There’s a part of your roof that is called a ‘valley’. A valley is where one slope of your roof meets another slope of your roof. It’s concave in structure and, therefore, similar to a valley in nature.
Valleys carry water down the roof to the gutters/eavestroughs.
These days most houses have metal valleys. These metal valleys are about 12’’ wide (although only approx 4”-8’’ of it are showing) and 8 to 10 feet in length. They work very well, certainly better than the old asphalt membrane rolls.
Recently I was called upon to resolve some roof leaks that caused significant damage to the interior of a house. The source of these leaks was a tiny hole. A hole of ⅛ or ¼ of an inch on a roof is tiny but a hole of such size on a valley where the water runs continuously is significant.
The water flowed into the attic, saturated the insulation and continued down into the living space.
How does such a tiny hole or tear happen? Let’s just say the metal valley got bent when being transported from the ground to the roof or from one part of the roof to another. The wind most likely caught the lengthy valley and bent it causing a crease. Following this a hole formed, ever so slightly, right in the middle of the 12” wide metal valley.