What type of flashing overlaps the top edge of the base flashing on a chimney?

Counter flashing is the correct answer because it is specifically designed to work in conjunction with base flashing to prevent water intrusion at roof joints, especially in areas like chimneys. Counter flashing is installed above the base flashing and consists of a metal or a similar waterproof material that overlaps the top edge of the base flashing. This arrangement ensures that any water that runs down the chimney is directed away from the roof and into the gutters or away from the structure, thus maintaining the integrity of the roofing system and preventing leaks.

Cap flashing is typically used at the top of parapet walls or the edges of roof openings, serving a different purpose. Step flashing relates to the design used around structures where the roof meets vertical walls, using a layered approach to ensure water runoff. Drip edge flashing is installed at the eaves of a roof to direct water into the gutters and is not directly associated with chimney structures. Each type of flashing has its unique application in roofing, but the function of counter flashing is specifically to enhance the sealing of the joint between the chimney and the roof.