Why is it leaking? If you look close, you can see in the wetter/darker spot on the shingle that there are two nail heads that are rusty. This valley looked great from the top, but once we started digging into it, we noticed poor nail placement by the previous installer. This is very common on your home’s first roof, as the builder usually uses the cheapest roofer possible. This roof was about 11 years old. The whole roof will have to be replaced soon, because the workmanship is failing. The shingles are in great shape, but it’s always the workmanship that will fail first. Remember, it’s all about workmanship.