What is the consequence of insufficient head lap in roofing?

Insufficient head lap in roofing is the vertical overlap of shingles or roofing materials that is needed to ensure water runs off the surface properly. When the head lap is inadequate, water can seep underneath the roofing materials instead of draining off smoothly. This can lead to excessive water leakage, which may result in significant damage to the roof structure, insulation, and interior spaces of the building.

In contrast, an adequate head lap helps direct water away from the roof system's layers, which enhances the roof's ability to keep the building dry and prevents potential leaks and moisture problems. Thus, the consequence of insufficient head lap is primarily related to water management, making excessive water leakage the correct answer to the question posed.