Repair or replace a deteriorated chimney crown, sizing the new cast-in-place crown correctly (overhang, drip edge, and bond break)

Steps

1. Inspect the existing crown for cracking, spalling, or a thin/undersized original crown (common on older chimneys) versus a full rebuild being needed
2. Reference chimney-industry guidance (e.g., Chimney Safety Institute of America publications) for standard crown design: overhang of roughly 2 to 2.5 inches beyond the chimney's outside face on all sides, with a drip edge formed on the underside of the overhang so water sheds clear of the masonry face
3. Form a bond break (foam, membrane, or building paper) between the top course of masonry and the new crown so the crown can expand/contract independently and not crack the bond at the brick
4. Slope the crown surface away from the flue toward the outer edge to actively shed water, and leave an expansion gap (roughly 3/8 inch) around the flue liner rather than parging the crown tight to the tile
5. Use a crack-resistant, reinforced mix (or a proprietary crown coating/sealant product per manufacturer instructions) rather than a thin unreinforced mortar cap prone to shrinkage cracking
6. Seal the cured crown with an appropriate water-repellent sealant and inspect the flashing at the crown/roof intersection while the work is exposed