use mortar to seal up any holes around crown
Prevent Chimney Leaks
Mix the mortar; it should be the consistency of cookie dough, not too dry and not soggy.
Note: Refer to the resources listed below for tips on mixing mortar.
While the mortar rests, prep the area by clearing away any loose debris.
When applying the mortar to the crown, form the mortar into an apex with the peak at the middle of the crown sloping down toward the edges. Think of it like a roof: the idea is to have the water drain away from the crown (Image 1).
Note: Don’t make the edge too thin; keep the edge at an approximately 1/2"-3/4" height to prevent it from possibly detaching later. Also, use the mortar to seal up any holes around the crown (Image 2).
Let the mortar to cure for several hours before doing any further work on the crown.