One reason your chimney crown may have cracked is related to thermal expansion. When your fireplace is active it heats up your flue tile causing it to expand slightly in both the horizontal and vertical directions. With no expansion joint, your flue tile will push against the chimney crown on all sides and something has to give, usually your chimney crown. This very slight expansion is enough to create hairline cracks in your chimney crown, which doesn’t get hot enough to expand at the same rate as your flue tile. Those hairline cracks will become worse over time as water enters them and then freezes and expands.
This thermal expansion is only a problem if your chimney crown was built without a proper expansion joint. An expansion joint is a ¼” to ⅜” gap between your top flue tile and chimney crown filled with foam that is wrapped around your clay flue tile when your chimney crown is built around it. There is no magic in the foam. The cheap foam roll you can buy at Home Depot and other big-box stores is also used as an underlayment for sill plates and window installation. The expansion joint should be sealed with a flexible, durable sealant to ensure that water does not seep in around the expansion joint. Remember, your flue tiles will shrink when they cool so whatever sealant you use needs to move with your tiles as they expand and contract due to thermal expansion.
For some reason, many masons throughout the years have attempted to use tar paper as an expansion joint material. Tar paper does not allow for the proper horizontal expansion of the heated chimney flue and will not be as effective in preventing cracks in a chimney crown. Unlike foam, tar paper is not compressible and will not allow for expansion.
It’s very difficult to add an expansion joint after the chimney crown is already built. At that point, if enough damage is present it would be better off to build a new chimney crown and add the expansion joint.
There are many chimney crowns still that have been built without an expansion joint. These often have hairline cracks that have been sealed and treated with a product like CrownCoat and will last for years to come.