How do you fix a chimney crown?
Short answer. For hairline cracks, clean the crown and brush on an elastomeric crown sealer that bridges the cracks and stays flexible. If the crown is broken through, crumbling or was built as a thin layer of mortar, it should be rebuilt as a properly formed concrete crown that slopes away from the flue, overhangs the brick with a drip groove, and has a flexible bond break where it meets the flue liner.
Crown or wash?
Many chimneys were finished with a thin, sloped layer of mortar troweled over the top bricks, often called a mortar wash. Mortar is not meant to be a cap, so it cracks and lets water into the brick below. A true crown is a cast concrete slab, thicker, sloped to shed water, and extending past the face of the brick.
Repairing a sound crown with hairline cracks
- Pick a dry, mild day with no rain in the forecast.
- Remove the cap if needed, and clear debris, loose material and old sealant.
- Fill any crack wider than a hairline with a patching product made for crowns.
- Brush the elastomeric crown sealer over the whole crown, carrying it onto the flue tile slightly as the product directs.
- Reinstall the cap, ideally one that covers the whole crown.
When to rebuild
- Pieces breaking off, or cracks that go all the way through.
- Stains or spalled brick in the top courses below the crown.
- Water in the firebox even with a good cap.
What a good rebuilt crown has
- Concrete thick enough to resist cracking, sloped away from the flue.
- An overhang past the brick face, with a drip groove underneath so water falls clear instead of running down the brick.
- A bond break, a flexible gap or sleeve around the flue tile, so the hot liner can expand without cracking the crown, finished with a flexible sealant.
Crown work happens at the very top of the chimney, so steep or two-story roofs are a job for a mason or chimney professional with fall protection. See chimney leaks and crickets and common roof leak causes.