Fireplace cracking along mortar joints is a common issue, even with relatively new fireplaces. They’re often caused by normal movement, improper installation, or different materials expanding and contracting at different rates. Fortunately, these cracks are usually cosmetic and can be repaired with a few masonry tools and fresh mortar.
Mason Mark McCullough demonstrates two common repairs:
A separated hearth joint crack
A mortar crack between the fireplace surround and the adjacent wall
How to Repair a Separated Hearth Joint Crack
This type of crack often forms where the hearth meets the fireplace surround. Movement between these surfaces can cause the mortar joint to separate.
- Remove the Old Mortar
- Use a brick hammer and masonry chisel or plugging chisel to carefully remove the damaged mortar. Work slowly to avoid damaging the surrounding brick. Wear safety glasses and hearing protection.
- Vacuum or brush out all loose debris so the new mortar can bond properly.
- Dampen the Brick
- Lightly brush water onto the surrounding brick using a flat chip brush.
- This step is important—dry brick can pull moisture out of fresh mortar too quickly, which weakens the repair and may cause the joints to become sandy, crumbly, and cracked over time.
- Mix the Mortar
- Using a brick trowel, mix Type N mortar in a mortar tub with water until it reaches an oatmeal-like consistency.
- Type N mortar is commonly used for interior brick and fireplace surrounds because it provides the right balance of strength and flexibility.
- Pack the Joint with Mortar
- Pick some mortar up with the brick trowel and hold it near the joint.
- Use a tuck pointing trowel to pack the new mortar into the joint. Use a slightly thinner trowel than the joint itself.
- Pack the joint completely.
- Finish the Joint
- After finishing the joint to match the existing mortar joint profile of your fireplace surround. Check the shape and size of the joint you’re repointing to find the right finishing tool.
- Most fireplace surrounds have concave profiles, meaning there’s an inward curve in the mortar joint. Use a concave jointer to replicate the look.
- Apply pressure to the heel of the tool, not the tip, for a smooth, consistent finish.
- Clean Excess Mortar
- Wipe away any excess mortar using a damp masonry sponge. Be careful not to pull mortar out of the joint.
- How to Repair a Mortar Joint Crack Between a Wall and Fireplace Surround
- This crack typically forms where brick meets drywall or another material. Because these materials move differently, a direct mortar bond can fail. Adding a bond breaker helps prevent future cracking.
- Remove the Old Mortar
- Use a brick hammer and masonry chisel or plugging chisel to carefully remove the damaged mortar. Work slowly to avoid damaging the surrounding brick. Wear safety glasses and hearing protection.
- Vacuum or brush out all loose debris so the new mortar can bond properly.
- Remove the Old Mortar and Install a Bond Breaker
- Apply a strip of self-adhering stainless steel flashing to the drywall behind the joint.
- This creates a bond breaker, preventing the drywall from absorbing moisture and reducing the chance of future cracking.
- Add Backer Rod
- Insert foam backer rod into the joint from top to bottom.
- The backer rod acts as a base, so the mortar doesn’t fall deeper into the gap and helps control how much mortar is needed.
- Dampen the Brick
- Lightly brush water onto the brick with a chip brush to prevent moisture loss.
- Mix the Mortar
- Using a brick trowel, mix Type N mortar in a mortar tub with water until it reaches an oatmeal-like consistency.
- Type N mortar is commonly used for interior brick and fireplace surrounds because it provides the right balance of strength and flexibility.
- Pack the Joint with Mortar
- Pick some mortar up with the brick trowel and hold it near the joint.
- Use a tuck pointing trowel to pack the new mortar into the joint. Use a slightly thinner trowel than the joint itself.
- Pack the joint completely.
- Finish the Joint
- After, finish the joint to match the existing mortar joint profile of your fireplace surround. Check the shape and size of the joint you’re repointing to find the right finishing tool.
- Most fireplace surrounds have concave profiles, meaning there’s an inward curve in the mortar joint. Use a concave jointer to replicate the look.
- Apply pressure to the heel of the tool, not the tip, for a smooth, consistent finish.
- Clean Excess Mortar
- Wipe away any excess mortar using a damp masonry sponge. Be careful not to pull mortar out of the joint.
These simple repairs can restore your fireplace’s appearance and prevent cracks from worsening. If cracks continue to return or grow larger, it may be worth consulting a professional mason to check for structural movement.
Materials
- Type N mortar mix
- Bond breaker (self-adhering stainless steel flashing)
- Foam backer rod
