How to Repair a Brick Chimney

Fixing Chimney
Lee Wallender
Overview
  • Working Time: 2 hrs
  • Total Time: 4 hrs
  • Skill Level: Intermediate
  • Estimated Cost: $20 to $40

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 in the masonry joints between bricks are early warning signs 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 the summer can become surprisingly large cracks by next spring. Letting those larger cracks in the outer brick go unattended will deepen the penetrating effect of rain, snow, and ice. 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 can accumulate and result in disaster for roof systems and interior ceilings, insulation, wall studs, and even floors.

Repair Cracks to Prevent Chimney Fires

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.

In most communities, minor brick chimney repair that does not involve replacing any elements of the chimney should not require a permit. If you are removing and replacing bricks in your chimney, check with your local permitting office. In some areas, this kind of work requires that you apply for a permit and have the work inspected once you complete it.

Brick chimney repair is dependent on seasons and weather. Do not apply mortar when temperatures are below 40 degrees Fahrenheit. 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.

Roof Safety

Any time you are working on a roof there is the potential for falls and serious injury, and the higher the roof or the steeper the pitch of the roof, the greater the danger will be. If you choose to do your own chimney repair, make sure to work on a dry day, and to wear shoes or boots with firm-grip soles. Never work on a roof that is damp. A mild, dry day is ideal for working on a roof.

Consider using a safety harness, also called a fall-arresting harness, whenever working on a roof—especially if the roof is very steep or very high. The equipment includes a metal ridge anchor that is attached to the peak of the house, a body harness that fits around your back and hips, and a rope with an automatic locking mechanism that prevents you from falling a long distance. Fall-arresting harnesses can be rented at home improvement centers and tool rental outlets.

Work slowly and carefully when making chimney repairs. Impatience and hurried work invite accidents.

What You'll Need

Equipment / Tools

  • Joint raker
  • Tuckpointing tool
  • Ladder
  • Masonry chisel
  • Hammer
  • Pointing trowel
  • Wire brush
  • Putty knife
  • Garden hose
  • Caulk gun
  • Paintbrush
  • Safety glasses
  • Fall-arresting harness (optional)

Materials

  • High heat mortar
  • Latex gloves
  • Mortar
  • Clean rags
  • Water-based silane/siloxane water repellent such as Prosoco Sure Klean

Instructions

Not all repair projects will require you to complete all the following steps. Minor chimney damage may require only cursory work. But it is a good idea to examine the chimney carefully and complete whatever work is called for.

  1. Repoint the Chimney Brick Mortar

    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 the process of scraping out the crumbly mortar and replacing it with new mortar.

    Use a joint raker to scrape out loose, weak mortar, leaving the good mortar in place. For more difficult crumbly mortar, gently tap it out with a hammer and masonry chisel. With a wire brush, sweep out all remaining bits of mortar.

    Spray down the brick with the garden hose, and let it sit for 30 minutes. Make sure the roof is dry before you continue work.

    Mix up a batch of mortar until it has the consistency of stiff peanut butter. With a pointing trowel, press the mortar into the open joints so that it matches the look of the existing mortar. Smooth down the joints with a tuckpointing tool. The joint should be slightly concave, recessed in from the surface of the brick.

    Person using a tool to repoint chimney bricks.
    Ian Lishman/Getty Images
  2. Caulk Around the Flue and the Crown

    One major avenue for water to enter your home and cause damage is at the joint between the chimney flue and the crown—the concrete cap that covers the top of the chimney brick. Typically, gaps form in this area after several seasons. Water that enters this gap can flow all the way down the sides of the flue.

    With a wire brush, clean out any debris such as ​moss and loose mortar. Use a caulk gun and a tube of high-heat mortar to fill this gap with a complete bead of caulk.

    Person using caulk and caulk gun around chimney.
    Lee Wallender
  3. Patch Cracks in the Large Crown Area

    The chimney crown is a sloping cap that forms the top-most section of your chimney. Its purpose is both to protect the masonry section of the chimney and to prevent water from pooling on top. Cracks in the crown are even more problematic than cracks on the vertical sections of the chimney, since water, snow, and ice can gather there. 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 and 1 inch in diameter 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 it may take longer for wider cracks.

    Person filling cracks near a chimney.
    Lee Wallender
  4. Patch Hairline Cracks With Sealant

    Hairline cracks, sometimes called 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, which can 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.

    Scraper pointing out hairline cracks on a chimney.
    Lee Wallender
  5. Repair Cracked Bricks

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

    Clean out the crack with the edge of a putty knife or with a screwdriver. Sweep the crack clean with a wire brush. Squeeze high-heat mortar into the crack as far as possible. Generally, the mortar will not extend very far unless it is a large crack. With a 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 water repellent.

    Person using a caulking gun to repair chimney cracks.
    Lee Wallender
  6. Replace Cracked Bricks

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

    With a masony chisel or old flat-head screwdriver, slowly chip away mortar from between the bricks. Using a drill and masonry bit to bore holes into the mortar will speed up the process and allow you to reach farther back.

    Once the brick is loosened, pull it out by hand. Use the chisel to chip away residual mortar from the adjacent bricks so that they are smooth.

    Use a small trowel to butter all four sides of the replacement brick (not including the front or back) with mortar. Push the replacement brick in place, centering it so that the seams on all sides are of equal width. Remove any excess mortar and smooth the joints with a tuckpointing tool.

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