumbly mortar and replacing it with new mortar.

    1. Use your joint raker to scrape out loose, weak mortar. Leave good mortar in place.
    2. For more difficult crumbly mortar, gently tap it out with your hammer and chisel.
    3. With your wire brush, sweep out all remaining bits of mortar.
    4. Spray down the brick with the garden hose, and let it sit for 30 minutes.
    5. Mix up the mortar until it has the consistency of stiff peanut butter.
    6. With your pointing trowel, press the mortar into the open joints so that it matches the look of the existing mortar.
    7. Smooth down the joints with the tuckpointing tool.
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  • 03 of 07

    Caulk Around the Flue and the Crown

    Person using caulk and caulk gun around chimney.
    Lee Wallender

    One major avenue for water to enter your home and cause damage is between the flues and the crown. Typically, a gap will form in this area after several seasons. Water that enters this gap can move all of the ways down the flue.

    With the wire brush, clean out any debris such as ​moss and mortar. Load up the caulking gun with high heat mortar, snip the end of the nozzle, and pierce the interior seal. Squeeze enough caulk to fill but not exceed the area of the gap.

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  • 04 of 07

    Patch Cracks in the Large Crown Area

    Person filling cracks near a chimney.
    Lee Wallender

    A chimney crown is a type of curved, sloping hat that forms the top-most section of your chimney. Its purpose is both to cap the masonry section of the chimney and to prevent water from pooling on top.

    When cracks develop, they can become a problem faster than with cracks on the side vertical sections of the chimney. This is because water, snow, and ice remain longer on these largely horizontal areas. Even with a recommended 1:4 ratio slope on the crown, moisture will linger on this area.

    Large cracks on the chimney crown between 1/8-inch and one-inch can be repaired with pre-mixed cement patch or mortar. Inject the repair mix into the crack with a squeeze bottle or force into the crack with a putty knife. Curing time typically ranges from four to six hours but may take longer for wider cracks.

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  • 05 of 07

    Patch Hairline Cracks With the Sealant

    Scraper pointing out hairline cracks on a chimney.
    Lee Wallender

    Hairline or spider cracks are treated differently from large cracks since mineral particles in the patch or mortar are too large to fit in the hairline crack's narrow space. The solution is to use a water repellent that is highly liquid in consistency, as this will penetrate the hairline cracks. As a bonus, this type of product will seal up all other areas of the porous crown, preventing additional hairline cracks from forming.

    Use the corner of the putty knife to scrape away any loose particles but do not enlarge the crack. Apply the water repellent undiluted with a brush. One gallon will be adequate for two coats on an average-sized chimney crown.

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  • 06 of 07

    Replace Cracked Bricks

    Person using a putty knife to fix cracks on a chimney.
    Lee Wallender

    Severely damaged bricks should be entirely removed and replaced. It is possible to carefully remove a single brick without affecting adjacent bricks.

    1. With a cold chisel or old flat-head screwdriver, slowly chip away mortar from between the bricks. Using a masonry-bit-equipped drill to bore holes into the mortar will speed up the process and allow you to reach farther back.
    2. Once the brick is loosened, pull it out by hand.
    3. Use your chisel to chip away residual mortar from the adjacent bricks so that they are smooth.
    4. Butter all four sides of the replacement brick (not including the front or back) with a trowel and brick mortar.
    5. Push the replacement brick in place, centering it so that the seams on all sides are of equal width.
    6. Remove any excess mortar and smooth it down with the tuckpointing tool.
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  • 07 of 07

    Repair Cracked Bricks on Your Chimney

    Person using a caulking gun to repair chimney cracks.
    Lee Wallender

    Individual chimney bricks that have a small crack or two can be repaired with a high-heat mortar and a caulking gun.

    1. Clean out the crack with the edge of the putty knife or with the screwdriver.
    2. Sweep the crack clean with the wire brush.
    3. Squeeze the high-heat mortar into the crack as far as possible. Generally, the mortar will not extend very far unless it is a large crack.
    4. With your gloved finger, wipe the mortar clean from the crack. Use a rag to clean the area around the crack.

    If the brick cracks are very thin hairline cracks, you may be able to close them up by brushing them with the silane/siloxane water repellent.