The concept of flashing exists for one purpose: To keep water out of your structure. Flashing is necessary especially at roof to wall intersections, crickets, chimneys and roof and wall penetrations such as plumbing vents, windows and doors. Different types of flashing, sometimes in multiple layers are employed to keep water out.
As simple as this sounds, the basic elements of proper flashing such as weather-lapping are often overlooked having the opposite desired effect of diverting water into your structure rather than away.
Visual Consulting finds the problems time and again.