So initially, the shingles went underneath the metal porch roof, and the previous roofer used sealant to make it waterproof. Well, that didn’t work. The owner said it leaked ever since he added the porch roof.
We thought about using a rubber membrane but decided against it as using a large section of valley metal would have a much higher likelihood of leaking. Here is the area of metal we had bent. We put a 2-inch rise in the middle instead of the standard 1-inch rise, and we made each side 16 inches wide with a 1/2 inch open hem. My metal guy said it is the giant valley metal he’s ever made.
Here is what it looked like after we pulled up the metal panels to cut them. You can see the old roof shingles go under the porch roof's wood.
Here is what it looked like after we installed it. We marked the metal panels, popped a straight line on them, and then took off the metal roofing panels and cut 4 inches off them. Laid the valley metal down and sealed each side. Then we laid the shingles and the metal panels down on top of the valley metal.