Final answer:
In re-roofing, penetrations are the most likely areas to leak due to the breaks in roofing material they create, followed by valleys which handle more water.
Step-by-step explanation:
When re-roofing, the areas of a roof that are most likely to leak are the penetrations. These are areas where items such as vents, chimneys, or skylights pass through the roof. Since these spots require breaking the continuity of the roofing material, they are particularly vulnerable to water ingress if not properly sealed. Second to penetrations, the valleys of a roof, where two slopes meet, can also be prone to leaking due to the higher volume of water they handle during rain. The edge of the roof and the gable ends are generally less prone to leaks when properly installed with appropriate flashing and overhangs.