How to Repair or Replace a Chimney Crown

Chimneys are the unsung heroes of a home, but they take a constant beating from the weather. A chimney crown is the solid slab of concrete at the very top of the masonry stack, designed to shed water away from the bricks and flue. Over time, freeze-thaw cycles cause this slab to crack, crumble, or separate, which allows water to seep directly into the chimney structure, leading to costly masonry damage and interior leaks. Getting this right means ensuring water stays outside where it belongs. A well-executed repair or replacement doesn't just patch the surface; it creates a watertight seal that lasts for years. You are looking for a smooth, sloped surface that hangs over the edge of the chimney bricks to act as an umbrella, directing rain away from the vulnerable mortar joints below.

  1. Scrub Away All Loose Material. Use a stiff wire brush to scrub away all loose debris, moss, and crumbling mortar from the top of the chimney. Vacuum up the dust so that your repair material has a clean, solid surface to grip.
  2. Seal Hairline Cracks First. For cracks less than a quarter-inch wide, use a masonry-specific polyurethane sealant. Inject the sealant deep into the crack and smooth it out with a pointing trowel to ensure a flush finish.
  3. Build Your Concrete Mold. If the crown is badly damaged, wrap heavy-duty flashing around the top of the chimney to act as a mold. Ensure the mold extends about two inches past the brick to create a protective drip edge.
  4. Mix Reinforced Concrete Batch. Mix high-strength, fast-setting concrete in a bucket until it reaches a thick, moldable consistency. Add a concrete bonding agent to the mix for added durability and weather resistance.
  5. Create a Sloped Drip Edge. Pour the concrete into your mold, working it well around the chimney flue. Use a trowel to slope the surface downward from the flue to the edges so water drains away from the chimney center.
  6. Cure and Protect 48 Hours. Smooth the surface with a steel trowel once the concrete begins to set. Let the crown cure for at least 48 hours before removing the flashing mold.