Short Answer
Roof planes and penetrations create changes in direction where water can accumulate or move underneath materials. Proper flashing provides controlled water pathways.
Detailed Explanation
Every roof has places where water naturally concentrates or changes direction — valleys where two roof planes meet, transitions where a roof meets a vertical wall, penetrations where pipes or chimneys pass through the roof surface. At each of these points, there's a gap or joint that water could enter. Flashing is what bridges those gaps and creates a controlled pathway for water to flow over and away from the joint. Without flashing — or with damaged, improperly installed, or deteriorated flashing — water follows the path of least resistance, which is often directly into the building. Because flashing covers the most vulnerable points on the roof, it's where most leaks originate. A roof is only as good as its flashing.
Why It Matters
Understanding that flashing is the most leak-prone part of the roof helps homeowners prioritize flashing quality when evaluating proposals and contractors.