How to Repair a Brick Chimney
Repairing Cracks in Chimney Crowns, Bricks, and Mortar
Masonry fireplaces, whether wood-burning or gas, are cozy home features but they can require chimney repair as they age. Cracks in brick, or in the masonry joints between bricks, are early warning signs that your chimney needs attention. 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. The good news is that cracks in a chimney's brick, mortar, crown, and cap are 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.
Small cracks in the summer can become surprisingly large cracks by next spring. Unattended larger cracks in the outer brick will deepen the penetrating effect of rain, snow, and ice. Then, the water works its way down between the outer brick and the flashing but sometimes farther inside between the outer brick and the flue. The water can accumulate and result in substantial damage for roof systems and interior ceilings, insulation, wall studs, and floors.
Repair Cracks to Prevent Chimney Fires
Cracks in a brick chimney that continue from the flue to the exterior can cause an extremely damaging chimney fire. Chimney fires can ignite instantly—they can sound like an explosion followed by a sound similar to a freight train. Once a chimney fire has started, only the fire department can stop it by extinguishing it from the top down.
One of the major causes of chimney fires is damage to the interior clay flue liner and joints. Creosote builds up in the cracks and eventually will catch fire. These problems are hard to fix. Most contractors will install a metal flue liner to correct the problem.
Before You Begin
In most communities, a permit shouldn't be required for minor brick chimney repair that does not involve replacing any elements of the chimney. However, if you are removing and replacing bricks in your chimney, check with your local permitting office. In some areas, this kind of work requires a permit and inspection.
Brick chimney repair is also 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 for favorable temperatures.
Any time you're working on a roof, especially a high or steep roof, there's the potential for falls and serious injury. If you choose to do your own chimney repair, make sure to work on a dry day and wear shoes or boots with firm-grip soles. Never work on a damp or wet roof. Work slowly and carefully when making chimney repairs. Impatience and hurried work invite accidents.
Be wary of heights and consider using a safety harness, also called a fall-arresting harness, whenever working on a roof—especially one that's steep or 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. Rent fall-arresting harnesses at home improvement centers and tool-rental outlets.
What You'll Need
Equipment / Tools
- Joint raker
- Tuckpointing tool
- Masonry chisel
- Pointing trowel
- Wire brush
- Putty knife
- Garden hose
- Caulk gun
- Safety glasses
- Fall-arresting harness (optional)
- High-heat mortar
- Latex gloves
- Clean rags
- Masonry sealer
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.
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.
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.
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.
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.
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.
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.
Both the National Fire Protection Association and the Chimney Safety Institute of America recommend an annual inspection of all chimneys, stacks, and vents.
FAQs. Chimney Safety Institute of America.