Why the front edge is critical
Water running down the roof hits the chimney's front face directly, so the apron flashing here takes more direct water flow than the step flashing along the sides.
Common problems
Aprons can crack from thermal movement, or lift if the fixing along the top edge corrodes — either issue lets water track behind the flashing and into the roof structure.
Getting the overlap right
A well-fitted apron overlaps the roof covering below it by a proper margin, so water is directed onto the tiles or slates rather than finding a way underneath them.