The majority of homes in the United States use some type of asphalt based shingles to cover their roof. Those shingles have some very specific installation instructions. One of those is that the shingles should be laid across the slope lengthwise. NOT up and down the slope lengthwise. MOST people know this. Some don’t and proceed to do silly things that cause roof leaks. See the pictures below of how NOT to lay shingles across a front facing gable. The water stains gave it away first but I sure was surprised when I climbed onto the roof and found the way the shingles were laid.