Why do valleys leak?

Valleys can leak primarily because the corners are not dubbed, which refers to the technique of shaping the roofing material at the corner of the valley to ensure proper water flow and drainage. A dubbed corner typically means that the materials are cut and designed to channel water smoothly down into the valley, preventing pooling or backflow that might lead to leaks.

When the corners are not properly shaped, water can accumulate and create gaps between the materials, allowing for the possibility of leakage. Properly dubbed corners help to direct water effectively toward the gutter system or the next downspout, ensuring that the water is not trapped where it can cause potential damage.

In contrast, while factors such as width and flatness can influence drainage, they are not as directly tied to leaking as the treatment of the corners of the valley. Additionally, while valleys need to be sealed adequately to prevent leaks, the lack of dubbing is often a more critical point in the physical construction of a valley that directly leads to leaking issues.