This roof had been replaced 20 years ago with a 40-year roof. The shingles were doing OK, but the problem was with the installation. The step flashing on the back on both sides wasn’t there, instead, they just had a single piece of L-metal that ran up the whole wall that was about 1″ wide. As you can see from the picture below, that’s not even close to step-flashing, which is required. There was also a leak coming through near a pipe on the back side and a piece of compromised plywood that was causing problems. Unfortunately, this 40-year roof only lasted 20 years (with leaks) because some shortcuts were taken by the previous installers.