How to Repair a Brick Chimney

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. Here’s what to do:

   • 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 and recessed from the surface of the brick.

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. Typically, gaps form in this area after several seasons. Water that enters this gap can flow 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.

   Tip

   Another source of water damage is the step flashing. It is sheet metal pieces “stepped” into the brick where it meets the roof line and is laced into the roofing. Sometimes the mortar or bricks need to be repaired or replaced around the metal pieces. Sometimes a good silicone caulking will do the trick.

3. Patch Cracks in the Large Crown Area

   The chimney crown is a sloping cap that forms the topmost 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 in 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 it into the crack with a putty knife.

   Tip

   Curing time for a repair mix typically ranges from four to six hours, but it may take longer for wider cracks.

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 a masonry sealer that has a very liquid consistency which can easily 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 sealer undiluted with a brush. One gallon will be adequate for two coats on an average-sized chimney crown.

5. Repair Cracked Bricks

   Individual chimney bricks that have a small crack or two can be repaired with 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.

   Tip

   If the brick cracks are very thin hairline cracks, you may be able to close them up by brushing them with masonry sealer.

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 masonry 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.

When to Call a Professional

If you think there may be problems with your chimney that go beyond small cracks and minor repairs, it’s best to call in a professional. In addition, a certified chimney sweep service can look down inside your chimney with the right tools and spot obstructions and damaging creosote.