A properly constructed chimney crown is very rarely found on the vast majority of chimneys.  In most cases, a chimney crown consists of the sloped application of mortar at the top of the chimney with the purpose being to divert water away from the flue tiles so that it drains off the chimney.  Mortar is the appropriate material when laying up masonry units, be they brick, block, or stone. However, it is not an acceptable material for use on a chimney crown in as much as it will develop hairline cracks during the drying process. These cracks serve as entry points for water which will result in damage to the chimney during freeze/ thaw cycles in the cold weather months.  CrownCoat is an elastomeric material that has been developed to address this very problem when properly applied over the entire chimney crown.