Fix the Crown or Call an Expert
Cracks in the crown allow water in (top left). Water that gets inside the chimney through cracks in the crown can cause efflorescence and damage the bricks.
Seal the crown (bottom). Small cracks in the crown can be sealed with an elastomeric masonry sealer, but a crumbling crown will have to be replaced. Smear on the sealant by hand, then smooth it with a brush.