What is used to waterproof roof valleys and the area between a chimney and a roof?

Flashing is the correct answer because it is specifically designed to prevent water infiltration at roof intersections, such as where a chimney meets the roof or within roof valleys. Flashing consists of thin pieces of metal or other waterproof materials that are installed in joints or around projections on a roof to direct water away and protect against leaks. This is particularly important in chimney applications, as the chimney is a common area where water can seep in, potentially causing significant damage over time.

In contrast, a cricket is a structure used to divert water around a chimney and is placed behind the chimney to help manage water flow, but it does not serve the same primary purpose as flashing. A corbel, on the other hand, is a supportive architectural element used to support a structure above it and is not related to waterproofing. Finally, a flue refers to the duct or pipe that carries exhaust gases from a chimney but does not provide waterproofing. Thus, flashing is the most crucial component in addressing waterproofing in these specific areas.