Why is a layer of ice and water shield recommended under the flashing in open valleys?

A layer of ice and water shield is recommended under the flashing in open valleys primarily to extend protection against ice and water. This waterproofing membrane is designed to prevent water infiltration that can occur due to melting snow or ice, especially in areas where water may back up in the roofing system.

In open valleys, the design often channels water away from the roof, but it also creates vulnerable points where water can infiltrate, particularly if there are any gaps or weaknesses in the roofing materials. The ice and water shield adheres closely to the substrate, creating a reliable barrier that captures any water that might otherwise seep into the underlying structure. This is critical in regions that experience freezing temperatures, as the potential for ice dams is higher, and it ensures that potential leaks during the winter are mitigated effectively.

This layer not only protects the valley area but enhances the longevity of the roof structure by preventing damage caused by water intrusion. Other options, such as providing a vapor barrier, enhancing roof color, or creating a thermal barrier, do not address the specific concern of water intrusion in the context of open valleys and the need for enhanced waterproofing under challenging weather conditions.